Dynamic Planet · 2026–27 Flashcards

The combined water loss to the atmosphere through evaporation (from water/soil surfaces) and transpiration (through plant stomata). Over land, ET often exceeds 50% of annual precipitation and is the largest output in most watershed water budgets.

P = ET + R + ΔS • P = Precipitation (total input) • ET = Evapotranspiration (atmospheric loss) • R = Runoff (streamflow/subsurface outflow) • ΔS = Change in storage (soil moisture, groundwater, snowpack) Over multi-year periods ΔS ≈ 0, simplifying to P = ET + R.

~3.5% of Earth's water is freshwater: • ~1.7% in glaciers/ice caps (~69% of freshwater) • ~1.7% in groundwater (~30% of freshwater) • ~0.03% as surface water (lakes, rivers, swamps) — <0.4% of freshwater • ~0.001% in the atmosphere Liquid freshwater accessible for human use is less than 1% of all freshwater.

• Atmosphere: ~9 days • Soil moisture: weeks to months • Rivers/streams: days to weeks • Shallow groundwater: years to decades • Deep groundwater: hundreds to thousands of years • Glaciers/ice caps: thousands to hundreds of thousands of years

The time delay between the peak rainfall intensity (or center of the storm event) and the peak discharge on the hydrograph. Short lag time = flashy watershed response (urbanized, steep, impervious). Long lag time = dampened response (forested, flat, permeable soils).

Baseflow is the steady background discharge in a stream between storm events, sustained by groundwater (saturated zone) seeping into the channel. On a hydrograph, it appears as a relatively flat line before and after the storm pulse. Baseflow separation divides total streamflow into direct runoff (storm response) and baseflow components.

Hortonian (infiltration-excess): rainfall intensity exceeds soil infiltration capacity; occurs on compacted/impervious surfaces; can occur anywhere during intense rain. Saturation-excess: soil is completely saturated (water table at surface); even gentle rain cannot infiltrate; concentrated in valley bottoms and variable source areas near streams. Both generate surface runoff but have different spatial patterns.

Contour interval (CI): the fixed elevation difference between adjacent contour lines (stated in map margin). Common values: 10, 20, 40, or 100 ft. Index contours: every 5th contour line, drawn thicker and labeled with elevation — used as anchor points for reading elevation. Regular contours are thinner and unlabeled.

Gradient = Rise / Run = (elevation change) / (horizontal distance) Units: ft/mi, m/km, or dimensionless ratio (e.g., 0.005). Example: 600 ft drop over 3 miles = 200 ft/mi. Steep slopes → closely spaced contours; gentle slopes → widely spaced contours.

When contour lines cross a stream, they bend into a V-shape pointing upstream (toward higher elevation). Water flows in the direction away from the V-point — toward the open end of the V (downhill). Steps: (1) Find the stream (blue); (2) Locate where contours cross; (3) Identify V-shape; (4) V-point = upstream; (5) Flow is away from V-point.

1. Identify the pour point (outlet/gauge).
2. Trace all stream tributaries upstream from the pour point.
3. Draw the drainage divide: follow ridgelines and hilltops connecting high points; the divide crosses contour lines perpendicularly.
4. Ask at each point: 'Does rain here drain to the pour point?' If yes = inside; if no = outside.
5. Close the loop back to the pour point, encircling all contributing area.

1. First-order streams have NO tributaries (unbranched headwater channels).
2. When two streams of EQUAL order join, the resulting stream increases by ONE order (e.g., 1+1=2, 2+2=3).
3. When two streams of UNEQUAL order join, the higher order is retained (e.g., 1+3=3 — NOT 4). Stream order never increases when a smaller stream joins a larger one.

Dendritic: tree-like branching, random tributary angles; forms on homogeneous rock (flat-lying sedimentary strata or massive igneous rock) — most common pattern. Trellis: parallel main streams with right-angle tributaries; forms on alternating resistant/weak rock bands (folded strata — e.g., Appalachian Valley & Ridge). Rectangular: angular right-angle bends in channels; forms where two sets of joints/faults intersect at ~90°.

Radial drainage pattern — streams flow outward in all directions from a central high point (like spokes on a wheel). Forms on isolated elevated landforms such as volcanic cones, domes, or isolated mountains where topography alone (not underlying rock structure) controls drainage direction.

Sinuosity = channel length / straight-line valley length A sinuosity >1.5 is generally classified as meandering. Straight channels: sinuosity ~1.0–1.05 (rare in nature) Braided channels: may have sinuosity near 1 (channels shift constantly within a wide belt) Meandering rivers can reach sinuosities of 3–4+.

Cut bank (outer bend): highest velocity, active erosion, steep/vertical bank face, channel is deepest here — thalweg follows outer bank. Point bar (inner bend): lowest velocity, active deposition of sand/gravel, gently sloping ramp, scroll bar ridges record meander migration over time. Helical (corkscrew) flow drives sediment from outer bank across the bed toward the inner bank, maintaining this coupled erosion-deposition pattern.

1. Meander growth: bend amplifies as outer bank erodes and point bar builds.
2. Neck narrowing: meander loop tightens; meander neck (land between limbs) narrows.
3. Cutoff: river breaks through the neck during a flood; new shorter, steeper channel forms.
4. Oxbow lake: sediment plugs the abandoned loop at both ends; isolated crescent-shaped lake.
5. Infilling: oxbow gradually fills with silt/organic material, becoming wetland/clay plug.

River-dominated (bird-foot): strong river discharge overwhelms coastal processes; distributaries extend as long fingers — classic example: Mississippi River delta. Wave-dominated: wave energy redistributes sediment; smooth, rounded arcuate or cuspate shoreline — classic example: Nile delta. Tide-dominated: tidal currents reshape sediment into elongated ridges parallel to tidal flow — classic example: Ganges-Brahmaputra delta.

Q = A × v • Q = discharge (m³/s or ft³/s [cfs]) • A = cross-sectional area of stream perpendicular to flow (m² or ft²) • v = mean flow velocity (m/s or ft/s) For a rectangular channel: A = width × average depth. For irregular channels, divide into strips: Q = Σ(w_i × d_i × v_i).

Capacity: the TOTAL AMOUNT (volume or mass) of sediment a stream can carry at a given discharge — increases with discharge and velocity. Competence: the MAXIMUM PARTICLE SIZE a stream can transport — proportional to the 6th power of velocity (very sensitive to velocity changes). Memory trick: 'Competence = Clast size' (both start with C-S). A stream can have high capacity (carrying lots of fine silt) but low competence (unable to move gravel).

A log-log graph plotting flow velocity (y-axis, cm/s) vs. grain diameter (x-axis, mm) with three regions:

1. Erosion (above upper curve): velocity sufficient to entrain/pick up grains.
2. Transportation (between curves): grains in motion but not being eroded from bed.
3. Deposition (below lower curve): velocity too low to carry grains; they settle. Key anomaly: clay requires HIGHER velocity to erode than silt (cohesion between clay particles), but once in suspension, clay settles only at very low velocities. Sand erodes at the LOWEST velocity (~20–30 cm/s).

Rp = (n + 1) / m • n = total years of discharge record • m = rank of flood event (1 = largest, 2 = second-largest, etc.) • Rp = recurrence interval in years (return period) Annual exceedance probability = 1/Rp 100-year flood: Rp = 100, annual probability = 1%. Probability of at least one occurrence in t years: P = 1 - (1 - 1/Rp)^t

Rp = (n + 1) / m = (29 + 1) / 3 = 30 / 3 = 10 years Annual exceedance probability = 1/Rp = 1/10 = 10% This is a '10-year flood' — expected to be equaled or exceeded on average once every 10 years.

v = (1/n) × R^(2/3) × S^(1/2) • v = mean flow velocity (m/s) • n = Manning's roughness coefficient (dimensionless; higher n = rougher channel) • R = hydraulic radius = A/P (m), where A = cross-sectional area, P = wetted perimeter • S = channel slope (dimensionless, rise/run) Discharge: Q = v × A Typical n values: smooth concrete ~0.012; natural gravel channel ~0.035; heavily vegetated ~0.08+

R = A / P • A = cross-sectional area of flow (m²) • P = wetted perimeter — channel bottom + both submerged side walls (NOT the water surface) For a 4 m wide × 1 m deep rectangular channel: • A = 4 × 1 = 4 m² • P = 4 + 1 + 1 = 6 m • R = 4/6 = 0.667 m For very wide channels (w >> d), R ≈ mean depth.

Boulder > 256 mm Cobble: 64–256 mm Pebble: 4–64 mm Granule: 2–4 mm Sand (very coarse to very fine): 0.0625–2 mm Silt: 0.004–0.0625 mm Clay: < 0.004 mm (<4 µm) Memory: 'Boulder Cobbles Produce Good Sand Since Centuries' (B, C, P, G, S, S, C)

Roundness: smoothness of a particle's EDGES and CORNERS — high roundness = smooth/worn corners (increases with transport distance and abrasion); low roundness = angular, fresh corners. Sphericity: how closely a particle approximates a SPHERE — equidimensional vs. elongated shape. A pebble can be well-rounded but elongated (high roundness, low sphericity), or freshly broken into a nearly equant shape (low roundness, high sphericity). They are INDEPENDENT properties.

1. Traction (rolling/sliding): coarsest particles (cobbles, gravel) dragged along the bed.
2. Saltation: sand-sized particles (~0.1–2 mm) hop/bounce short distances; dominant for sand.
3. Suspension: silt and clay (<0.0625 mm) kept aloft indefinitely by turbulence; gives rivers brown flood color.
4. Dissolution (solution): chemically weathered ions dissolved in water (invisible dissolved load). All except dissolved load are forms of clastic (particulate) transport.

