Lab 4 - Sharing a River: The Colorado River Story

Download a Student Activity Worksheet here. (Microsoft Word 2007 (.docx) 68kB Mar14 19)


All of us take water for granted because we can just turn on a tap and use as much as we want. This is not true everywhere. One out of every ten people on our planet don't have access to fresh, clean water. Ultimately, water is our most valuable resource and conserving water is extremely important for our survival. In desert areas like the American Southwest, water is like gold, a rare commodity that enables large cities and agricultural areas to exist in arid regions where only cactus and small shrubs lived before.

In the Southwest, the Colorado River watershed is the major source of water. The Colorado River supplies 30 million people in seven states and Mexico with water (see diagram right). Denver, Las Vegas, Phoenix, Tucson, Los Angeles, and San Diego all depend on it, and starting this year (2017) so will Albuquerque. It irrigates four million acres of farmland, much of which would otherwise be desert, but now produces billions of dollars' worth of crops. Gauges installed in the 19th century provide a measure of the river in acre-feet, one acre-foot being a foot of water spread over an acre, or about 326,000 gallons (see the image below to see how many acre-feet a family of five uses in a year). In 2002, water managers all along the river began to wonder if one-hundred years of recorded data was sufficient to understand the changing nature of the river. Since then, levels of water in the Colorado River and its major storage areas like Lake Mead, have diminished each year. 2015 was a wet year and for the first time in 16 years, water from the Colorado River reached its final, natural destination—the Gulf of California. See the usually dry terminus of the river in the image below.

Many of the issues dealing with a decrease in available water, due to climate change and an increasing population in the region, are surrounded by controversy. As we face shortages of water imposed by the finiteness of our natural resources, we must make decisions involving different stakeholders--people who have a vested interest in obtaining water--with different priorities. For example, how do we allocate water between farmers and people living in cities? How can cities make common sense decisions relating to preventing population growth in areas where water supplies are already scarce? Stakeholders protecting their economic interests often argue that the constraints are not real or that they are exaggerated. To best solve these problems, we rely on science to give us a more complete picture of the available water resources that we have, how they have varied in the past, and how they may change in the future. This deeper understanding will lead to better decision making and policy.

What will I learn?

In this lab, you will take a journey to the Colorado River Basin and learn about the demands on the water resources that the river provides. You will learn about how scientists work in the field and collaborate to search for clues to determine what the nature of the stream flow was in the Colorado Basin in the past. You will evaluate the ring patterns of samples of wood collected from sites in the basin to determine how streamflow has varied over the past using some of the same methods that dendrochronologistsa scientist that focuses on dating events, environmental changes, and archaeological artifacts by using the characteristic patterns of annual growth rings in timber, tree trunks, and other wood samples use in their research.

Learning Objectives

After completing this lab, you should be able to:

  • describe how tree rings can be used to determine past stream flow and hydrological conditions in the Colorado River Basin.
  • measure tree-ring data and compare those measurements to published work.
  • explain how proxy data, such as measurements of annual growth rings in trees, reveal the nature, scale, and duration of past climate events like droughts.
  • explain how the analysis of science data leads to a deeper understanding of the challenges we face as a society and helps to guide our decision making.

References and Additional Resources

For more information about the science in this lab, consult the following papers and articles.

  • Climate Fluctuations, Drought, and Flow of the Colorado River (2004) Webb, R., Hereford, R., McCabe, R. USGS Factsheet
  • Climate Change Impacts on Hydropower in the Colorado River Basin (2012) Thiel, Aaron/Center For Water Policy/University of Wisconsin-Milwaukee
  • Gray, S. T., J. J. Lukas, and C. A. Woodhouse (2011), Millennial-length records of streamflow from three major Upper Colorado River Tributaries 1, J. Am. Water Resour. Assoc., 47(4), 702–712.
  • Jacoby, G. C., and C. W. Stockton (1976 ), Long-term surface-water supply and streamflow trends in the Upper Colorado River basin based on tree-ring analyses, Natl. Sci. Found. Lake Powell Res. Proj. Bull., 18, 70.
  • Meko, D. M., C. A. Woodhouse, C. A. Baisan, T. Knight, J. J. Lukas, M. K. Hughes, and M. W. Salzer (2007), Medieval drought in the upper Colorado River Basin, Geophys. Res. Lett., 34, L10705, doi:10.1029/2007GL029988.
  • Woodhouse, C. A., and J. J. Lukas (2006), Drought, tree rings and water resource management in Colorado, Can. Water Resour. J., 31(4),297–310.
  • Woodhouse, C. A., S. T. Gray, and D. M. Meko (2006), Updated streamflow reconstructions for the Upper Colorado River Basin, Water Resour. Res., 42, W05415, doi:10.1029/2005WR004455
  • Drying of the West, National Geographic. February 2008. By Robert Kunzig, Photographed by Vincent Laforet
  • TreeFlow (a comprehensive web resource for tree-ring reconstructions of streamflow and climate, providing easy access to reconstruction data as well as information about how the data were developed and can be used):
  • TreeFlow: Colorado River Streamflow: A Paleo Perspective
  • IPCC Report, Chapter 2 - Future Climate Changes, Risks, and Impacts
  • David, Owen, Where The River Runs Dry, New Yorker May 25th, 2015 Issue: