Colorado River transformation

Major reservoirs, constructed along the Colorado River to capture and more efficiently use annual river flow, promote evaporative water loss that concentrates the mineral salts washed into the river. Climate change and high levels of water consumption have exacerbated this problem.

Background

The Colorado River is the major source of freshwater for a watershed of seven southwestern states in the United States and the adjacent nation of Mexico. In the early twentieth century, California was growing rapidly, and the other states realized that their future development depended upon water allocation from the river. A series of compacts, laws, and court rulings legally parceled out more water than annually flows down the river, which eventually cut off the river's natural flow to the sea.

Law of the River

In 1902, the Reclamation Act provided for development of irrigation projects throughout the West, including projects in California, Arizona, Utah, and Colorado in the Colorado River Basin. The Colorado River Compact (1922), the first of a series of laws and agreements specific to the Colorado River, divided the river into an upper and lower basin and allocated 9.25 cubic kilometers of water to each. The lower basin was given priority, so in dry years the upper basin allocation must be cut back to ensure that 9.25 cubic kilometers flows to the lower basin. The states in each basin negotiated a share of the water.

In a “good year,” state allocations of Colorado River water could be as high as the following: Colorado, 4.76 cubic kilometers; New Mexico, 1.04 cubic kilometers; Utah, 2.11 cubic kilometers; Wyoming, 1.28 cubic kilometers; Nevada, 0.37 cubic kilometer; Arizona, 3.45 cubic kilometers; and California, 5.43 cubic kilometers. In 1944, Mexico was granted an allocation of 1.85 cubic kilometers, and a subsequent agreement specified allowable salinity. However, in 2022, after two decades of severe drought in the Colorado River Basin, the federal government announced that the states receiving water from this source would receive less water in 2023. The US Bureau of Reclamation said it would determine the new water allocations for the affected states if they failed to decide on their own reduced water allocations.

The courts allocated 1.11 cubic kilometers to Native Americans in the lower basin. By the twenty-first century organized efforts to secure an increased water allocation for Native Americans had gained momentum, and in March 2022 representatives from twenty tribes, including the Southern Ute Tribe, met with Deb Haaland, secretary of the US Department of the Interior, to reopen negotiations regarding their water use in this region.

Dams and Reservoirs

In the late 1800s, agricultural development of the Imperial Valley of California was watered by a canal from the Colorado River running mostly through Mexico. A major flood in 1905 caused a breach in the canal, and for two years the river emptied into the valley, creating the Salton Sea. Another major flood in 1910 stimulated planning for a flood-control dam upstream: The Hoover Dam was completed in 1935, and Lake Mead formed behind it. The Colorado River Compact between the states was made in response to plans to build the Hoover Dam, which would primarily benefit California. The dam provided flood control and generated electricity for Las Vegas and Los Angeles.

There are now seven major dams on the Colorado River, from Glen Canyon Dam, just north of Lees Ferry (the divide between the upper and lower basins), to the Mexican border. An additional thirty-five dams are built on tributaries within the watershed. The four largest reservoirs expose a tremendous surface area to evaporation: Lake Powell, 658 square kilometers; Lake Mead, 640 square kilometers; Lake Mojave, 114 square kilometers; and Lake Havasu, 83 square kilometers. This evaporation has driven an increase in the river’s salinity. Indeed, river levels have increased so much that in 1973 an agreement was reached between the United States and Mexico to mandate maximum salinity levels in the water crossing the border. The following year, Congress authorized construction of a desalination plant on the US side of the border to ensure that water flowing into Mexico achieved treaty salinity specifications.

In addition to affecting water quality, dams and reservoirs change the physical characteristics of the river. Sediments normally deposited along the riverbanks during floods are now captured behind the dams. Absence of annual scouring by floods, with additional deposition of new sediments, has greatly altered habitat along the banks of much of the river.