Natural levee: ridge of sand and coarse silt flanking both sides of a river channel, built by repeated overbank flooding depositing coarsest sediment immediately adjacent to the channel. Can raise the channel above the surrounding floodplain (perched channel). Backswamp: low-lying, poorly drained areas behind natural levees. Receives only finest clay during floods; waterlogged frequently. Yazoo streams run parallel to the main channel in the backswamp, unable to enter due to levee height.

Vadose zone (zone of aeration): pore spaces contain BOTH air and water; subdivided into soil moisture zone, intermediate zone, and capillary fringe; water moves downward by gravity (percolation). Zone of saturation: ALL pore spaces filled with water; groundwater resides here; water flows laterally under hydraulic gradient. Boundary between zones = the water table (where pore pressure equals atmospheric pressure).

Porosity (n) = volume of voids / total volume × 100% — measures STORAGE capacity. Permeability (hydraulic conductivity, K) = ability to TRANSMIT fluid through connected pores. Clay paradox: clay has high porosity (up to 50%+) because of many tiny pores, but those pores are so small (microscopic platelet-shaped) that water molecules experience enormous resistance — permeability is extremely low. Well-sorted gravel has ~30% porosity AND high permeability because its large, well-connected pores allow easy flow.

Unconfined (water-table) aquifer: no impermeable cap; upper boundary IS the water table; recharges directly from surface precipitation; water in well rises only to water table level. Confined (artesian) aquifer: sandwiched between impermeable confining layers (aquitards/aquicludes); water under pressure; recharges only at upland outcrop zone; water in well rises to potentiometric surface — above the aquifer top and sometimes above ground (flowing artesian well).

I = (h₁ − h₂) / d • h₁ = hydraulic head at upgradient point (higher) • h₂ = hydraulic head at downgradient point (lower) • d = horizontal distance between points Groundwater flows from HIGH to LOW hydraulic head (high to low energy), perpendicular to water table contour lines. On a water table contour map, flow lines are drawn perpendicular to contours, pointing toward lower values.

Q = K × I × A • Q = discharge (m³/day or m³/s) • K = hydraulic conductivity (m/day) — aquifer property • I = hydraulic gradient (dimensionless) = (h₁−h₂)/d • A = cross-sectional area perpendicular to flow (m²) Actual pore velocity: v = (K × I) / n • n = porosity (decimal) Because n < 1, actual velocity > Darcy velocity. Contaminants travel at pore velocity, not Darcy velocity.

Q = K × I × A = 5 × 0.004 × 200 = 4.0 m³/day Darcy velocity (q) = K × I = 5 × 0.004 = 0.02 m/day Actual pore velocity = q / n = 0.02 / 0.25 = 0.08 m/day Contaminants travel at 0.08 m/day (~29 m/year), NOT the Darcy velocity of 0.02 m/day.

A cone of depression is a funnel-shaped lowering of the water table (or potentiometric surface) centered on a pumping well. Caused by withdrawing groundwater faster than it can be replenished from the surrounding aquifer. The hydraulic gradient steepens toward the well, drawing in groundwater from all directions. If pumping exceeds recharge, the cone deepens and widens; overlapping cones of depression from multiple wells can cause regional aquifer depletion.

z = 40 × h • z = depth of the freshwater–saltwater interface below sea level (m) • h = height of the water table above sea level (m) Derived from density ratio: ρ_f/(ρ_s − ρ_f) = 1.000/0.025 = 40 Meaning: for every 1 m of freshwater head above sea level, the interface lies 40 m below sea level. Over-pumping lowers h → interface rises → saltwater intrusion. The total freshwater lens thickness above sea level = h + 40h = 41h.

Urbanization effects on hydrograph: • Higher peak discharge (more runoff from impervious surfaces) • Shorter lag time (faster concentration via storm drains) • Steeper rising limb (rapid surface runoff) • Shorter recession limb (quick drainage) • Lower baseflow between storms (less groundwater recharge) • Increased flood frequency (smaller events now generate significant runoff) Forested watershed: lower, broader, later peak; longer lag; higher baseflow.

Base level: the lowest elevation to which a stream can erode its channel — the point of zero gradient. Ultimate base level: sea level — the absolute lower limit for erosion by streams reaching the ocean. Local (temporary) base level: any intermediate control point that temporarily limits downcutting — lakes, reservoirs, resistant rock layers, dams. Local base levels are temporary; when removed (dam breach, lake draining), the stream resumes incision toward the next lower base level.

Braided: high sediment load relative to discharge, steep gradient, non-cohesive (sandy/gravel) banks, variable discharge; multiple interwoven channels separated by shifting bars; wide and shallow; associated with glacial outwash, alluvial fans. Meandering: moderate sediment load, low gradient, cohesive fine-grained banks, more stable discharge; single sinuous channel with cut banks and point bars; deeper relative to width; dominant in lowland alluvial settings.

Key controls on ET rate: • Temperature: higher temps → greater vapor pressure deficit → faster evaporation • Humidity (relative): low humidity → steeper vapor gradient → faster ET • Wind: removes humid air near surface, maintaining vapor gradient • Solar radiation: energy source for phase change (evaporation requires ~2,450 J/g) • Vegetation type: deep-rooted trees transpire more than grasses; C4 plants are more efficient • Soil moisture: actual ET < potential ET when soils dry below field capacity

Orographic precipitation: air is forced upward over a mountain barrier → adiabatic cooling → condensation and precipitation on the windward slope. Rain shadow: the leeward (downwind) side of a mountain receives dramatically less precipitation because air descends, warms adiabatically, and becomes drier. Examples: Olympic Peninsula (wet windward) vs. eastern Washington (dry leeward); Sierra Nevada creates the Great Basin rain shadow.

INCREASE infiltration capacity: • Coarse-textured soils (sand, loam) — large pores • Dry antecedent moisture — pore space available • Dense vegetation — roots create macropores; intercept rain reducing impact • Organic matter — improves soil structure

DECREASE infiltration capacity: • Clay-rich soils — tiny pores, swelling when wet • High antecedent moisture — pores already filled • Compaction (livestock, machinery) — destroys structure • Impervious surfaces — concrete, asphalt • Hydrophobic soils — after fire (waxy compounds coat particles)

Local relief = the difference between the highest and lowest elevation within a defined area. Ridges: elongated high areas — contours form U- or V-shapes pointing AWAY from the high point (downstream); the open end of the U points downhill. Valleys (drainages): contours form V-shapes pointing UPHILL (upstream); opposite of a ridge. Saddle (col): low point on a ridge between two summits — contours form an hourglass/saddle shape with higher elevations on two sides and lower on two sides. Depressions: indicated by hachured contour lines (tick marks pointing inward).

1. Draw a straight line (A–B) across the map section of interest.
2. Place the edge of graph paper along A–B; mark where each contour line intersects the paper edge; label each with its elevation.
3. Set the y-axis scale (vertical) to include the full elevation range; choose vertical exaggeration if desired.
4. For each marked contour intersection, plot a point at the corresponding elevation above the x-axis.
5. Connect points with a smooth curve, maintaining relative steepness (closer marks = steeper slope).
6. Note: V-shaped troughs = stream valleys; U-shapes = glacial valleys; flat sections = floodplains/plateaus.

Watershed (drainage basin, catchment): the total land area that drains water, sediment, and dissolved materials to a common outlet (pour point) via a stream network. Key components: • Divide: ridge/hilltop boundary separating adjacent watersheds; water falling on each side drains to different outlets • Main channel (trunk stream): highest-order stream receiving all tributary flow • Tributaries: smaller streams joining the main channel • Mouth/outlet: point where the stream exits the watershed (ocean, lake, or larger river) Watersheds are nested: small sub-basins combine into larger basins (e.g., Ohio River basin within Mississippi River basin).

Junction angle: the angle at which a tributary enters the main channel. In most natural systems, tributaries enter at acute angles pointing downstream (consistent with flow direction). Barbed tributaries: tributaries that enter the main channel pointing UPSTREAM — a diagnostic feature of stream capture (piracy). They indicate the main channel's flow direction was reversed after a beheading event, leaving tributaries that originally flowed in the other direction looking 'barbed' relative to the new flow direction. Tribute confluence geometry also reflects structural geology — tributaries in trellis patterns enter nearly at right angles to the trunk stream.

1. Amazon River basin: ~7.0 million km² (South America) — largest by area AND discharge (~20% of all river discharge to oceans)
2. Congo River basin: ~3.7 million km² (Central Africa) — largest in Africa, second highest discharge globally
3. Mississippi-Missouri basin: ~3.2 million km² (North America)
4. Ob-Irtysh basin: ~3.0 million km² (Siberia, Russia)
5. Nile River basin: ~3.0 million km² (Africa) — longest river but modest discharge (flows through desert) The Great Divide in North America separates Pacific drainage from Atlantic/Gulf drainage.

Mechanical weathering: breaks rock into smaller pieces WITHOUT changing mineral composition. • Frost wedging: water expands 9% on freezing → enlarges cracks • Root wedging: tree roots pry open fractures • Thermal expansion/contraction: daily temperature cycling causes stress • Abrasion: rocks grind against each other in rivers/glaciers

Chemical weathering: changes mineral chemistry through reactions with water/acids. • Carbonation/dissolution: H₂O + CO₂ → H₂CO₃ dissolves limestone → caves, karst • Hydrolysis: water reacts with silicates (feldspar → kaolinite clay) • Oxidation: Fe²⁺ + O₂ → Fe₂O₃ (rust-red weathering rinds) • Hydration: minerals absorb water, expand (anhydrite → gypsum) Chemical rates increase with warm, moist climates; mechanical rates increase in cold climates with freeze-thaw cycles.

