EarthLabs > Drought > Lab 6: Drying of the American West > 6B: What's Responsible for Lower Reservoir Levels?

Drying of the American West

Part B: What's Responsible for Lower Reservoir Levels?


map of temperature trends Temperature trends in the continental U.S., 2000-2007 averages compared to 20th century averages. From Hotter and Drier: The West's Changed Climate. Used with permission from NRDC.

The Natural Resources Defense Council (NRDC) is an environmental action agency that represents 1.2 million members in courts of law using the expertise of more than 350 lawyers, scientists, and other professionals. In collaboration with the Rocky Mountain Climate Organization (RMCO), NRDC recently compiled a report titled Hotter and Drier: The West's Changed Climate. The report draws on over 100 scientific studies and government reports to document changes in temperature and precipitation across the west and a population "explosion" of people living in places such as Las Vegas and Phoenix where they are dependent on the river's water.

  1. Visit the overview page for the report, titled Hotter and Drier, The West's Changed Climate. Click the various states on the interactive map to see photographs of the effects of hotter, drier climate in the west.
  2. Chapter 3 of the Hotter and Drier publication is specifically about the Colorado River Basin. Your teacher may assign you to download the report and read this chapter. In addition to showing increasing temperature trends across the basin, it cites reports that indicate:

    • As early as 2030, the average flow of the river could be reduced to only half of the level on which the Colorado River Compact is based. The Compact is the legal agreement used to divide Colorado River water among the states through which it flows.
    • If current levels of water use continue, there is a 50 percent chance that by 2023, water levels in the river's two main reservoirs, Lake Powell and Lake Mead, will fall below their outlets. This means that the reservoirs will have effectively gone dry.
    • By the end of the century, "Dust Bowl" conditions will be the new climate norm of the Southwest.

    The report also cites trends in reduced volume and shorter duration of snowpackthe volume of water that exists as snow on the surface during winter monthsacross the basin. Historically, melting snowpack has fed the river gradually through the spring months. Since 1985, snow has been melting earlier and faster, flowing downhill in the late winter, leaving the land drier during the spring.

  3. Read more about the source of the water in the Colorado River at the site, Colorado River Streamflow: A Paleo Perspective.

Stop and Think

4. Compare snowpack in a watershed to a dam on a river. How are they alike? How are they different?
5. What effect does the El Nino Southern Oscillation (ENSO) have on water supplies to the Colorado River Basin?
6. How does increasing population of sunbelt cities in the Lower Colorado River Basin contribute to lowering reservoir levels?

Natural flow of the river

Since the early 1900s, dams on the Colorado River and its tributaries have diverted huge volumes of water away from the river. The dams have also increased the amount of water that is lost to the atmosphere by evaporation. By considering all the water that has been removed from the river system upstream, scientists have been able to reconstruct the "natural flow" record of the Colorado River at Lee's Ferry, the point directly below Glen Canyon Dam (the dam that forms the reservoir called Lake Powell). This natural flow record is important because it shows the variability in streamflow due to climate alone, apart from changes in the use and management of the river.

  1. Examine the graph below. Interpret the three lines to understand the climate trend of the past and project it into the future.
  2. Inflow gauge record into Lake Mead
    The graph shows water volume in million acre-feet (MAF). Acre-feet are units used in agriculture. One acre-foot is approximately 326,000 gallons, the amount of water it takes to cover one acre of land with one foot of water. Used with permission from Western Water Assessment, http://wwa.colorado.edu/

  3. The Colorado River Compact, the legal agreement that divides the river water among the basin states, was established based on the assumption that the average annual flow at Lee's Ferry was about 16.4 MAF. This was based on the 20 years of gage records available in 1922. However, the flow since 1922 has been generally lower than these early gaged flows, so the amount of water that the states have to share is mediumsmaller than expected. There is not enough water in the river, on average, to fulfill all of the legal entitlements that states have to the water. How the Colorado River Basin states will solve this issue remains to be seen.
  4. About the Colorado River Compact, the legal agreement

Stop and Think

7. In your own words, describe what's responsible for lower reservoir levels in the Colorado River Basin.
8. List at least 3 possible impacts of the decline in water storage on the Colorado River.

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