Watersheds

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Watersheds

• Reading
• Discussion issues facing Arizonas 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?
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Drainage Basin
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.

Source Glossary of Geology, 3rd Ed.,1987, AGI
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Watershed Definitions

• 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)

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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 - Importance

• 1. 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.
• 4Identify sub-watersheds of major tributaries

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Synonyms Basin Catchment Catchment
Area Catchment Basin Drainage Area Drainage
Basin Feeding Ground Gathering
Ground Hydrographic Basin Watershed
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 WSs
8,131,000 af
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Major Western Rivers
Strahler 4-7
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Major Western Rivers
ag.arizona.edu/watershed/
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Contributing Area

• Upper Basin
• CO
• WY
• NM
• UT
• Lower Basin
• AZ
• NV
• CA

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CRB Analysis