Degradation (downcutting/incision): stream cuts downward → former floodplain left as a terrace above current floodplain. Causes: base level fall (sea level drop, dam removal), increased discharge, decreased sediment load. Aggradation: stream deposits sediment, bed rises. Causes: increased sediment supply, reduced discharge, base level rise. Fills valley; creates thick alluvial deposits. Paired terraces: same elevation on both sides → formed by simple incision into own floodplain (climate-controlled). Unpaired terraces: different elevations on each bank → lateral migration + incision, or tectonic tilting. Epoch: Pleistocene glacial/interglacial cycles drove repeated aggradation–incision terrace sequences in many valleys.

Antecedent stream: a stream that existed BEFORE the mountain/ridge it cuts through was uplifted; the stream maintained its course by eroding as rapidly as the land rose → creates a water gap (gorge through a ridge). Example: Colorado River predates Basin-and-Range uplift; rivers cutting through the Appalachians. Superposed stream: a stream that established its course on a surface layer of rock, then cut downward through it into different, older rock below; the stream ignores the buried structure beneath. Water gap: a gap in a ridge where a stream cuts through. Wind gap: an abandoned water gap — former river course now dry (formed after stream capture diverted the flow).

A knickpoint is a sudden break in the smooth concave-up longitudinal profile of a stream — typically a waterfall, rapid, or steep reach. Forms where a stream crosses resistant bedrock or where base level drops suddenly (dam removal, fault scarp, sea level fall). Upstream migration: erosion is most intense at the base of the waterfall (plunge pool undercutting the resistant cap rock). The cap rock collapses, and the waterfall retreats headward while the gorge below lengthens. Example: Niagara Falls has retreated ~11 km upstream from its original position at the Niagara Escarpment since post-glacial time (~10,000 years ago) at ~1 m/year.

Recharge areas: locations where water infiltrates downward into the aquifer — typically upland areas with permeable soils, minimal impervious cover. For confined aquifers, recharge occurs only at the exposed outcrop (outcrop zone), often far from where the water is used. Discharge areas: locations where groundwater exits to the surface — springs, seeps, wetland fens, stream channels, lakes. Gaining stream: stream receives groundwater inflow (water table higher than stream bed) — baseflow-dominated; stays wet even in droughts. Losing stream: stream loses water to groundwater (water table lower than stream bed) — common in arid regions and near cones of depression; stream stage higher than water table.

Contact spring: groundwater emerges where a permeable layer meets an impermeable layer at the surface — water is forced to discharge laterally. Fault spring: groundwater moves along a fault plane to the surface, or a fault juxtaposes permeable rock against impermeable rock. Artesian spring: pressurized water in a confined aquifer reaches the surface through a natural opening. Karst spring: large discharge where underground conduit flow emerges — can vary greatly in flow (responds rapidly to rainfall). Sustainable yield: the rate of groundwater withdrawal that can be maintained indefinitely without depleting the aquifer — withdrawal must not exceed long-term recharge rate. The Ogallala Aquifer is being mined far above sustainable yield (recharge rate ~0.5 cm/yr vs. extraction of meters/yr).

Stream capture (piracy): headward erosion by an aggressive stream (captor) breaks through a divide and intercepts the headwaters of an adjacent stream (beheaded stream). Evidence left behind:

1. Elbow of capture: abrupt right-angle bend where the captured channel joins the captor stream
2. Wind gap: the abandoned valley through the ridge, now dry (no stream), that was formerly occupied by the beheaded stream
3. Barbed tributaries: tributaries of the beheaded stream that now enter the captor in the 'wrong' direction (pointing upstream)
4. Misfit stream: the beheaded stream is now under-fit — its valley is too large for its reduced flow

Perennial stream: flows year-round; water table above channel bed; sustained by baseflow → gaining stream (receives groundwater). Intermittent stream: flows seasonally (spring snowmelt, wet season); alternately gaining/losing; goes dry during droughts or dry season. Ephemeral stream: flows only during or immediately after precipitation; water table always below channel; no baseflow; always a losing stream. Gaining (effluent) stream: groundwater seeps into channel; water table slopes toward stream; provides cool, stable baseflow. Losing (influent) stream: stream recharges groundwater; water table below channel; common in karst, arid regions, and near wells.

Sorting: the degree to which particles in a sediment sample are similar in size. A well-sorted sediment has particles all near the same size; poorly sorted sediment has a wide range of sizes. Interpretation: • Well-sorted: long transport distance or winnowing by wind/waves (beach sand = well-sorted); aeolian dunes = extremely well-sorted • Poorly sorted: short transport, rapid deposition, mixed sources — glacial till is the classic example (glacier deposits all sizes together without water sorting) • Moderate sorting: typical of fluvial deposits Textural maturity scale: immature (clay-rich, poorly sorted, angular) → submature → mature → supermature (pure, rounded, well-sorted quartz sand)

The summit elevation is between 760 ft and 800 ft — it is above 760 ft but below 800 ft. Best estimate: typically read as 'higher than 760 ft' — the actual elevation could be anywhere from 761 ft to 799 ft. If the hilltop contour (the highest closed contour) is at 760 ft: the summit is 760 ft < elevation < 800 ft, so commonly reported as ~780 ft (half-contour interval above the highest closed contour as a rough estimate). Rule: you cannot know the exact summit elevation from contours alone — the last closed contour gives a minimum, and the summit is less than one full contour interval above it.

Reaction sequence:

1. CO₂ + H₂O → H₂CO₃ (carbonic acid formation — in soil, atmosphere)
2. H₂CO₃ → H⁺ + HCO₃⁻ (dissociation to bicarbonate)
3. H⁺ + CaCO₃ → Ca²⁺ + HCO₃⁻ (dissolution of limestone) Net: CaCO₃ + CO₂ + H₂O → Ca²⁺ + 2HCO₃⁻ CO₂ control: Soil CO₂ concentrations are 10–100× higher than atmospheric (from root respiration and decomposition), making soil water far more aggressive (lower pH) than rainwater. Dissolution is most intense in the soil zone and at depth where CO₂-rich water contacts limestone. Temperature also matters: cooler water holds more dissolved CO₂, enhancing dissolution.

Solution sinkhole: slow, gradual dissolution of bedrock downward; forms a bowl-shaped depression in exposed or thin-soil karst; no abrupt failure. Decades to millennia. Collapse sinkhole: cave roof failure — underground void grows until overburden can no longer support itself; sudden, dramatic, can be catastrophic. Cover-subsidence sinkhole: overlying sediment is unconsolidated and granular (sand); slowly ravels downward through bedrock openings; gradual sagging at surface. Cover-collapse sinkhole: cohesive clay overburden forms a 'bridge' over the void; bridge eventually fails suddenly → dramatic surface collapse (most dangerous type; common in Florida). Cenotes: water-filled sinkholes in the Yucatán Peninsula (limestone), used as freshwater sources by Maya civilization.

Polje (polye): the largest karst landform — a broad, flat-floored closed depression, sometimes km to tens of km wide, with steep margins; may flood seasonally through internal ponors (swallow holes). Common in Dinaric Alps (former Yugoslavia). Blind valley: a stream valley that terminates abruptly where the stream sinks underground (at a swallow hole/sinkhole); valley has no through-drainage. Tower karst (fenglin): tall, steep-sided residual limestone towers rising above lowlands; formed in tropical humid regions with rapid dissolution leaving isolated pinnacles. Classic in Guilin, China. Cockpit karst (fengcong): irregular star-shaped depressions surrounded by rounded hill masses; more moderate dissolution in tropical karst. Classic in Jamaica, Puerto Rico.

Phreatic passage: formed BELOW the water table in the zone of saturation; dissolution occurs on ALL sides of the tube (rounded, oval cross-section — 'plumbing pipe' shape); walls show scalloping by turbulent flow. Vadose passage: formed ABOVE the water table; water flows on the floor of the passage, cutting a canyon-shaped incision with a flat or trenched floor and passage extending upward. Speleothems (cave deposits — CaCO₃ precipitated from supersaturated water):

1. Stalactite: hangs from ceiling (c = ceiling)
2. Stalagmite: grows from floor up (g = ground)
3. Column: stalactite + stalagmite meet
4. Flowstone: sheet of calcite coating cave floor/walls
5. Helictite: twisted/curved speleothem defying gravity (capillary forces)
6. Cave pearl: concentric calcite around a nucleus in a shallow pool

Dye tracing: a tracer dye (typically fluorescent rhodamine WT or fluorescein) is introduced at a swallow hole or sinkhole; detectors (charcoal packets or instruments) are placed at springs, wells, and streams. Breakthrough time and concentration curves reveal: • Which springs are hydraulically connected to which sinkholes • Flow velocity through conduits (often 100–1000 m/day — much faster than porous media) • Multiple connections (one sinkhole may supply several springs) Rapid storm response: karst springs can respond within hours of a storm because water travels through conduits (open cave passages) at high velocity, with minimal storage and filtration. This also means contaminants introduced at the surface reach springs rapidly — unlike porous-media aquifers that filter and attenuate pollutants.

