streams and floods Flashcards
flooding due to climate change
higher temp -> ocean temo higher -> more evaporation -> more cloud cover and moisture -> more precipitation
hydrologic cycle
EPTSIMR
evaporation
precipitation
transportation
sublimation (solid to gas)
infilitration
melting
runoff
drainage networks
configuration of tributaries and trunk streams
geometry influenced by topography, rocks types, jointing, and differences in chemical weathering and erodibility
patterns: dendritic, radial, rectangular, parallel, trellis
drainage basin
land areas drain into trunk stream or body of water
watershed: area of land that drains into stream
ridges and peaks separate drainage basins
discharge
amount of water flowing in channel
m^3 /s
cross-sectional area x average water velocity
not uniform in a channel (friction)
turbulence
twisting, swirling motion of fluid
caused by
irregularities in bank and bottom
objects in water
steep gradients
how do streams erode?
scour: water picks up and move sediment
breaking and lifting: fast water breaks chunks of rock off channel, lifts rock off channel bottom
abrasion: sandblasting of rock by particles in fast water
dissolution: running water dissolves soluble minerals
sediment load
material moved by running water load
dissolved: ions from chemical weathering
suspended: fine particles in water
bed: large particles move along bottom
sediment transport (competence and capacity)
slow water: fine sediments and solutes
fast water: move boulders and clasts
COMPETENCE: ability to move large clasts
CAPACITY: amount of sediment carried
how does decrease in water velocity affect sediment transport?
competence reduced, sediment drops out
channel bottoms filled
sands form inside banks
silts and clays drape floodplains
sediment deposition
fluvial deposits are sediments transported by streams (alluvium)
sediment accumulation in channel and point bars
flood: sediment accumulates on floodplain
stream gradient
change in elevation per distance flowed (ft/mile)
longitudinal changes (gradient, dischargem competence, channels)
character of stream changes along its length
near source: steep gradient, low discharge, high competence, straight channels
near base: flat gradient, high discharge, low competence, curved channels
base level
lowest point to which stream can erode
velocity drops to zero at base level
ultimate base level is sea level but a lake is a temp or local base level
valley
- gently sloping sidewall and wider bottom
- _/ shaped
canyons
- steep sidewalls and narrow bottom
- variation in resistance to erosion results in stair-step profile
- strong rocks -> steep, weak -> gentle slopes
- |_| shaped
waterfalls
- super steep gradient so water is in free fall
- scours a deep plunge pool
- temp base levels
alluvial fans
- build at canyon mouths
- sediment drops out as water spreads from mouth (coarsest material first, close to mouth)
- strong flood smoothes fan surface
braided stream
interfingering channels
high sediment load, shallow channel
sediment is deposited, chokes channel in normal flow
meandering streams
sinuous looping curves
form where gradient is low and substrate is soft and easily eroded
meanders evolve during floods
how is a channel modified during periods of flood?
curves migrate downstream
fast water erodes the outside stream bank
slow water deposits points bars on inner part of curve
floodwaters may cut an outside bank to create shorter pathway downstream
cut off meander neck -> oxbow lake
deltas
forms when a stream enters standing water
stream divides into a fan of distributaries
velocity slow, sediment drops out
morphology is balance of sediment load, waves and storms, tides and slumping
avulsion
when a river’s flow is abruptly diverted from its established channel to a new course (more direct path)
stream piracy
one stream captures the flow of another
stream with vigorous headward erosion and steeper gradient intercepts stream with gentler gradient
captured stream flows into new stream
