Initial Publication Date: August 12, 2008

Drying of the American West

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


GIS Temperature Anomalies 2012-2016 v. 1951-1980 Global temperature trends, 2012-2016 averages compared to 1951-1980 averages. From NASA/GSFC

As you've looked at the data on climate and precipitation, you've learned about how drought is considered a common part of the long-term climate of the American West. Part A looked at two of the water reservoirs for the Colorado River Basin and the current drought situation. Now, you're going to look at one of many recent studies that are helping decisionmakers plan for the future of this region's water.

Precipitation is a necessary factor in determining water levels, but temperature is also a factor. Temperature helps influence evaporation, snowmelt, and drives up water use both in cities and for agriculture. Studies indicate that temperatures are rising across the country, and in the Upper Basin of the Colorado River, from 2000 to 2014 temperatures were 1.6 degrees Fahrenheit higher than the 20th-century average. One recent research study looked at what rising temperatures mean for the next century and the Colorado River and what they call a "hot drought." This article Climate Change is Shrinking the Colorado River is based on a research study written by the same two authors. Your teacher may ask you to read this article.

The study found:

  • Continued low water flows combined with current operating rules at the reservoir could lower Lake Mead to 1000 feet. (Hydropower generation ceases at 1050 feet, and water ceases to flow out at the level of 895 feet.)
  • Temperatures could rise by 5 degrees Fahrenheit over the 20th-century average by 2050 with modest or high GHG emissions. Higher emissions could result in a rise of temperatures by 9.5 degrees Fahrenheit.
  • Water flow in the Colorado could be reduced by 20% or more below the 20th-century average by 2050, and by up to 40% by 2100.
  • Large precipitation increases could offset the rising temperatures, but precipitation would have to increase by an average of 8% by 2050 and a 15% by 2100.
  • Likelihood of a megadrought, lasting 20-50 years, occurring in the Basin is better than 80% sometime during the 21st century.

The study also cites warming temperatures affecting snowpack and runoff. Snowpack is the volume of water that exists as snow on the surface during winter months. Historically, melting snowpack has fed the Colorado 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.

Watch the video Water: A Zero Sum Game from the Institute of Arctic and Alpine Research at the University of Colorado, Boulder. This video looks at how snow impacts the watershed.

Read more about the source of the water in the Colorado River at the TreeFlow 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). As it states on the TreeFlow site, "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/

  2. 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 smaller 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.

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.

Optional Extensions

Watch the animation of temperature anomalies from 1880 to 2016 on NASA's visualization site.

Look at how recent snowpack for the Upper Colorado River Basin compares with recent years. (snowpack data live Nov – mid-June) You can turn individual years on and off by clicking on the key icons below the graph.