Karst aquifer: • Flow: rapid conduit flow (meters to km per day); turbulent in conduits • Storage: limited (most water in conduits, not matrix porosity) • Response: flashy — springs respond within hours of rainfall • Contamination: extremely vulnerable — no filtration; contaminants travel fast; spills can rapidly reach springs • Examples: Florida (Floridan Aquifer), Kentucky/Tennessee (Mammoth Cave), Yucatán (Maya cenotes), Dinaric Alps, South China Karst

Porous-media (granular) aquifer: • Flow: slow (meters per year to decade); laminar, Darcian • Storage: large (water distributed throughout matrix pores) • Response: slow — weeks to months lag after recharge • Contamination: more protected — physical filtration of bacteria; travel time gives opportunity for attenuation

1. Florida (especially central Florida): dissolution of Eocene-Miocene limestone (Floridan Aquifer System); cover-collapse sinkholes common due to thick unconsolidated sandy overburden over cavities
2. Kentucky/Tennessee (Mammoth Cave region): Mississippian-age limestone; home to the world's longest known cave system (~676 km mapped passages)
3. Pennsylvania/Virginia/West Virginia (Valley & Ridge Province): dolomite and limestone in folded Appalachian strata; many springs and disappearing streams
4. Texas (Edwards Plateau / Edwards Aquifer): Cretaceous limestone; major karst spring systems including Barton Springs (Austin) and Comal Springs (New Braunfels)
5. Missouri (Ozark Plateau): Ordovician-Mississippian carbonates; dense cave and spring network

1. Glacial moraine-dammed: moraine acts as dam behind glacial retreat — Great Lakes, Finger Lakes (NY)
2. Kettle lake: formed in depression left by melting buried ice block in glacial outwash — abundant in Minnesota, Wisconsin
3. Tectonic (rift/graben): formed in down-dropped block between faults — Lake Baikal (world's deepest, ~1,642 m), East African Rift lakes (Tanganyika, Malawi)
4. Volcanic crater lake: formed in volcanic crater or caldera — Crater Lake, Oregon (Mazama caldera, 589 m deep)
5. Oxbow lake: abandoned meander loop sealed by sediment — Mississippi River floodplain
6. Solution lake: sinkhole fills with water — Florida lakes; Dead Sea basin

Closed-basin lake: a lake with no surface outflow — water exits ONLY by evaporation. Because evaporation removes pure water but leaves dissolved salts behind, solutes accumulate over time → salinity increases. Salinity rate depends on balance between: • Inflow of dissolved salts (streams, groundwater) • Evaporation rate (hot, arid climates accelerate salinization) Examples: • Great Salt Lake, Utah: ~3–8× seawater salinity (varies with water level); shallow closed basin; historically higher lake (Pleistocene Lake Bonneville) • Dead Sea (Jordan/Israel): ~10× seawater (~340 g/L); hypersaline; deepest point ~306 m below sea level — world's lowest lake surface elevation

Summer stratification development:

1. Spring overturn (see 9.4) eliminates stratification — lake isothermal at ~4°C
2. Surface water warms → becomes less dense → floats above cold deep water
3. Three layers form: • Epilimnion: warm (15–25°C+), well-mixed by wind, oxygenated • Thermocline (metalimnion): rapid temperature drop (≥1°C per meter depth) • Hypolimnion: cold (4°C), still, often oxygen-depleted (decomposition)
4. Density difference resists wind mixing → stratification stable through summer

Fall breakdown: 5. Surface cools → density increases → sinks 6. Thermocline weakens → wind mixing penetrates deeper 7. Complete overturn when surface reaches ~4°C (densest) → isothermal mixing

Thermocline gradient = ΔT / Δz = (12 − 6) / (9 − 5) = 6°C / 4 m = 1.5°C/m A thermocline gradient ≥1°C/m is typically considered a strong (stable) thermocline. This calculation is a common exam question: read off two depths from a profile and divide. Context: in summer, the thermocline in a small temperate lake often exceeds 2–3°C/m; large deep lakes may have gentler gradients.

Dimictic (two turnovers): temperate lakes — spring overturn (ice melts, 4°C surface sinks) + fall overturn (surface cools to 4°C, sinks again). Stratified in summer (warm epilimnion) and winter (ice cover, inverse stratification). Monomictic: one overturn per year. Warm monomictic (subtropical — never freezes, overturns in winter). Cold monomictic (subpolar — frozen most of year, only summer open water allows mixing). Meromictic: NEVER fully mixes. Permanent chemocline separates oxygenated surface (mixolimnion) from anoxic bottom (monimolimnion); dense bottom water may be saline, high in dissolved gases (CO₂, H₂S). Lake Nyos, Cameroon (volcanic CO₂ limnic eruption 1986). Amictic: never mixes — permanently ice-covered (Antarctic polar lakes); year-round inverse stratification.

            Oligotrophic        Eutrophic

Nutrients: Low (N, P) High Transparency: High (Secchi 6–10m+) Low (<1–2 m) DO (deep): High (well-oxygenated) Low/anoxic in hypolimnion Algae: Sparse, low biomass Dense blooms (cyanobacteria) Fish: Cold-water species Warm-water, tolerant species (trout, cisco) (carp, catfish) Examples: Lake Superior, Lake Erie (western basin), Lake Tahoe Lake Apopka (FL) Key driver of eutrophication: phosphorus is the limiting nutrient in most freshwater lakes (Liebig's law of the minimum). Even small P additions can trigger algal blooms.

TMDL (Total Maximum Daily Load): the maximum amount of a pollutant (most often phosphorus or nitrogen) that a water body can receive while still meeting water quality standards (set under the Clean Water Act, Section 303(d)). Process:

1. Water body is listed as impaired (e.g., algal blooms violating DO/recreation standards)
2. EPA or state calculates the TMDL: how much total load the lake can absorb
3. TMDL is allocated among point sources (wastewater plants) and non-point sources (farms, stormwater)
4. Permits (NPDES) are revised to enforce point-source limits; BMPs required for non-point sources TMDLs are legally binding pollution reduction targets. Phosphorus TMDLs have driven nutrient reductions in Lake Erie, Chesapeake Bay, and many Florida lakes.

Wave action: wind-generated waves strike the shoreline at an angle → swash runs up beach at the wave approach angle; backwash runs straight down the beach slope under gravity. Littoral (longshore) drift: net sediment transport parallel to the shoreline when waves approach at an angle; sediment is moved in a zigzag path along the shore. Beach: accumulation of sediment in the surf zone where wave energy is dissipated. Spit: an elongated deposit of sand/gravel extending from the shoreline into open water — formed where the shoreline curves (e.g., into a bay) and littoral drift continues in the original direction, depositing sediment into deeper water. Spits often curve at their tips (recurved spit) due to refracted waves. Common lake shore features: tombolos (bars connecting an island to shore), bay-mouth bars, and cuspate forelands.

Natural lake: • Sediment trapping: traps fine suspended sediment, but inlet areas coarsen; basin fills slowly over geological time • Water quality: stratifies naturally; minimal management • Downstream: natural flow regime; bedload continues past outlet (if stream outflow)

Reservoir: • Sediment trapping (trap efficiency): dams trap 80–100% of incoming sediment → reservoir loses capacity over time; Aswan Dam has trapped ~100% of Nile sediment → downstream delta erosion and loss of agricultural fertility • Water quality: selective withdrawal at depth can discharge cold hypolimnetic water or anoxic water — alters temperature and DO downstream • Downstream: peak flows reduced (flood control), but also baseflow may be manipulated; sediment-starved stream below dam incises its bed and erodes banks ('hungry water' effect)

Hydrarch succession (lake filling):

1. Open deep lake: phytoplankton, zooplankton communities; sediment accumulates from inflow and organic matter
2. Shallow lake: rooted aquatic macrophytes colonize edges (water lilies, pondweeds); more sediment trapping
3. Marsh stage: lake is very shallow; emergent vegetation (cattails, reeds) encroaches from shore; peaty organic matter builds up
4. Wet meadow/Fen: standing water disappears; wet grassy/sedge community develops on organic soils
5. Shrub swamp: woody shrubs (willows, alders) invade
6. Forested swamp/bog: trees colonize (in humid climates); eventually...
7. Climax forest: terrestrial community Timeline: small lakes in temperate regions may complete this in hundreds to a few thousand years; large lakes persist much longer. Most glacial lakes are <15,000 years old.

Fjord: a long, narrow, deep inlet carved by a glacier that has since been drowned by sea-level rise (marine fjord) or filled with freshwater (fjord lake). Carved by valley glaciers — typical U-shaped cross-section, over-deepened below sea level. Locations: Norway, British Columbia, southern Chile, New Zealand, Greenland. Glacial scour lake: formed in a rock basin EXCAVATED by glacial erosion — bedrock overdeepened by moving ice; Great Lakes are partly scour lakes. Moraine-dammed lake: water backed up behind a terminal or lateral moraine deposit left by glacial retreat — the moraine is a dam of unconsolidated sediment (can be unstable → glacial lake outburst floods/GLOFs). Key difference: Scour lakes are in bedrock basins; moraine-dammed lakes have sediment dams that can fail.

Inverse stratification: in winter, surface water is coldest and LESS dense (water density maximum is at 4°C; below 4°C, water becomes less dense again as it approaches 0°C for freezing). This 'anomalous expansion' of water means: • Dense 4°C water sinks to the bottom • Cold (<4°C) water accumulates near surface • Ice forms at 0°C at the top • Bottom water remains at ~4°C all winter Why this matters: if water behaved like most substances (denser when cold), lakes would freeze from the bottom up, killing all benthic life every winter. The anomalous density maximum at 4°C protects aquatic life by insulating deep water under an ice layer. Inverse stratification: surface ≈ 0°C (ice), just below ice ≈ 1–3°C, bottom ≈ 4°C.

The three-parameter approach (all three must be present under normal conditions):

1. Hydric soils: soils formed under prolonged waterlogged conditions — show gleying (blue-grey reduced iron), mottling, or organic accumulation (Histosols, Aquents)
2. Hydrophytic vegetation: plant species adapted to saturated/flooded conditions — listed as OBL (obligate wetland, >99% frequency in wetlands), FACW, FAC (facultative)
3. Wetland hydrology: evidence of inundation or saturation at/near the surface for ≥14 consecutive days during the growing season — water marks, drift lines, oxidized root channels Regulatory framework: Clean Water Act Section 404 requires a permit to fill 'waters of the US' including wetlands; Ramsar Convention (1971) is the international treaty protecting wetlands of international importance.

