River Channels Flashcards
provide a summary of meandering rivers
- gentler gradient
- local sediment supply from bank erosion
- fine sediment
- suspended load
- single sinuous channel
provide a summary of braided rivers
- “flashy” regime
- steeper gradient
- abundant sediment supply from valley sides
- coarse sediment
- episodic bedload transport
- multiple channels, frequent avulsion
what is the Hjulstrom curve
shows the relationship between particle size (in well sorted sediment) and the critical entrainment velocity (velocity required to entrain a particle in the water)
used to determine whether a river will erode, transport, or deposit sediment
What are flood plains
moderately flat land which is formed from debris deposited when the river is in flood
width is proportional to river discharge
what are river terraces
Remnants of former flood plain abandoned by channel incision
can be single or “flights”
slope down stream but not necessarily at same grade as flood stream
paired terraces
vertical down cutting by river is faster than lateral migration of river channel. Terraces at same altitude
3 changes of stream condition
- Aggradation → alluvial fill
- Degradation → paired terraces
- renewed aggradation → inset alluvial fill
- renewed degredation → inset paired terraces
unpaired terraces
formed during degradation of the valley fill at levels below the highest point of aggradation
single change in stream condition
aggradation → degredation
Describe aggradation and degradation
Aggradation
GAINS EXCEED LOSSES
- deposition of sediment → increase in land elevation
- supply of sediment greater than the amount of material the system is able to transport
Degradation
LOSSES EXCEED GAINS
- Erosional processes → lowering of fluvial surfaces (stream beds and flood plains)
- occurs where there is bed rock erosion or sediment starvation
- leaves behind fluvial terrace
Describe the characteristics of a graded river/profile
- grade attained by removing irregularities (waterfalls, rapids, lake basins)
- concave with steep headwaters and gentle downstream
- eroded its bed to achieve the gradient necessary to provide energy to transport all its sediment
- All tributaries graded to level of main stream and join at the same level
- hard to actually achieve in nature
How does a graded profile get rid of topographic irregularities
- infilling hollows with sedimentation
e. g lake basins + alluvial valley fills - removing bedrock steps by erosion
e. g at waterfall, rapids etc.
Explain the process of river capture
river is diverted from its own bed and flows down the bed of another
Antecedent stream cuts across strata, separate stream cuts weaker strata → this stream cuts head ward to intercept original stream → combined they accelerate head cutting (positive feedback)
Not a Q but important
The long-term evolution of river valleys involves a conflict between the fluvial system working to achieve grade, and geological and (in some regions) glacial forces disrupting this evolutionary pathway
what factors cause aggradation
- deforestation (causes soil erosion which results in aggradational problems)
- landslides
- earth flow
- debris flow
- river bank failure
