Transition zones between deserts
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Transition zones between deserts

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Transition zones between deserts




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Presentation on theme: "Transition zones between deserts"— Presentation transcript:

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Transition zones between desertsand humid areas are fragile

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Soil gets eroded by wind and water

Productive land becomes degraded

Main culprits:

Overgrazing

Overplowing

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Human activity can stress the ecosystem

• Grazing• Cutting trees for charcoal• Some agriculture

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Drainage BasinsDischargeSediment loadMeandering/Braided riversFloodplainsFlooding

Outline

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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

Evaporation434

Precipitation398

Runoff 36

Groundwatertable

Groundwaterflow

Infiltration

Surfacerunoff

Precipitation107

Evaporation71

Flux in and flux out over oceans is almost balanced.

Excess is moved toland and precipitates.

…or filters into soiland rock, where it moves as groundwater.

The precipitationruns off intolakes, streams,and oceans…

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Drainage Basin/Watershed

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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

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Rio Grande

Rio Grande

Pecos

Colorado

Gila

San Juan

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Rio Grande Watershed

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Stream profile depends on location

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Q=VAQ is dischargeV is average velocity (m/s)A is cross-sectional area (m2)units of Q?

Discharge/Total Flow

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Discharge/Total Flow

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Discharge/Total Flow

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Sediment Load and Grain Size

Streams can be provided with particles of any size from mud to giant bouldersVolume and velocity of flow limit size and amount of sediment that stream can carry

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Sediment Load and Grain Size

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Sediment Transport and Flooding

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Rivers

Meandering: low sediment content, gentle gradient, more vegetation

Braided: high sediment content, steep gradient, unstable channels

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Nenana River, Alaska

Smaller rivers

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Meandering Streams

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Green River, Utah

Meandering

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Meandering Streams

Size and shape of river meanders:Meander wavelength, width and arc lengthRatios are constant regardless of stream size

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Meandering Streams

Erosion/deposition of meanders creates floodplainSlow water velocity forms natural levee Keeps small floods within channel

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LOW-SEDIMENT LOAD,

LOW VELOCITY

Low-velocity, low-sedimentstreams 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.

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Meanders in an Alaskan river

Point bar

High-velocity

flow in channel

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Floodplain

Oxbow lake

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Braided Streams

Multi-channel pathsOverloaded with sediment that is deposited in islandsPromoted by dry climate, glacier outwash, alluvial fans

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Braided river,

Cook Inlet, Alaska

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Sandy braided River: Red River, Texas-Oklahoma

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Braided glacial outwash, Gulf of Alaska

Braided rivers are common near glaciers

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Alluvial fans have braided streams

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

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Delta reworked by wave action, Cook Inlet, Alaska