Freshwater tidal marsh: • Hydrology: daily tidal inundation (near river mouths, estuaries above salinity); predictable flooding rhythm • Vegetation: wild rice, arrow arum, spatterdock, pickerelweed • Nutrients: tidal exchange imports nutrients from estuary; highly productive • Found: tidal rivers, e.g., Delaware River, Hudson River freshwater reach

Inland (non-tidal) freshwater marsh: • Hydrology: seasonal flooding from precipitation, snowmelt, or groundwater; fluctuating water table • Vegetation: cattails (Typha), bulrushes (Scirpus), sedges, common reed (Phragmites) • Nutrients: nutrient-rich if fed by agricultural or urban runoff; productive • Habitat: Prairie potholes (glacial kettles), Everglades (Florida), Great Black Swamp (former Ohio) Both support high waterfowl and wading bird diversity.

Forested swamps (bottomland hardwood forests): • Flood attenuation: absorb floodwaters like a sponge, releasing slowly — reduce downstream peak flows • Water quality: uptake of nutrients (N, P) from floodwaters; sediment deposition • Wildlife habitat: high biodiversity — black bears, migratory birds (wood warblers, waterfowl), wood ducks, amphibians; bottomland hardwoods are among North America's most diverse habitats • Carbon storage: waterlogged soils accumulate organic matter (low decomposition under anoxic conditions) Vegetation examples: bald cypress (Taxodium distichum) and tupelo gum (Nyssa aquatica) in southeastern US swamps (e.g., Atchafalaya Basin, Great Dismal Swamp) Threats: drainage and conversion to agriculture (lost ~50% of US wetlands since 1600s, mostly bottomland hardwoods); levee construction that cuts off flood pulses; logging for cypress timber.

          BOG                          FEN

Water source: Precipitation only (ombrotrophic) Groundwater + precipitation (minerotrophic) pH: Acidic (3.5–5) Circumneutral to alkaline (5.5–7.5+) Nutrients: Nutrient-poor Moderate to nutrient-rich Vegetation: Sphagnum mosses dominate; Sedges (Carex), grasses, rushes; ericaceous shrubs (Labrador tea, brown mosses; orchids in rich fens; sundew, pitcher plant) peat-forming but more diverse Peat type: Sphagnum peat (light, low bulk Sedge/reed peat (darker, denser, density, highly acidic) less acidic) Transition: Bog → fen if mineral groundwater Fen → bog if basin becomes isolated is cut off; fen → bog if domed from groundwater by sphagnum growth Boreal peatlands (Canada, Russia) store ~30% of global soil carbon despite covering <3% of land area.

Hydroperiod: the annual pattern of water depth fluctuation in a wetland — specifically the timing, duration, frequency, and depth of surface inundation and soil saturation. Why it's the 'master variable': • Determines what soil processes occur (aerobic vs. anaerobic; redox conditions) • Controls what plants can survive (tolerates only species suited to that flooding regime) • Influences nutrient cycling rates (denitrification, methanogenesis, sulfate reduction all require anoxic soils) • Shapes wildlife use (waterfowl need specific water depths; amphibians need permanent vs. temporary water) Examples of hydroperiod variation: • Temporary wetland: wet 2–4 months/year (vernal pools) → supports fairy shrimp, wood frogs • Permanent wetland: water year-round → supports fish, supports different vegetation • Prairie potholes: highly variable (wet years vs. dry years) → 'duck factory of North America'

1. Flood attenuation: wetlands store excess water during high flows and release it slowly, reducing peak discharge downstream. Key mechanism: high soil porosity + surface storage volume. A 1-ha wetland can store thousands of m³.
2. Water quality improvement: nutrients (N, P) and sediment are retained as floodwaters slow and deposit; denitrifying bacteria in anoxic soils convert NO₃⁻ to N₂ gas (nitrogen removal); wetlands can remove 80–90% of nitrate from agricultural runoff.
3. Carbon sequestration: anaerobic decomposition is slow → organic matter (peat) accumulates; peatlands store ~550 Gt C globally (comparable to atmospheric CO₂).
4. Biodiversity habitat: ~40% of world's species live in or depend on freshwater habitats, many tied to wetlands; waterfowl, amphibians, freshwater mussels all use wetlands.
5. Shoreline stabilization: wetland root systems bind sediment and dissipate wave energy, reducing erosion along lake/stream margins.

River Continuum Concept (Vannote et al., 1980): physical and chemical gradients from headwaters (small streams) to large rivers predict predictable shifts in biological communities and organic matter processing.

Headwaters (orders 1–3): shaded by riparian canopy → little in-stream photosynthesis → dominated by allochthonous inputs (leaf litter, wood); shredders (break down leaves: stonefly, caddisfly larvae) and collectors dominant; P:R ratio <1 (heterotrophic).

Mid-reaches (orders 4–6): wider, more light; algal mats and aquatic macrophytes develop → more autochthonous production; grazers (scrape algae: mayfly larvae) and collectors dominant; P:R ≈ 1.

Large rivers (orders 7+): turbid → low light penetration → dominated by fine particulate organic matter from upstream; collectors and filterers dominant; P:R <1 again (heterotrophic from upstream processing).

Hilsenhoff Biotic Index (HBI): a numerical water quality index based on the pollution tolerance of benthic macroinvertebrates. Each taxon has a tolerance value (0 = intolerant to pollution; 10 = highly tolerant). HBI = Σ(nᵢ × tᵢ) / N, where nᵢ = count of taxon i, tᵢ = tolerance value, N = total individuals. HBI interpretation: 0–3.50 = Excellent; 3.51–5.50 = Good; 5.51–6.50 = Fair; 6.51–8.50 = Poor; 8.51–10 = Very Poor.

EPT taxa (Ephemeroptera-Plecoptera-Trichoptera): • Mayflies (Ephemeroptera): gills on abdomen, 2–3 tails — sensitive to low DO and pollution • Stoneflies (Plecoptera): 2 tails, require cool, well-oxygenated water — most sensitive • Caddisflies (Trichoptera): build cases of sand/sticks; generally sensitive High EPT richness and abundance → high water quality. Chironomid midges and tubificid worms (oligochaetes) are tolerant taxa indicating impairment.

Oxygen sag curve (Streeter-Phelps model): documents DO depletion and recovery downstream of an organic pollution point source. Zones:

1. Zone of degradation (immediately below outfall): BOD (biochemical oxygen demand) from organic waste consumes dissolved oxygen faster than reaeration can replenish; turbid, few organisms; DO falls rapidly.
2. Zone of active decomposition (DO minimum/sag): DO reaches minimum (can hit 0 — septic zone); anaerobic decomposition; H₂S odors; primarily tolerant organisms (tubificid worms, rat-tailed maggots).
3. Zone of recovery: reaeration from atmosphere + photosynthesis replenishes DO; algae increase; organic matter is mostly oxidized; DO rises.
4. Clean zone: DO restored to saturation; EPT organisms return; recovery complete. Rate of recovery depends on: river velocity, turbulence (reaeration), temperature, dilution, and nature/amount of waste.

Five functions of riparian zones:

1. Thermal regulation: streamside trees shade the channel → reduce water temperature → critical for cold-water fish (trout, salmon); logging that removes riparian trees increases stream temperatures dramatically
2. Bank stabilization: root networks bind soil, resist erosion during high flows
3. Allochthonous organic input: leaf litter, insects, woody debris fall into stream → food for shredders and provide habitat complexity (large woody debris = pools, cover)
4. Nutrient filtering: uptake of nitrogen and phosphorus in vegetation and soil before it reaches the stream; denitrification in riparian soils
5. Wildlife corridor: linear habitat connecting upland patches; critical for amphibians, mammals, and birds

Key threat: livestock grazing in riparian zones → trampling of streambanks → increased sediment, reduced shade, compacted soils that reduce infiltration.

Trophic cascade: indirect effects of a predator on lower trophic levels that 'cascade' down the food web — typically reducing herbivore abundance → releasing primary producers from grazing pressure. Classic freshwater example — the wolf-fish-algae cascade: • Remove top predator fish (e.g., bass): mid-level predators (small fish, crayfish) increase → zooplankton decrease → algae (phytoplankton) increase → eutrophication. Explicit lake example: Lake Mendota, Wisconsin • Biomanipulation: stocking predatory bass/walleye → suppresses planktivorous fish → zooplankton (Daphnia) increase → zooplankton graze algae → water clarity improves Chain: predator fish → [suppress] → planktivores → [suppress] → zooplankton → [graze] → phytoplankton Magnitude: a single large Daphnia (water flea) can clear 1 mL of algal water per hour — grazing is powerful when zooplankton are released from fish predation.

BOD (Biochemical Oxygen Demand): the amount of dissolved oxygen consumed by microorganisms in decomposing organic matter in a water sample over a set time (standard: 5 days at 20°C = BOD₅), measured in mg/L. High BOD = high organic load = high oxygen demand: • Clean water: BOD₅ < 2 mg/L • Moderately polluted: 3–8 mg/L • Untreated sewage: 200–400 mg/L Relationship to DO: as microbes decompose organic matter, they consume O₂ → DO decreases. If decomposition rate > reaeration rate, DO drops below 4 mg/L (stressful to fish) or to 0 (hypoxic/anoxic → fish kills). COD (Chemical Oxygen Demand): measures ALL oxidizable material (chemical oxidation with dichromate) — faster test, higher values than BOD, includes non-biodegradable fraction.

