Overview of runoff
- surface - - overland flow
- stream flow, channel flow
- subsurface - throughflow - contributes to runoff peaks in stream hydrographs
- groundwater flow - contributes only to base flow
Streams are the most ubiquitous and dominant agents of landscape alteration
In general streams have formed the valleys they are in.
Start with overland flow.
- Very rapidly develop channels and a master channel is formed. The initial slope is replaced by slopes that slant toward the main drainage line. New slopes subject to erosion resulting in development of tributaries and expansion of channel's drainage area.
3 parts to river longitudinal profile
- headwaters - small steep streams - net erosion
- middle - less steep - often no net erosion or deposition - develop flood plain seasonally inundated by runoff
- lower reaches - flat
- ends at sea - delta - net deposition
- profile is a complex balance between river's erosive force and capacity to deposit sediments.
Can consider development in terms of maximum equilibrium slope - analogous to angle of repose.
anything gentler = stable
anything steeper = graded to lower angle
reach equilibrium grade
Departure of some or all of stream from ideal profile -
- due resistant rock units
- tributaries providing influx of sediments and/or high discharge
- sea level variations (mostly lower reaches)
- tectonic uplift and/or tilting
Erosion by streams involves
corrosion (chemical) - solution of soluble material
corrasion (mechanical) - particles in transport grind and abrade
cavitation - hydraulic impact. Removes material by force of water along (ie plunge pool)
Material transported by streams:
dissolved load
suspended load - finer particles, maintained by turbulence. Up to 90% of mechanical load of river
saltation load - lifted but shortly fall back. Bounce. Occurs where high energy conditions
bed load - roll, slide along river bed. Generally only in times of high discharge
Channel shape
straight
braided - where 2 or more channels around alluvial bars, islands.
sinuous - - generally happens where stream at equilibrium profile. No net erosion or deposition. meanders –
shape determined by
time
climate - amt water, vegetation
available relief
rock type - resistance to erosion
geologic structure
Alluvial sediments:
channel - bed
- bars
channel margin - point bars
overbank - vertical accretion
Floods = temporary non-equilibrium states. More water than channel can handle. Consequence of temporary increase in water flow
When stream overflows –
width increases greatly, rapidly
depth increases slightly, gradually
velocity adjusts by decreasing greatly
then transport capability < or <<
deposition occurs - vertical accretion
Deposition results in:
For reference click here for some slides related to deposition of sediments transported by streams.
Water engineering - channels, levees, etc modify natural system
Dams
- water supply
- hydropower
- flood control
- recreation
Levees - flood control
Channel straightening - navigation