Rivers and Streams River Systems A river or

Rivers and Streams

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

• 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 • Discharge (Q) • Gradient • Average velocity (V) • Channel cross-sectional area (A) • Load • Base level

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

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

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

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

• Base Level - level to which a stream will erode

Graded Stream

Stream Erosion • 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

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

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

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

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

Alluvial Fans • 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