Point-source (PS) pollution: • Origin: single, identifiable discharge location — pipe, outfall, ditch; factories, municipal wastewater plants, CAFOs (large animal operations) • Regulation: Clean Water Act Section 402 (NPDES permits) — regulated since 1972; relatively well-controlled in US • Monitoring: continuous sampling at pipe; compliance measurable

Non-point-source (NPS) pollution: • Origin: diffuse; no single pipe — agricultural fields, urban lawns, roads, construction sites, logging operations, atmospheric deposition • Regulation: no direct permit system; managed through BMPs (Best Management Practices) — cover crops, buffer strips, detention ponds, street sweeping • Monitoring: difficult to attribute to specific land owner • Scale: NPS is now the leading cause of water quality impairment in US lakes and streams; agricultural NPS (nitrogen, phosphorus, sediment) is dominant

Eutrophication cascade:

1. Nutrient loading: excess N (mostly from agriculture/urban) and P enter waterbody via runoff or wastewater
2. Algal bloom: nutrients stimulate rapid growth of phytoplankton (especially cyanobacteria in warm, calm, stratified conditions)
3. Light limitation: dense algal mats block sunlight → submerged aquatic vegetation (SAV) dies
4. Algal die-off: when nutrients or light are exhausted, algal biomass dies and sinks
5. Decomposition: bacteria decompose algal cells → consume large amounts of DO
6. Hypoxia/anoxia: DO falls below 2 mg/L (hypoxic) or 0 (anoxic) — especially in stratified hypolimnion where O₂ cannot be replenished
7. Fish kills and dead zone: motile organisms flee; sessile benthos die Cyanobacteria add risk: produce toxins (microcystins) → harmful algal blooms (HABs) toxic to fish, wildlife, humans

Excess sediment impacts: • Increases turbidity → reduces light penetration → suppresses algae and submerged vegetation • Settles in streambed → fills interstitial spaces → smothers salmon eggs in gravel redds → kills benthic invertebrates • Abrades fish gills → physiological stress • Fills reservoirs → reduces storage capacity (sedimentation is the principal long-term threat to reservoir function) • Covers cobble/gravel substrate → destroys EPT habitat Sources: construction sites (can export 200× more sediment than forested land per unit area), agriculture, logging, streambank erosion

Thermal pollution: • Source: power plants withdraw river water for cooling; return it 5–10°C warmer; also loss of riparian shade • Effects: reduces DO saturation (warm water holds less O₂); disrupts spawning cues; favors warm-water nuisance algae; can exceed thermal tolerance of cold-water species (trout: T_lethal ~25°C)

Bioaccumulation: the buildup of a contaminant in an organism's tissues to concentrations HIGHER than in the surrounding water/environment. Occurs when the rate of uptake exceeds excretion — especially for lipophilic (fat-soluble) compounds that bind to fatty tissues. Biomagnification: the increase in contaminant concentration at each step UP the food chain, because predators consume many prey items, concentrating the contaminant. Freshwater example — Mercury (methylmercury): • Water: 0.0001 µg/L • Phytoplankton: 0.01 µg/kg • Zooplankton: 0.1 µg/kg (~1000× water) • Small fish: 1 µg/kg • Large piscivorous fish (walleye, pike): 1–10 µg/kg (~100,000× water) Mercury enters via atmospheric deposition (coal combustion); converted to methylmercury by sulfate-reducing bacteria in anoxic sediments; bioaccumulates up the fish food chain. FDA advises limits on consumption of top predator fish.

Hungry water effect: the reservoir traps nearly all sediment moving downstream (trap efficiency 80–100% for large dams). Water released from the dam is clear and sediment-free, but still has energy (velocity). This sediment-free water: • Erodes and incises the river bed downstream — 'hungry' for sediment • Undercuts and erodes streambanks (bank scalloping) • Armors the streambed — fine material removed, coarse lag deposit remains • Reduces bedload to river mouth → delta erosion and shoreline retreat (e.g., Nile delta retreating since Aswan Dam, 1970; Colorado River delta nearly eliminated) Other dam effects: • Altered thermal regime: hypolimnetic releases are cold in summer → thermal shock for downstream fish • Blocked fish migration (salmon, shad cannot pass without fish ladders) • Simplified flow regime — loss of flood pulses that maintain floodplain ecosystems

Local protection: levees confine river flow within a narrower channel → prevents overbank flooding of protected areas → reduces floodplain inundation.

Downstream amplification ('levee effect'): • Levees prevent floodplain storage → all flood water stays in channel → water travels faster downstream at higher stage • Peak discharge INCREASED downstream because natural floodplain attenuation is eliminated • Downstream communities (beyond levee system) experience larger floods • Levee failure: when overtopped or breached, failure is sudden and catastrophic — water pours into area that has been protected (and densely developed) → severe damage

Floodplain disconnection: • Floodplain-river exchange eliminated → loss of nutrient cycling, fish spawning habitat, sediment deposition • Example: Lower Mississippi River levee system — much of New Orleans is below river level; entirely dependent on levee integrity (Hurricane Katrina 2005 demonstrated vulnerability)

Aral Sea case study: • Formerly the 4th largest lake in the world (~68,000 km²) on the Kazakhstan-Uzbekistan border • Soviet irrigation schemes in the 1960s–1980s diverted the Amu Darya and Syr Darya rivers (the two main inflows) almost entirely to cotton and rice irrigation • By 2000: reduced to ~10% of original volume; split into separate North and South Aral Sea remnants • Impacts:

• Fishing industry (24 native fish species) collapsed — 40,000–60,000 fishing-related jobs lost
• Exposed lake bed became a salt and pesticide desert — dust storms carry toxic material across the region
• Local climate changed (hotter summers, colder winters without the lake's moderating effect)
• Salinization of irrigated soils → declining crop yields • Partial recovery: Kazakhstan built the Kok-Aral Dam (2005) → North Aral Sea partially refilled; fish returned • Key lesson: inland closed basins are highly sensitive to diversion; recovery requires international cooperation

Channelization outcomes (short vs. long term): Short-term: faster drainage, reduced flooding of adjacent land, lowers water table Long-term (unintended): • Increased stream gradient → higher velocity → accelerated bed and bank erosion • Loss of habitat diversity (riffles, pools, meanders → uniform straight ditch) • Loss of riparian vegetation, streamside habitat • Water drains faster → lower groundwater → drains adjacent wetlands • Increased downstream flooding (flashier hydrograph)

Stream restoration goals (reverse channelization effects): • Re-meandering: reintroduce bends → lower gradient → reduce erosion • Reconnecting floodplain: re-establish overbank flow events • Large woody debris placement: create pools, habitat complexity • Riparian planting: restore shade, bank stabilization, allochthonous inputs • Results measurable in years: EPT taxa return; fish diversity increases; water temperature decreases

Elwha River, Washington (completed 2014): • Two dams removed: Elwha Dam (1913) and Glines Canyon Dam (1927) • Combined the largest dam removal in US history at the time • 21 km of river and 70 km of tributary reopened to salmon • Outcome: within 2 years, Chinook salmon recolonized upstream reaches; sediment delta rebuilt; vegetation recolonized delta; river channel re-established natural braided pattern in former reservoir

Klamath River, California/Oregon (completed 2024): • Four dams removed: Iron Gate, Copco 1, Copco 2, J.C. Boyle • Largest dam removal in US history (and world at time of completion) • ~480 km of Klamath River and tributaries reopened • Goals: restore Coho and Chinook salmon runs; reduce toxic algal blooms (Microcystis) in reservoirs; restore tribal (Karuk, Yurok, Klamath tribes) cultural resources • Early monitoring (2024–): salmon entering newly accessible reaches within weeks of dam breach

Hydrograph effects of deforestation: • Interception eliminated: rain hits soil directly → increased surface impact → more surface runoff • ET reduced (no transpiration) → more soil moisture → faster saturation → more runoff • Result: higher, flashier peaks; shorter lag time; more frequent flooding; LOWER baseflow (less groundwater recharge if soil is compacted)

Sediment yield: • Road construction during logging: highly erosive; sediment yield from logging roads can be 100–1,000× background rates • Loss of root network → mass wasting (debris slides) on steep slopes • Direct log skidding → soil compaction, ruts, erosion • Buffer strip requirement (Oregon Plan, Riparian Forest Practice): mandatory no-cut zones beside streams

Stream temperature: • Forest canopy shading maintains cool water temperatures • Logging removes shade → streams warm by 5–10°C in some studies • Lethal to cold-water salmonids (trout, salmon) → thermal refugia become critical

Critical Zone (CZ): the thin outer layer of the Earth extending from the top of the forest canopy to the bottom of active groundwater — where rock meets life. Defined by the US NSF Critical Zone Observatory network (established 2007). Why 'living skin': it is the interface where: • Physical weathering of bedrock → regolith → soil → sediment • Chemical weathering transforms rock minerals → releases nutrients • Water moves through rock, soil, biosphere, and atmosphere • Life (microbes, roots, burrowing animals) actively drives weathering and nutrient cycling • All freshwater that humans use (surface water, soil moisture, groundwater) is generated and stored here Thickness: millimeters (bare rock) to tens of meters (thick weathering profiles in humid tropics) Key products: soil, freshwater, atmospheric CO₂ drawdown via silicate weathering (long-term carbon cycle)

Field capacity (FC): the water content remaining in a soil after excess water has drained away under gravity (~1–3 days after saturation). Represents the maximum water a soil can hold against gravity. Sandy soil FC ~10%; clay loam FC ~35%. Wilting point (WP, permanent wilting point): the soil water content at which plants can no longer extract water against the strong matric suction holding it to soil particles (~1.5 MPa); leaves wilt permanently. Sandy soil WP ~4%; clay WP ~20%. Plant-available water (PAW) = FC − WP: the fraction of soil water accessible to plants. • Sandy soil: FC 10% − WP 4% = ~6% PAW (low storage) • Clay loam: FC 35% − WP 20% = ~15% PAW • Loam (optimal): balances water-holding and drainage Capillary rise: water moves upward through small pores against gravity (matric potential) — can raise water 0.3 m (coarse sand) to >30 m (very fine clay) above water table.

Permafrost: ground that remains at or below 0°C for at least two consecutive years (ice not necessarily present, but usually is).

