• From fluvius (L. river), fluvial geomorphology is the geomorphic work of rivers, but also the erosion of soil and rock on hillslopes by running water, particularly in semiarid environments (badlands)
  
  

  alluvium

  the sediment transported and deposited by streams

Stream Erosion and Sediment Transport

erosional processes

hydraulic action

the force applied by turbulent water

abrasion

erosive force of sediment suspended in the flow

corrasion

the dissolving of rocks in water; chemical weathering

stream competence

• the maximum particle size transported
• increases with velocity because competence is a function of bed shear stress
• small particles are cohesive and thus require a high erosional velocity but remain in suspension in running water
• large particles are continuously transported and deposited because the erosion threshold is only slightly higher than the transport threshold
• intermediate particle sizes (coarse silt and fine sand) are most easily eroded by running water

stream capacity

• the theoretical maximum amount of suspended sediment transported by a stream
• difficult to determine because a sediment laden stream is transitional to a debris flow
• increases with the 2-3rd power of discharge

types of sediment load

dissolved

sediment in solution, reflecting solubility of rocks in the watershed and rates of corrasion

suspended

fine material held above the stream bed by turbulence; accounts for most stream sediment and most of the work performed by streams

traction (bedload)

coarse fraction (pebbles, cobbles and gravel) that rolls and slides along bed

saltation (L. jump)

combination of traction and suspension; coarse particles are briefly suspended, transported a short distance and fall back to bed

Fluvial Landforms

• most structurally-controlled landforms represent the exposure of geologic structures by stream erosion

stream channels

• meandering
  
  • the most stable and efficient channel geometry
  • formed and maintained by erosion of banks and deposition on point bars
  
  
• braided
  
  • multiple superimposed meandering channels as discharge and sediment load vary seasonally and diurnally, e.g. semiarid and proglacial streams
  
  
• straight
  
  • either artificial or structurally controlled

graded river

• independent factors
  
  
  • runoff and sediment from the basin and base level, the lowest elevation on the stream which determine the potential energy (=mgh, h = height or difference in elevation) of the stream
  • determined by climate and geology, i.e. external to the fluvial system
  • a change in any one of these controlling factors results in an adjustment of the stream channel by degradation or aggradation towards a new graded geometry
  
  
• semi-dependent factors
  
  
  • channel width, depth, roughness, velocity, pattern and load grain size
  • depend on the independent factors but also the stream, that is, there is some self-regulation of the system as these factors depend on each other
  
  
• dependent factors
  
  
  • slope of the water surface
  • this is the final adjustment of streams to the semi-dependent variables
  • it cannot change abruptly like the other factors, but rather requires a change in the stream channel geometry which then results in a change in the slope of the stream

drainage networks

• all streams in erodable materials will from a dendritic (tree-like, branching) pattern
• other drainage patterns (e.g., radial, trellis, rectangular and annular) reflect the structural control of stream networks
• a distributary drainage pattern is associated with deltas, where the stream is subdividing in distributaries (the opposite of tributaries)
  
  

river valley

superimposed stream

maintains its course as it cuts down and encounters different rocks and geologic structures

antecedent stream

predates a geologic structure that formed slowly such that the river maintained its course

water gap

a gorge cut in a ridge by a superimposed or antecedent stream

stream capture (piracy)

diversion of streams from one channel to another as the divide between them is breached by erosion

underfit (misfit) stream

a small stream occupying a large valley, because the original larger river was captured or was fed by a glacier (e.g. the glacial meltwater that created the Qu'Appelle valley)

floodplain

• the surface of low relief adjacent to a stream
• becomes the stream bed during flood
• floodplain features include meander scars, levees, and oxbow lakes

terraces

• elevated sections of former floodplain
  
  

  unpaired terraces

  fragment of former floodplain preserved only on one side of the valley
  
  

  paired terraces

  occur at same elevations on opposite valley sides

alluvial fan

• segment of a low-angle cone with its apex at the mouth of a canyon
• as a stream (usually ephemeral) leaves a canyon, drainage becomes distributary; these wider, shallower, lower-gradient streams have less transport capacity
• adjacent alluvial fans coalesce to produce a piedmont plain at the base of mountain fronts

delta

• similar morphology to an alluvial fan but deposition results from sharp reduction in velocity as a stream enters standing water
• also tends to include finer sediments and turbidity currents