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Watersheds

• Reading:– Discussion: issues facing Arizona’s rivers

• Lecture:– How you identify a watershed– Why are they important

• Activity:– Using tributaries to find a watershed boundary

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What separates watersheds?How do you identify watershed boundaries?

Drainage Divides

boto.ocean.washington.edu/gifs/purus.gif

River NetworksContour Lines

Reynolds and Johnson

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Source: Glossary of Geology, 3rd Ed.,1987, AGI

Drainage Basin

http://www.alpinezone.com/hiking/01images/older/KNIFEDGE.jpg

Drainage Divide

• A region or area bounded by a drainage divide and occupied by a drainage system; • specifically, the tract of country that gathers water originating as precipitation and contributes it to a particular stream channel or system of channels, or to a lake, reservoir or other body of water.

• The original meaning of the term signifies a “water parting” or the line, ridge, or summit of high ground separating two drainage basins.

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• A region draining into a river or lake (American Heritage Dictionary)

• The area that produces runoff to a downstream point (Handbook of Hydrology)

• The area contained within a drainage divide above a specified point on a stream (Dictionary Of Geologic Terms)

• The upstream area that can contribute runoff to a point below.

• A drainage basin that divides the landscape into hydrologically defined areas. (Environment Canada)

Watershed Definitions

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http://www.nationalatlas.gov/Images/condivm.gif

The Continental Divide is a line separating waters that flow into the Atlantic Ocean or Gulf of Mexico from those that flow into the Pacific Ocean. It runs north-south along the crest of the Rocky Mountains (in Mexico and Canada too) and is sometimes called the Great Divide. This map layer was compiled by the U.S. Geological Survey by extracting the appropriate lines from the Hydrologic Unit Boundaries layer of the National Atlas.

www.nationalatlas.gov/ condivm.html

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Watershed - Importance1. Understand what a watershed is both literally and conceptually (including

the mapped representation of a watershed and the issue of scale). 2. Understand the components and processes of a watershed including runoff,

soil, geology, geography, permeability, storage, land cover, land use, vegetation, precipitation, stream flow, flooding, drought (climate), fire, drainage patterns, erosion, deposition and population.

3. Understand a watershed as a system (e.g. a change in one area will affect the dynamics of the entire system) and how that system functions.

4. Understand that watershed management is complex because of culture, economics, politics, social constructs, scientific studies and aesthetics. Some water users include urban, rural, agricultural, business & industry, energy, recreation, fish and wildlife and earth systems.

5. Understand that watersheds change over time both naturally (e.g. flooding, fire) and due to anthropogenic causes (e.g. damming a river, water rights, water withdrawals).

6. Know some of the issues facing the watershed managers of the Colorado River Watershed as well as other Southwestern Watersheds.

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The drainage pattern allows you to understand the watershed boundaries and directions of stream flow even without topography

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… although a shaded DEM helps!

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Seeing Watersheds Activity

1: trace the main channel of the river from its mouth to the headwaters.

2: trace the major tributaries (start at the coast/Gulf).

3a: Find the drainage divides by marking a dot above the top of each river, midway to the adjacent watershed.

3b: Connect the dots (start at the mouth) to form the watershed boundary.

4:Identify sub-watersheds of major tributaries

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Synonyms:Basin

Catchment Catchment AreaCatchment Basin

Drainage AreaDrainage Basin

Feeding GroundGathering Ground

Hydrographic BasinWatershed

Source: Glossary of Geology, 3rd Ed.,1987, American Geophysical Institute

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Watershed – Sub-watershed

HUC: 14-15

HUC: 1401-1508

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Colorado – “source” of 4 WS’s

= 8,131,000 af

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Major Western Rivers

Strahler:4-7

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ag.arizona.edu/watershed/

Snake

SanJoaquin

Platte

Colora

do

RioGrande

Missouri

Columbia

Sacramento

Gila

Yellowstone

Klamath

Major Western Rivers

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Contributing Area

Upper Basin• CO• WY• NM• UTLower Basin• AZ• NV• CA

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CRB Analysis