Distribution: • Continuous permafrost: >90% of area underlain; northern Alaska, Canada, Siberia — temperatures so cold that permafrost exists everywhere except beneath lakes/rivers • Discontinuous permafrost: 50–90% coverage; south of continuous zone; permafrost absent under lakes, rivers, south-facing slopes • Sporadic permafrost: <50% coverage; only in cold/shaded locations

Active layer: seasonally thaws each summer (0.3–3 m thick) — where all soil biological activity, plant roots, and surface hydrology occur.

Thermokarst: irregular, hummocky terrain created when ice-rich permafrost thaws → ice volume lost → ground subsides unevenly → creates lakes (thermokarst lakes), ponds, and depressions. Climate warming is expanding thermokarst in the Arctic, releasing stored organic carbon as CO₂ and CH₄ (positive feedback).

~1,500–1,700 Gt of organic carbon is stored in permafrost soils (northern hemisphere) — roughly twice the amount currently in the atmosphere (~850 Gt C as CO₂). Why thaw matters: • Frozen organic matter (dead plants, animals) is preserved in cold, anoxic conditions • As permafrost thaws, microbial decomposition resumes → releases CO₂ (aerobic) and methane CH₄ (anaerobic, in waterlogged thermokarst lakes) • CH₄ has 80× the warming potential of CO₂ over 20 years • This creates a positive climate feedback: warming → permafrost thaw → more greenhouse gases → more warming (permafrost carbon feedback) • Current estimates: could release 100–200 Gt C by 2100 under high-emissions scenarios — equivalent to decades of current global fossil fuel emissions • Also affects hydrology: drainage patterns change drastically as ice melts (some areas get wetter, some drier); infrastructure (roads, buildings on permafrost) destabilizes

Pingo: a dome-shaped, ice-cored hill in permafrost regions, ranging from <1 m to >50 m high and up to 500 m diameter.

Closed-system (hydrostatic) pingo: • Forms where a drained lake exposes and refreezes saturated talik (unfrozen zone) • Porewater is squeezed upward as permafrost closes in from all sides → freezes to form massive ice core • Dome shape develops as ice grows → ground is lifted • Found on flat lake-plain terrain; common in Mackenzie Delta region (thousands present)

Open-system (hydraulic) pingo: • Groundwater under artesian pressure (hydraulic head) is forced upward through a thin permafrost zone • Freezes near the surface → forms ice core • Found near slopes where groundwater escapes from unfrozen subpermafrost aquifer • Common in Greenland, Svalbard, parts of Alaska

Collapse: when a pingo's ice core melts (climate warming, disturbance), the dome collapses → leaves a circular pond or 'pingo scar' — diagnostic of former permafrost

Alpine (valley) glacier: • Forms in mountain valleys; flows downslope through a valley • Mass balance: accumulation zone (net snow gain) at head; ablation zone (net loss by melt/calving) at terminus • Movement: internal deformation (ice crystal creep) + basal sliding (lubricated by meltwater) • World's mountain glaciers hold ~170,000 km³ ice (~0.4 m sea-level equivalent if all melted) • Freshwater role: 'water towers' — many major rivers (Ganges, Indus, Colorado, Rhine) depend on glacial meltwater for summer low-flow; glacier retreat = short-term flow increase → long-term decrease (peak water concept)

Ice sheets (Greenland + Antarctica): • Continental scale; flow outward from ice domes to calving fronts/ice shelves • Hold ~26.5 million km³ = ~65 m sea-level equivalent (Antarctic: 58 m; Greenland: 7 m) • Jökulhlaups (glacial outburst floods): sudden release of subglacial or ice-dammed lake water — extremely high peak discharge; historically shaped many landscapes (Channeled Scablands, Washington)

Frost heave: upward displacement of soil/objects caused by the expansion of water as it freezes — ice lenses grow in fine-grained soils (silts), lifting objects (fence posts, boulders, buildings) upward. Requires: frost-susceptible soil (silts), moisture supply, freezing temperatures.

Solifluction: slow downslope flow of water-saturated soil in periglacial regions — active layer becomes waterlogged during spring thaw (cannot drain through frozen permafrost below); flows downslope at 1–10 cm/yr. Creates lobes and terraces on hillslopes.

Patterned ground: regular geometric forms created by frost action: • Sorted stone circles/polygons: frost heave pushes coarse clasts to margins → rings of stones around fine-grained centers • Ice-wedge polygons: thermal contraction cracks fill with water → freeze → form wedges; surface expression is a polygonal network • Tundra polygons: ubiquitous Arctic landform; controls surface hydrology (high-center polygons = drained; low-center polygons = flooded, pond-filled)

Hydrological significance: permafrost creates an impermeable layer → active layer saturates rapidly → overland flow even on gentle slopes → flashy hydrographs

Peak water: the point in time when glacial meltwater runoff reaches its maximum as a glacier retreats and thins — before the glacier shrinks so much that it can no longer provide significant meltwater. Phases:

1. Early retreat phase: increasing meltwater as warm summers melt the expanding ablation zone → river discharge increases
2. Peak water: maximum runoff; glacier still provides substantial meltwater but is shrinking rapidly
3. Post-peak phase: glacier too small to sustain high meltwater → summer flows DECREASE → streams become more dependent on precipitation and snowpack Impacts: • Indus, Ganges river basins: hundreds of millions of people depend on glacier-fed rivers for dry-season flow and agriculture; crossing peak water = long-term water stress • Timing varies: tropical/mid-latitude glaciers predicted to cross peak water mid-21st century; some have already passed it (European Alps, some tropical Andes) • Adaptation required: reservoir storage, groundwater development, demand management

1. Littoral zone: shallow, near-shore area where light penetrates to the bottom; rooted aquatic plants (macrophytes) can grow; highest biodiversity — invertebrates, amphibians, small fish; most productive.
2. Limnetic zone: open surface water away from shore, within the photic zone; dominated by phytoplankton (algae, cyanobacteria) as primary producers; zooplankton as grazers; pelagic fish (open-water species).
3. Profundal zone: deep, open water below the photic zone (aphotic — no photosynthesis); cold, often low DO; limited life — mostly cold-tolerant invertebrates (chironomid larvae, tubifex worms); corresponds to the hypolimnion.
4. Benthic zone: the lake bottom substrate; extends from shore to deepest point; includes benthos (organisms living on/in the sediment); more productive in shallow benthic than deep; decomposers, burrowers, filter feeders.

NPDES (National Pollutant Discharge Elimination System): the regulatory permit program under Clean Water Act Section 402 that controls point-source discharges of pollutants to US waters. Requirements: • Technology-based effluent limits: at minimum, Best Available Technology (BAT) for industrial sources; Secondary Treatment for municipal wastewater (removes ~85–95% of BOD and suspended solids) • Water quality-based limits: if technology limits are insufficient to protect receiving water standards, stricter limits are imposed (tied to TMDLs) • Monitoring and reporting: permittee must measure and report effluent quality (flow, pH, DO, BOD, metals, nutrients) regularly • Public disclosure: violations are public record Timeline: Clean Water Act passed 1972; dramatically improved point-source control — rivers like the Cuyahoga (which caught fire in 1969) and the Thames have shown remarkable recovery since NPDES implementation.

Pre-development (forested/natural): • Evapotranspiration: ~40% of precipitation • Infiltration/groundwater recharge: ~50% of precipitation • Surface runoff: ~10% of precipitation • Interception by canopy: 15–40% (never reaches ground)

Post-urbanization (>75% impervious cover): • Evapotranspiration: ~30% (less vegetation) • Infiltration: ~15% (most surfaces sealed) • Surface runoff: ~55% of precipitation • Deep groundwater recharge: minimal Consequences: • Streams receive dramatically more runoff per rainfall event → flashier, higher peaks • Reduced groundwater recharge → lower dry-season baseflow → streams go dry or 'shrink' in summer • Urban stormwater carries pollutants: oil, road salt, heavy metals, bacteria, nutrients from lawns • Urban stream syndrome: combination of flashy hydrology, pollutant pulses, loss of riparian habitat → simplified, degraded stream communities

Potential ET (PET): the rate of evapotranspiration that WOULD occur if water supply were unlimited — determined by atmospheric demand (temperature, radiation, humidity, wind). Measured with pan evaporimeters or calculated with Penman-Monteith equation. Actual ET (AET): the evapotranspiration that actually occurs, limited by available water (soil moisture, precipitation). AET ≤ PET always. • In humid climates: AET ≈ PET (water not limiting; energy is limiting) • In arid climates: AET << PET (water severely limiting; energy is abundant)

Budyko framework: the aridity index = PET/P (P = precipitation). • Ratio < 1 (humid): water-limited ET dominates; P > PET → runoff exists • Ratio > 1 (arid): energy-limited ET; most P evapotranspires; little/no runoff The Budyko curve plots AET/P vs. PET/P and describes the long-term partitioning of precipitation into ET and runoff across thousands of watersheds globally.

1. Kariba Reservoir (Zimbabwe/Zambia, 1959): ~180 km³ — created by Kariba Dam on Zambezi River; displaced 57,000 Tonga people; massive invasion by water hyacinth (Eichhornia crassipes) after impoundment; now a major fishery (Kariba bream/Limnothrissa)
2. Bratsk Reservoir (Russia, Siberia): ~169 km³ — Angara River; severe loss of forest under flooded area; altered climate locally
3. Aswan High Dam / Lake Nasser (Egypt, 1970): ~169 km³ — controls Nile floods; provides irrigation and hydropower; but traps 100% of Nile sediment → delta erosion; downstream loss of nutrient-rich flood silt → fertilizer dependency; schistosomiasis (bilharzia) parasite spread in slow irrigation canals
4. Lake Volta (Ghana, 1965): ~148 km³ — largest by surface area (~8,500 km²); displaced ~80,000 people; significant methane emissions from flooded tropical vegetation

Ramsar Convention (1971): the Convention on Wetlands of International Importance — the only global intergovernmental treaty focused on a specific ecosystem. Named for Ramsar, Iran, where it was signed. Currently 172 contracting parties; ~2,400 Ramsar sites covering ~254 million hectares.

Criteria for designation (any ONE criterion sufficient):

1. Representativeness: a rare, unique, or representative wetland type
2. Biodiversity: supports vulnerable, endangered, or critically endangered species
3. Waterbirds: regularly supports ≥20,000 waterbirds
4. Waterbirds: supports ≥1% of a biogeographic population of a waterbird species
5. Fish: important to fish diversity, spawning, nursery, or migration
6. Indigenous fish: important for indigenous people's subsistence
7. Hydrology: plays important hydrological role in watershed
8. Wetland-dependent species: supports ≥1% of a biogeographic population of a non-bird animal species

Famous Ramsar sites: Everglades (USA), Danube Delta (Romania), Pantanal (Brazil — world's largest wetland ~150,000 km²)

Permafrost as an impermeable layer: • Frozen ground is hydraulically sealed — water cannot percolate downward into frozen substrate • Active layer (seasonal thaw zone) becomes saturated quickly after snowmelt → widespread ponding and wetlands even on gentle slopes • Drainage density is LOW — few integrated stream networks; water ponds rather than running off

Permafrost thaw effects on hydrology: • Thermokarst lake formation: subsidence creates new ponds; total lake area can increase or decrease depending on basin geometry and drainage • Talik formation: unfrozen zone beneath lakes grows; eventually breaches permafrost → lake drains rapidly through talik → lake disappears (common in Alaska's North Slope: satellite data show net loss of lakes since ~1950s) • Increased deep drainage: thawing permafrost allows deeper groundwater flow paths → increased baseflow to arctic rivers • Stream network expansion: new drainage channels form as soil becomes erodible • Arctic rivers are carrying more dissolved organic carbon (DOC) and nutrients as permafrost thaws → affecting coastal ocean chemistry

Ephemeroptera (mayflies): • Morphology: 3 caudal filaments (2 cerci + terminal filament), prominent compound eyes, abdominal gills; nymphs aquatic, adults live <24 hours • Ecology: collectors-gatherers, scrapers, or predators depending on genus; 2-3 year nymphal stage • Sensitivity: moderately to highly intolerant of low DO and pollution; good bioindicators

Plecoptera (stoneflies): • Morphology: 2 cerci, flat body, wing pads in late instars, tarsal claws for clinging to rocks • Ecology: shredders in headwater streams (cold, forested); some predators; require clean, cold, fast-flowing water with high DO • Sensitivity: MOST SENSITIVE EPT order; presence = excellent water quality; absent from impaired streams first

Trichoptera (caddisflies): • Morphology: many build portable cases of sand, pebbles, or plant material using silk; campodeiform larvae with hooks • Ecology: net-spinners (filterers), scrapers, shredders, predators — functionally diverse • Sensitivity: generally sensitive to moderate; some tolerant genera exist

All three orders require dissolved oxygen >5–6 mg/L and clean substrate for successful reproduction.

Liebig's Law of the Minimum: plant/algal growth is limited by the nutrient in shortest supply relative to demand — the 'limiting nutrient.'

Freshwater lakes: PHOSPHORUS is typically limiting • N:P ratio in most lake water is higher than algal demand (Redfield-like ratio ~16:1 by atoms) • Phosphorus is naturally scarce; adding any P stimulates growth • Wetland and riparian soils are poor at removing P (it binds to sediment, but internal loading from anoxic sediments can recycle it) • Management: phosphorus detergent bans, wastewater P removal, agricultural BMPs to reduce P runoff

Marine/estuarine systems: NITROGEN is typically limiting • Seawater naturally contains abundant dissolved P from oceanic upwelling • Nitrogen is typically in short supply relative to algal demand • Chesapeake Bay, Gulf of Mexico dead zone: driven by nitrogen loading from Mississippi Valley agriculture

Exception: some water bodies are co-limited by both N and P; reducing BOTH is safest management approach.

Post-dam removal recovery sequence:

Geomorphic (hours to years):

1. Drawdown: reservoir empties → soft, unstable sediment exposed on former reservoir bed
2. Sediment pulse: massive load of trapped fine sediment released downstream (can temporarily worsen turbidity, bury habitat)
3. Channel incision: river re-establishes gradient through reservoir sediment; new channel forms in delta/reservoir fill
4. Bed coarsening: fine sediment flushed; gravel/cobble substrate re-established (essential for salmon redds)
5. Longitudinal profile re-grading: knickpoint migrates upstream through reservoir fill; gradient normalizes over years

Ecological (weeks to decades): • Within weeks: migratory fish attempt upstream passage; first spawning in newly accessible reaches • Within 1–3 years: EPT macroinvertebrates recolonize from upstream; riparian vegetation establishes on exposed sediment • Within 5–10 years: salmonid populations rebuilding (evidence from Elwha, Sandy River) • Decades: full geomorphic stability; complex habitat (riffles, pools, large woody debris) developing

Mechanism: during summer stratification, the hypolimnion is isolated from the surface by the thermocline — no wind mixing, no photosynthesis (aphotic), no atmospheric O₂ supply.

Eutrophic lake (low DO in hypolimnion): • High algal biomass produced in epilimnion • Dead algal cells sink into the hypolimnion → decomposition by bacteria → large O₂ demand • Isolated hypolimnion cannot replenish O₂ → goes hypoxic or anoxic by late summer • Anoxic sediment releases phosphorus (internal loading) → feeds next algal bloom

Oligotrophic lake (well-oxygenated hypolimnion): • Little algal production → little organic matter sinks • Decomposition demand is low → the limited O₂ in the isolated hypolimnion is sufficient • Hypolimnion remains cool (4–6°C) and oxygenated → supports cold-water fish (cisco, lake trout)

Key concept: the driving variable is the ORGANIC LOADING to the hypolimnion, not the stratification itself.

Temperate karst (e.g., Kentucky, Dinaric Alps): • Moderate dissolution rates; well-developed underground drainage • Surface features: closed depressions (dolines/sinkholes), sinking streams, dry valleys, karst springs • Cave systems common and extensive • Relief moderate; sinkholes typically meters to tens of meters deep

Tropical karst (e.g., South China, Yucatán, Jamaica): • High temperatures + high precipitation → very high chemical weathering rates (warm water holds more CO₂) • More intense dissolution → spectacular surface forms • Tower karst (fenglin, Guilin China): isolated steep towers 100–300 m tall above flat alluvial plain • Cone/cockpit karst (Jamaica, Puerto Rico): star-shaped depressions surrounded by rounded hills • Cenotes (Yucatán): flooded sinkholes through thin limestone cap; connected by phreatic passages → famous cave diving sites; supplied freshwater to Maya civilization • Dissolution ~10× faster in tropics than temperate regions per unit area

Stage measurement: • A staff gauge or automated pressure sensor/buoyancy gauge records water surface elevation (stage/gauge height) continuously. • Modern USGS gauges use acoustic Doppler, bubbler systems, or radar sensors for stage.

Velocity-area method (direct discharge measurement):

1. At a cross-section, measure total stream width.
2. Divide width into 20+ vertical subsections.
3. At each subsection: measure depth (d_i) and flow velocity (v_i) at 60% depth (or average of 0.2 and 0.8 depth for deep channels).
4. Discharge for each strip: q_i = v_i × d_i × w_i
5. Total Q = Σq_i

Rating curve: • Multiple Q measurements at different stages over time are plotted (stage on y-axis, Q on x-axis). • Best-fit curve (power function: Q = a × stage^b) is fitted → called the stage-discharge rating curve. • Once calibrated, continuous stage record → continuous Q record automatically. • Rating curves must be updated after floods that shift the streambed.

Sphagnum (peat moss): the keystone organism of boreal and temperate bogs. Its ecological dominance arises from unique properties:

1. High water-holding capacity: dead hyaline cells (large empty cells) hold 10–25× their dry weight in water — sphagnum carpets regulate hydrology by holding water and keeping the bog surface wet.
2. Acidification: sphagnum releases H⁺ ions in exchange for cations (nutrient uptake) → lowers bog water pH to 3.5–5 → inhibits decomposition by bacteria (acid suppresses microbial activity)
3. Slow decomposition: acidic, cold, anaerobic, and phenolic chemistry of sphagnum → very slow decomposition → carbon accumulates as peat (thousands of years of partially decomposed sphagnum)
4. Nutrient competition: by lowering nutrients and pH, sphagnum outcompetes vascular plants that need neutral, nutrient-rich soils Peat accumulates at ~1 mm/year; boreal peatlands may be >10 m thick; radiocarbon dating shows some Canadian peat >10,000 years old (post-glacial).

Ogallala Aquifer (High Plains Aquifer System): • Location: underlies ~450,000 km² across 8 states (South Dakota to Texas) • Volume: ~3,700 km³ of groundwater — one of the world's largest aquifers • Recharge rate: extremely slow — ~0.5–1 cm/year of water-table rise from natural recharge (Pleistocene deposits with low modern recharge in semiarid climate) • Use: irrigates ~30% of US groundwater irrigation; supports corn, wheat, cotton, cattle feedlots; ~$20 billion/year in agricultural production • Depletion: water table has fallen 30–60 m in parts of Kansas and Texas since 1950s; some areas already 'mined out' (wells going dry) • Recovery rate: even if pumping stopped, refilling to pre-development levels would take thousands of years • Conservation response: managed depletion strategies, irrigation efficiency improvements, rotational fallow • Lesson: fossil groundwater (recharged during Pleistocene) cannot be replenished on human timescales — extraction is mining, not harvesting