The Lower Colorado watershed region.
The watershed drainage area includes: (a) the Colorado River Basin below the Lee Ferry compact point which is one mile below the mouth of the Paria River; (b) streams that originate within the United States and ultimately discharge into the Gulf of California; and (c) the Animas Valley, Willcox Playa, and other smaller closed basins, includes parts of Arizona, California, Nevada, New Mexico, and Utah.
About the Watershed
Population based on 1970-2000 U.S. Census Data.
Prior to development, the Colorado River flowed unimpeded some 1,700 miles with a vertical elevation drop of more than 14,000 feet from its beginnings in the southern Rocky Mountains and eastern Great Basin to its terminus at the Gulf of California. The lower portion of the river from the Grand Canyon downstream was typically low gradient and flowed through a rather broad alluvial valley with relatively few confined reaches. At its mouth was an alluvial delta containing vast marshes, riparian forests and backwaters. Such habitats were present along the entire reach of the lower river. The riparian belt extended away from the river for up to several miles where the water table was relatively shallow.
The Colorado River supplies water and power to the heavily populated and rapidly growing areas of southern California, southern Nevada, Arizona, and Mexico. The availability and quality of water deliveries are critical considerations. In addition, numerous plants and animals depend upon the river for subsistence. Recreation is rapidly gaining importance as a major use of the river and its reservoirs. The treaty with Mexico provides requirements for water delivery south of the border. A number of Indian tribes depend upon the river for economic and cultural values.
- The Colorado Plateau: A physiographic province which encompasses much of Utah, Western Colorado, Northwestern New Mexico, and the northern tier of Arizona, the Colorado Plateau evokes images of redrock canyons, ribbons of creeks, clear blue skies, ancestral homelands for diverse Native American Nations, and dramatic elevational and biological diversity. The Plateau is characterized by one of the West’s great rivers: The Rio Colorado and its thousands of miles of tributaries, large and small. The Colorado Plateau is described roughly as an area bounded on the east by the crest of the Continental Divide, on the west by the Wasatch Line, on the north by the Uinta Mountains in Utah, and to the southern edge along Arizona’s Mogollon Rim.
- U.S.-Mexico Treaty: The United States and Mexico entered into a treaty on February 3, 1944, which guarantees Mexico 1,500,000 acre-feet of Colorado River water annually. This entitlement is subject to increase or decrease under certain circumstances provided for in the treaty.
- The Colorado River Compact: The Colorado River Compact paved the way for the construction of storage dams and delivery facilities on the Colorado River, and, in 1928,Congress passed the Boulder Canyon Project Act, authorizing construction of Hoover Dam. Hoover Dam was without precedent, the greatest dam of its day; it is still a world-renowned structure. Located in Black Canyon between Nevada and Arizona, the dam is a National Historic Landmark and a National Historic Civil Engineering Landmark. In 1994, the American Society of Civil Engineers named it one of America’s Seven Modern Civil Engineering Wonders.
- Major rivers and streams: The river and its tributaries – the Green, the Gunnison, the San Juan, the Virgin, the Little Colorado, and the Gila Rivers – are called the "Colorado River Basin." These rivers drain 242,000 square miles in the United States, or one-twelfth of the country’s continental land area, and 2,000 square miles in Mexico. Seven western states and Mexico have beneficial interests in the Colorado River Basin.
- Salinity: The Colorado River and its tributaries provide municipal and industrial water to about 27 million people and irrigation water to nearly four million acres of land in the United States. The river also serves about 2.3 million people and 500,000 acres in Mexico. The threat of salinity is a major concern in both the Unites States and Mexico. Salinity affects agricultural, municipal, and industrial water users.
In June 1974, Congress enacted the Colorado River Basin Salinity Control Act, Public Law 93-320, which directed the Secretary of the Interior to proceed with a program to enhance and protect the quality of water available in the Colorado River for use in the United States and Republic of Mexico.
- Changing Habitat: The Colorado River was once an environment of extremes. The river flooded wash spring as the Rocky Mountain snowpack melted, bringing with it millions of tons of sediment. It was warm, even hot, in late summer as runoff decreased, and in the winter it was shallow and cold. The native fish that inhabited the river were well adapted to these physical and chemical extremes. The construction of mainstream dams reduced the sediment in the river, allowing sunlight to penetrate through the water and produce an abundance of algae and plant growth. Without the abrasive sediments, small animals were able to colonize the riverbed and bank lines as well as the newly formed reservoirs. Introduced fish species such as carp, largemouth bass, channel catfish, and various sunfishes took advantage of these new plant and animal food sources and quickly established and expanded their populations. Native fish like the razorback sucker, were not well adapted to these biological changes, and their populations slowly declined. Today, this fish is listed as an endangered species, with the last large population in the Colorado River Basin inhabiting Lake Mohave. This group is rapidly declining because the remaining fish are old, generally in excess of 40 years of age, and they are dying off. It is expected that all these old fish will be gone from Lake Mohave by the year 2000.