Transition zones between deserts and humid areas are fragile

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

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

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

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

Drainage Basin/Watershed

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

Rio Grande Pecos Colorado Gila San Juan

Rio Grande Watershed

Stream profile depends on location

Q=VA Q is discharge V is average velocity (m/s) A is cross-sectional area (m 2 ) units of Q? Discharge/Total Flow

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

Sediment Load and Grain Size

Sediment Transport and Flooding

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

Nenana River, Alaska Smaller rivers

Meandering Streams

Green River, Utah Meandering

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

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

LOW-SEDIMENT LOAD, LOW VELOCITY Low-velocity, low-sediment streams form meanders. Meanders shift from side to side in a snaking motion. 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 current is faster at outside banks, which are eroded. The abandoned loop remains as an oxbow lake.

Meanders in an Alaskan river Point barHigh-velocity flow in channel

Floodplain Oxbow lake

Braided Streams Multi-channel paths Overloaded with sediment that is deposited in islands Promoted by dry climate, glacier outwash, alluvial fans

Braided river, Cook Inlet, Alaska

Sandy braided River: Red River, Texas-Oklahoma

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

Alluvial fans have braided streams

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

Delta reworked by wave action, Cook Inlet, Alaska