River Systems: Dynamics and Patterns

1. Rivers andStreams

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

6. Divided into 3 subsystems • Collecting system • Transporting system • Dispersing system

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

10. Common Drainage Patterns

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

13. 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

15. 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

18. 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)

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

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

24. Graded Stream

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

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

28. Headward Erosion • Erosion rates are greatest where stream gradients are high

30. Headward erosion of a plateau

32. Stream Deposition • Floodplains • Deltas • Alluvial valleys and fans

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

36. River at Flood Stage Floodplain Main Channel

37. 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

38. 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

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

43. 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

44. 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

46. 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

47. Alluvial fans in Death Valley