Human Uses

Shortly after completion of the Hoover Dam, plans were made to build additional dams, aqueducts, and canals. The Parker Dam, creating Lake Havasu, was completed in 1938, and within three years the Colorado River Aqueduct was supplying water to Los Angeles, 400 kilometers to the west. The following year, the Imperial Diversion Dam was completed to divert water through the All-American Canal, which replaced the old through Mexico. The new canal provided reliable irrigation water to the Imperial Valley and water to the city of San Diego.

Almost from the beginning, California began using more than its allocation. By 1990, California had to begin reducing its consumption, as other states used greater and greater amounts of their allocations. In 1963 Denver completed the Harold D. Roberts Tunnel under the continental divide to divert water from the Colorado River watershed to the city. The Central Arizona Project, whose backbone is an aqueduct from Lake Havasu through Phoenix to Tucson, was essentially complete by 1993.

Despite efforts to reduce its consumption, California’s use remained greater than its allocation at about 5.92 cubic kilometers through the end of the 1990s, even as other states made efforts to increase their own access. As a result, by 1980, following years of heavy water usage, the Colorado River had stopped regularly reaching the ocean. This situation led to additional consequences in the former delta region in Mexico, where freshwater input was cut off. In 2014 the gates of the Morelos Dam on the US-Mexico were opened to provide a two-month pulse flow of Colorado River water into the delta region, which provided much-needed water to the region's dehydrated ecosystem and reunited the river with the sea for the first time in years.

In 2023, the Colorado River provided water for consumption by more than 40 million people in Denver, Phoenix, Salt Lake City, Las Vegas, Los Angeles, San Diego, and hundreds of other communities, including Native American reservations, cities, and towns. By that time, the river also provided water to irrigate 5.5 million acres of agricultural land. 2023 marked a significant reduction in human water usage. Arizona, California, and Nevada each pulled less water from the struggling river, resulting in a 13 percent reduction in water usage from 2022.

Context

Early in the twentieth century, politicians realized the importance of achieving an equitable allocation of Colorado River water for the future development of their states and the region. Their agreements were based on available data suggesting an annual flow of 20.23 cubic kilometers per year. It would later be discovered that the average flow is considerably less, about 16.65 cubic kilometers. Disputes over Colorado River water rights in the twenty-first century continued to be shaped by that initial overestimation. Meanwhile, severe droughts placed further strain on the area's resources and further complicated efforts to determine future water use in the region.

Key Concepts

• aqueduct (irrigation canal): a human-made channel or pipe to carry water for consumption or industrial or agricultural use
• desalination: the process of removing dissolved salts and minerals from water to produce freshwater for consumption or irrigation
• evaporation: transformation of liquid water to water vapor, which is absorbed into the air
• reservoir: a body of water that forms behind a dam
• watershed: the area of land drained by a river and all of its tributary streams

Bibliography

Clay, Chandler. "Bringing the River Back to the Sea." Environmental Defense Fund, www.edf.org/sites/default/files/pulseflow/index.html. Accessed 13 Dec. 2024.

"Colorado River." Feeding Ourselves Thirsty, feedingourselvesthirsty.ceres.org/regional-analysis/colorado-river. Accessed 13 Dec. 2024.

Estabrook, Rachel, and Michael Elizabeth Sakas. "The Colorado River is Drying up — But Basin States Have ‘No Plan’ on How to Cut Water Use." CPR News, 17 Sept. 2022, www.cpr.org/2022/09/17/colorado-river-drought-basin-states-water-restrictions/. Accessed 13 Dec. 2024.

Mimiaga, Jim. "Native American Tribes Assert Water Rights on Colorado River Basin." The Journal, 7 Apr. 2022, www.the-journal.com/articles/native-american-tribes-assert-water-rights-on-colorado-river-basin/. Accessed 13 Dec. 2024. Walton, Brett. "Colorado River Water Use in Three States Drops to 40-Year Low." Circle of Blue, 14 June 2024, www.circleofblue.org/2024/world/colorado-river-water-use-in-three-states-drops-to-40-year-low/. Accessed 13 Dec. 2024.