Rivers and Streams

River Systems A river or stream: any body of water flowing downhill in a well defined channel A river or stream: any body of water flowing downhill in a well defined channel Drainage basin Drainage basin Drainage divide Drainage divide

Divided into 3 subsystems Divided into 3 subsystems –Collecting system –Transporting system –Dispersing system

Stream System Order - Tributaries decreases downstream -Tributary length increases downstream -Gradient decreases downstream -Channels are wider deeper downstream

Common Drainage Patterns

Stream Flow Dynamics Factors affecting stream behavior Factors affecting stream behavior Discharge (Q) Discharge (Q) Gradient Gradient Average velocity (V) Average velocity (V) Channel cross-sectional area (A) Channel cross-sectional area (A) Load Load Base level Base level

Stream Flow Dynamics Discharge Discharge The volume of water flowing past a given point in the river The volume of water flowing past a given point in the river Discharge will vary with time and weather conditions Discharge will vary with time and weather conditions Measured in cubic meters per second Measured in cubic meters per second

Gradient Gradient –The slope of the stream channel –Change in elevation divided by distance (m/km) –Longitudinal profile is a concave upward Steep at headwaters Steep at headwaters Nearly flat at discharge area Nearly flat at discharge area

Velocity varies within the channel Velocity varies within the channel –Highest velocity is in the center of the channel the center of the channel –Drag slows water along the bottom, banks, and the bottom, banks, and top (water-air interface) top (water-air interface)

Load Load –Material carried along by the stream Suspended load (fine particles) or Suspended load (fine particles) or Bed load (coarse particles) Bed load (coarse particles) Dissolved load (ions in solution) Dissolved load (ions in solution)

Base Level - level to which a stream will erode Base Level - level to which a stream will erode

Graded Stream

Stream Erosion River systems erode the land surface by: River systems erode the land surface by: –Removal of weathering products –Downcutting of stream channel –Headward erosion

Downcutting of Channels Bottom of stream channel abraded by transported sand & gravel Bottom of stream channel abraded by transported sand & gravel

Headward Erosion Erosion rates are greatest where stream gradients are high Erosion rates are greatest where stream gradients are high

Headward erosion of a plateau

Stream Deposition –Floodplains –Deltas –Alluvial valleys and fans

Meandering Streams Rivers flow in a sinuous pattern Rivers flow in a sinuous pattern –Turbulent flow & velocity changes eroded one bank, deposit on the other –Erosion changes course of river

River at Flood Stage Floodplain Main Channel

Floodplains Floods Floods –Normal stage - water level below the bank –Bankfull stage - water level even with the bank –Flood stage - water level above the bank Floodplains Floodplains –Areas that will be submerged when a river is at flood stage

Braided Streams Multiple channel system Multiple channel system –Interlaced channels with islands –High sediment load –Common in arid & semi-arid regions Seasonal high flows Seasonal high flows –Common in front of glaciers Large sediment load, fluctuating water flow Large sediment load, fluctuating water flow

Alluvial Valleys Streams fill part of their valleys with sediment Streams fill part of their valleys with sediment –Changes in hydrologic conditions initiate deposition –Subsequently cut through deposits Create terraces Create terraces

Deltas Formed by sudden drop in velocity Formed by sudden drop in velocity –Stream enters ocean or lake –Stream drops sediment load –Evolve by three mechanisms –Growth may be influenced by ocean waves

Delta Growth Distributaries Distributaries –Levee and in-stream deposit formation –Channel splits into two smaller channels Splays Splays –Crevasse allow water to leave main channel –Water spreads out forming a splay deposit Avulsion Avulsion –Shift in main course to follow steeper slope

Alluvial Fans Accumulation of sediment in a dry basin Accumulation of sediment in a dry basin –Deposition due to rapid velocity drop –Usually arid climate –High sediment load - braided streams –Form fan-shaped deposit

Alluvial fans in Death Valley