As North America approaches the end of the 2021 water year, the two largest reservoirs in the United States stand at their lowest levels since they were first filled.
After two years of intense drought and two decades of long-term drought in the American Southwest, government water managers have been forced to reconsider how supplies will be portioned out in the 2022 water year.
Lake Mead is the largest reservoir in the United States and part of a system that supplies water to at least 40 million people across seven states and northern Mexico. It stands today at its lowest level since Franklin Delano Roosevelt was president. This means less water will be portioned out to some states in the 2022 water year.
As of August 22, 2021, Lake Mead was filled to just 35 percent of its capacity. The low water level comes at a time when 95 percent of the land in nine Western states is affected by some level of drought (64 percent is extreme or worse). It continues a 22-year megadrought that may be the region’s worst dry spell in twelve centuries.
These natural-color images were acquired in August 2000 and August 2021 by Landsat 7 and Landsat 8. The tan fringes along the shoreline in 2021 are areas of the lakebed that would be underwater when the reservoir is filled closer to capacity. The phenomenon is often referred to as a “bathtub ring.”
The lake elevation data above come from the U.S. Bureau of Reclamation, which manages Lake Mead, Lake Powell, and other portions of the Colorado River watershed. At the end of July 2021, the water elevation at the Hoover Dam was 1067.65 feet above sea level, the lowest since April 1937, when the lake was still being filled. The elevation at the end of July 2000—around the time of the Landsat 7 images above and below—was 1199.97 feet.
At maximum capacity, Lake Mead reaches an elevation 1,220 feet near the dam and would hold 9.3 trillion gallons of water. The lake last approached full capacity in the summers of 1983 and 1999. It has been dropping ever since.
In most years, about 10 percent of the water in the lake comes from local precipitation and groundwater, with the rest coming from snow melt in the Rocky Mountains that melts and flows down to rivers, traveling through Lake Powell, Glen Canyon, and the Grand Canyon on the way. The Colorado River basin is managed to provide water to millions of people—most notably the cities of San Diego, Las Vegas, Phoenix, and Los Angeles—and 4-5 million acres of farmland in the Southwest. The river is allotted to states and to Mexico through laws like the 1922 Colorado River Compact and by a recent drought contingency plan announced in 2019.
With the Lake Mead reservoir at 35 percent of capacity, Lake Powell at 31 percent, and the entire Lower Colorado system at 40 percent, the Bureau of Reclamation announced on August 16 that water allocations would be cut over the next year. “The Upper [Colorado] Basin experienced an exceptionally dry spring in 2021, with April to July runoff into Lake Powell totaling just 26 percent of average despite near-average snowfall last winter,” the USBR statement said. ”Given ongoing historic drought and low runoff conditions in the Colorado River Basin, downstream releases from Glen Canyon Dam and Hoover Dam will be reduced in 2022 due to declining reservoir levels. In the Lower Basin the reductions represent the first “shortage” declaration—demonstrating the severity of the drought and low reservoir conditions.”
For the 2022 water year, which begins October 1st:
• Mexico will receive 80,000 fewer acre-feet, approximately 5 percent of the country’s annual allotment
• Nevada’s take will be cut by: 21,000 acre-feet (about 7 percent of the state’s annual apportionment).
• The biggest cuts will come to Arizona, which will receive 512,000 fewer acre-feet, approximately 18 percent of the state’s annual apportionment and 8 percent of the state’s total water use (for agriculture and human consumption).
An acre-foot is enough water to supply one to two households a year.
NASA Earth Observatory images by Lauren Dauphin, using Landsat data from the U.S. Geological Survey and lake elevation data from the Bureau of Reclamation. Story by Michael Carlowicz and Kathryn Hansen.
Straddling the border of southeastern Utah and northeastern Arizona, Lake Powell is the second largest reservoir by capacity in the United States. In July 2021, water levels on the lake fell to the lowest point since 1969 and have continued dropping. As of September 20, 2021, the water elevation at Glen Canyon Dam was 3,546.93 feet, more than 153 feet below “full pool” (elevation 3,700 feet). The lake held just 30 percent of its capacity. To compensate, federal managers started releasing water from upstream reservoirs to help keep Lake Powell from dropping below a threshold that threatens hydro power equipment at the dam.
The natural-color images above show Lake Powell in the late summer of 2017 and 2021, as observed by the Operational Land Imager on Landsat 8. The September 2017 image was chosen because it represents the highest water level (3,630.76 feet) from the past decade. The line plot below shows water levels since 1999, when Lake Powell approached 94 percent capacity.
With the announcement on September 22, the U.S. Bureau of Reclamation (USBR) explained that updated hydro-logical models for the next five years “show continued elevated risk of Lake Powell and Lake Mead reaching critically-low elevations as a result of the historic drought and low-runoff conditions in the Colorado River Basin. At Lake Powell, the projections indicate the potential of falling below minimum power pool as early as July 2022 should extremely dry hydrology continue into next year.” Minimum power pool refers to an elevation—3,490 feet—that water levels must remain above to keep the dam’s hydro power turbines working properly.
“Given ongoing historic drought and low runoff conditions in the Colorado River Basin, downstream releases from Glen Canyon Dam and Hoover Dam will be reduced in 2022 due to declining reservoir levels,” the USBR statement said. “In the Lower Basin the reductions represent the first “shortage” declaration—demonstrating the severity of the drought and low reservoir conditions.”
In a report and op-ed released on September 22, members of a NOAA Drought Task Force offered some context for the low water levels across the region. “Successive dry winter seasons in 2019-2020 and 2020-2021, together with a failed 2020 summer southwestern monsoon, led precipitation totals since January 2020 to be the lowest on record since at least 1895 over the entirety of the Southwest. At the same time, temperatures across the six states considered in the report (Arizona, California, Colorado, Nevada, New Mexico and Utah) were at their third highest on record. Together, the exceptionally low precipitation and warm temperatures reduced snowpack and increased evaporation of soil moisture, leading to a persistent and widespread drought over most of the American West.“
Sharing Colorado River Water: History, Public Policy and the Colorado River Compact
As a measure of its importance and stature, the compact became the keystone to the “Law of the River.” The Law of the River is a composite some might say an “assortment” to better describe its piecemeal assemblage of state and federal laws and regulations, court decisions, and international treaties made over time for the purpose of managing the Colorado River. Concerned with one of the West’s most important rivers, the compact clearly stands as a monument in U. S western water law.
The signing of the compact is a notable event which western water interests remember and celebrate. Accordingly, during May 28-31, 1997 a Colorado River Compact Symposium was conducted at Bishops Lodge in Santa Fe the site where compact delegates met in 1922 to celebrate the 75th anniversary of the signing of the compact. The symposium topic was “Using History to Understand Current Water Problems.”
READ MORE ABOUT: History & Public Policy, History of the Compact, Arizona Stands Firm, Colorado River Use Today, Indian Water Rights, Water Marketing, Colorado River Environmental Concerns and Future Uncertainties.
VIDEO: Unrelenting drought leaves millions who rely on Colorado River facing an uncertain future