Transition zones between deserts and humid areas are fragile

3. Transition zones between deserts and humid areas are fragile

5. Soil gets eroded by wind and water Productive land becomes degraded Main culprits: Overgrazing Overplowing

7. Human activity can stress the ecosystem • Grazing • Cutting trees for charcoal • Some agriculture

10. Outline • Drainage Basins • Discharge • Sediment load • Meandering/Braided rivers • Floodplains • Flooding

11. SEA SEA LAND LAND + 336 Runoff from land + 398 Precipitation over sea + 434 Evaporation + 434 Evaporation – 398 Precipitation + 436 Excess to land via precipitation + 107 Precipitation – 371 Evaporation + 436 Runoff to ocean + 107 Precipitation – 336 Runoff to ocean + 471 Evaporation Flux in and flux out over oceans is almost balanced. Precipitation 107 The precipitation runs off into lakes, streams, and oceans… Evaporation 71 Excess is moved to land and precipitates. Evaporation 434 Precipitation 398 Surface runoff Infiltration Runoff 36 Groundwater table Groundwater flow …or filters into soil and rock, where it moves as groundwater.

12. Drainage Basin/Watershed

14. Drainage Basins/Watersheds • Rio Grande River basin includes: • Chama • Puerco • Pecos (and all of its tributaries) • It doesn’t include the Gila, because it flows into Colorado • Continental Divide (in NM) separates: • Rio Grande drainage • Colorado River drainage

15. San Juan Rio Grande Colorado Gila Pecos Rio Grande

16. Rio Grande Watershed

20. Stream profile depends on location

21. Discharge/Total Flow • Q=VA • Q is discharge • V is average velocity (m/s) • A is cross-sectional area (m2) • units of Q?

22. Discharge/Total Flow

23. Discharge/Total Flow

24. Sediment Load and Grain Size • Streams can be provided with particles of any size from mud to giant boulders • Volume and velocity of flow limit size and amount of sediment that stream can carry

25. Sediment Load and Grain Size

26. Sediment Transport and Flooding

27. Rivers • Meandering: low sediment content, gentle gradient, more vegetation • Braided: high sediment content, steep gradient, unstable channels

28. Nenana River, Alaska Smaller rivers

29. Meandering Streams

30. Green River, Utah Meandering

31. Meandering Streams • Size and shape of river meanders: • Meander wavelength, width and arc length • Ratios are constant regardless of stream size

32. Meandering Streams • Erosion/deposition of meanders creates floodplain • Slow water velocity forms natural levee • Keeps small floods within channel

38. LOW-SEDIMENT LOAD, LOW VELOCITY Low-velocity, low-sediment streams form meanders. Meanders shift from side to side in a snaking motion. The current is faster at outside banks, which are eroded. Sediments get deposited in inside banks forming point bars. The bends grow closer and the point bars bigger. Point bars During a major flood, the river cuts across the loop. The abandoned loop remains as an oxbow lake.

39. Meanders in an Alaskan river Point bar High-velocity flow in channel

40. Floodplain Oxbow lake

41. Braided Streams • Multi-channelpaths • Overloaded with sediment that is deposited in islands • Promoted by dry climate, glacier outwash, alluvial fans

42. Braided river, Cook Inlet, Alaska

43. Sandy braided River: Red River, Texas-Oklahoma

44. Braided glacial outwash, Gulf of Alaska Braided rivers are common near glaciers

45. Alluvial fans have braided streams

46. Gravel delta at mouth of braided river, Cook Inlet, Alaska

47. Delta reworked by wave action, Cook Inlet, Alaska