Initial Publication Date: April 28, 2017

Part 2: Explore the Colorado River Basin

Have you been in a drought? Are you living through one now? Most places in the United States experience drought occasionally. Sometimes, drought causes widespread interruptions of daily life. Other times, drought is milder; people may not even notice it except for the lack of rainy days. The word "drought" means different things to different people. What visions does the term bring to your mind? Parched land, dried crops, dust storms, and starving livestock are some of the scenes that people associate with the term drought. Unlike most hazardous weather conditions, drought is not always obvious. Drought can be years in the making, as moisture in the soil evaporates and surface water sources disappear due to the lack of rain.

Common to all types of drought is the fact that they originate from a lack of precipitation resulting from an unusual weather pattern. If the weather pattern lasts a short time (a few weeks or a couple months), the drought is considered short-term. But if the weather or atmospheric circulation pattern becomes entrenched and the lack of precipitation lasts for several months to several years, the drought is considered to be a long-term drought. How do we measure the severity of a drought?

The Palmer Drought Severity Indexuses readily available temperature and precipitation data to estimate relative dryness. It is a standardized index that spans -10 (dry) to +10 (wet) (PDSI) measures the duration and intensity of the long-term drought and it is useful to compare the relative dryness/wetness from one place compared to another. Long-term drought is cumulative, so the intensity of drought during any particular month is dependent on the current weather patterns, plus the cumulative patterns of previous months. Since weather patterns can change, almost literally overnight, from a long-term drought pattern to a long-term wet pattern, the PDSI can respond fairly rapidly. Notice that on the date shown on the example map, the only serious drought conditions were in California, Wyoming, and Montana. Is your region or state presently experiencing drought? Check the Current Drought Index Map and see.

Approximately 1,400 miles long and flowing through seven U.S. States and into Mexico, the Colorado River drains roughly one-twelfth of the land area of the contiguous United States. The Colorado River Basin is divided into the Upper and Lower Basins at Lee's Ferry located in northern Arizona. The Upper Basin spans portions of Wyoming, Colorado, New Mexico, Utah, and northern Arizona. The Lower Basin covers parts of Nevada, Arizona, California, southwestern Utah, and western New Mexico. The Colorado River also supplies water to parts of the states of Baja California and Sonora in northwestern Mexico.

In this activity, your group will explore aspects of the Colorado River Basin using a variety of resources and tools such as Google Maps. It is important to know about the environment and ecology of the region that you are studying in order to get a better sense of the climate signal that the trees are recording. In the second part of the activity, you will focus your attention on one of three regions in New Mexico, you will examine the terrain and look for trees that would be good candidates for yielding core samples with a long-growth history.


1. Right click on the Colorado River Basin Interactive Map link and open the map in a new window. You will use the interactive map to investigate the components of the hydrologic basin formed by the Colorado River and its tributaries. Take note of major rivers in the system, dams and the lakes formed by them, and look at the series of human-made canals that draw water from the river. Note: The Colorado River forms the border between Arizona and California so that portion of the graphic is a dotted line instead of blue.

Stop and Think

2.1 What major rivers flow into the Colorado River in the Upper Basin?

2.2 What are the major rivers that branch from the Colorado River in the Lower Basin?

2.3 List the major dams that have been built in the basin and lakes/reservoirs that have formed behind them.

2.4 How many canal systems have been built to carry water to other portions of the basin? Where do they carry water to?

2. Explore the Colorado River Basin

Right click on the Google Maps link and open the program in a new window. Type "Glen Canyon dam" into the Search Box. When the map comes up, click on "satellite view."

3. Zoom out and examine Lake Powell--the body of water formed by the dam. Look for evidence of falling water levels. Note: Sediment and rocks exposed by falling water levels often appear white and are called "bathtub rings." The white bathtub rings seen on the rocky sides of the canyon shows the historical high water level in the reservoir. The ring is a coating of minerals, deposited on the rocks while they were covered by water. Take a screenshot of a good example of bathtub rings.

4. Type in the following sites along the course of the river in the search box and make observations. Note any evidence of diminishing water levels and note the terrain and types of vegetation at each site. Also note the type of land use (residential, urban, agriculture, desert, etc.) at each site. Zoom/drop in to verify.

  • Hoover Dam/Lake Mead
  • Davis Dam/Lake Mohave
  • Parker Dam/Lake Havasu (At this site locate the Colorado River Aqueduct and Central Arizona Project)
  • Imperial Dam (At this site locate the All-American Canal)
  • Morelos Dam (At this site locate the canal that takes almost all the water to the Mexicali Valley)

Stop and Think

2.5 At which sites did you find evidence of decreasing water levels in the system?

2.6 Paste in your picture of bathtub rings in Lake Powell on your answer sheet.

2.7 What do you think caused the flow of the Colorado River below Morelos Dam to be so drastically reduced?

2.8 How did the vegetation change as you traveled along the river in the lower basin?

2.9 Much of the water in the Colorado is being used to irrigate otherwise dry, unproductive desert regions. Take a screenshot of a good example of this practice.