Hydrologic Cycle and Fluvial Processes

sun/solar energy

what drives the hydrologic cycle - through evaporation of water from oceans, lakes, rivers → water vapor rises, cools, condenses into clouds → precipitation → runoff → infiltrate/groundwater

residence time

at steady state, = reservoir amount/flux

watershed

drainage basin, all the land drained by river/river system. bounded by mountains/high elevation points. can be nested/contained by another wastershed.

V shape

higher elevations, river valleys have this shape as the steep gradient (slope) causes the river to flow quickly and cut down into the rock

U shape

lower elevations, river valleys have this shape as the river moves towards flatter land, so the gradient (slope) becomes gentle causing the river to slow down, eroding the sides of valley more than the bottom

stream channels

passageways where water normally run in valleys

meandering stream

river or stream that winds and curves through a valley in a snake-like pattern, single channel → lower sediment load, fastest flow and most erosion on the outside banks, slowest flow and deposition on the inside banks → gentle gradients, fine sediments

braided stream

stream that occupy many channels instead of one → common for highly variable flow and high sediment load, associated with glacier drainage → steep gradients, coarse sediments

floodplains

flat surface next to the river that gets flooded occasionally, tend to have rich soil, levees (natural or manmade) are boundaries of flood plain

river sediment load

total load = quantity of sediment carried in a river = bed load (on bottom, ~10% total) + suspended load (~90%) + dissolved load (<1%, weathering products and ions)

per mil

‰, parts in 1000 parts. EX: 1 gram per 1000 grams = 1‰

river discharge

volume of water that flows through a river channel over a period of time, is calculated by width, depth, velocity

Q = WDV, where W = width, D = depth, V = velocity

stream competence

maximum size of particles a stream can transport, which is primarily dependent on the stream's velocity

dynamic equilibrium

steady state between sediment transported by a stream (erosion) and sediment delivered to stream (deposition), EX: more load → more deposition → steeper slope → higher velocity → difficult to carry load

Upstream floods

floods that are relatively local, shorter duration due to precipitation

Downstream floods

wider area, longer duration due to saturated soils.

magnitude/frequency concept

floods follow this concept where smaller floods occur more frequently, while larger, more extreme floods are rare but have severe consequences.

Effects of urbanization

reduced infiltration with more impervious surfaces (like roads, sidewalks, and buildings), increased runoff with more flows into streams as less water absorbed, flashy discharge or increased flow → more erosion and greater flood risks downstream

Channelization

adjustment to flood hazards by modifying a river’s flow through straightening, deepening, lining channels with concrete. can reduce flooding but increase flow velocity (risking downstream floods + erosion)

restoration

adjustment to flood hazards by reestablishing natural flow patterns, vegetation, and wetlands. slows water, increase infiltration, reduce flood impacts