Lake Mead upriver of Hoover Dam on 27 July 2009, showing the "bathtub ring" due to the depleted water level. Photo by Mjponso, licensed under the Creative Commons Attribution-ShareAlike 3.0 License.
Twelve years of persistent drought and increasing human demand for water in the American southwest have reduced Lake Mead to its lowest level in more than 50 years.
Created in the 1930s as the reservoir behind the Hoover Dam, Lake Mead is located on the Colorado River along the border of Nevada and Arizona. More than 90 percent of its water comes from snow melt in the Rocky Mountains of Utah, Colorado, and Wyoming.
Since 2000, however, below average winter snows have reduced inflow to the point that it cannot keep pace with evaporation and the amount of water flowing out of the lake. The result? As of August, the lake had been reduced to 37 percent capacity.
Satellite images of a shrinking Lake Mead. At left, August 1985; at right, July 2010. Source: NASA
The ramifications for states in the Colorado River’s lower basin – California, Arizona and Nevada – are potentially severe. Electricity from the power plant at Hoover Dam and the water from the Lake Mead reservoir made farms and thriving cities (such as Las Vegas) possible where once there was largely desert. If the reservoir continues to be depleted at a faster rate than it is replenished, the supply of electricity and water to these areas will almost certainly be threatened.
In 2000, water in Lake Mead stood at 1,215 feet above sea level; by last month, it had fallen to 1,087 ft. At 1,075 ft., regional plans call for water restrictions and the water outtake that serves Las Vegas approaches interoperability. At 1,050 ft., the ability of the power plant to produce electricity becomes compromised.
Adding to the problems of reduced inflow and increased demand is a quirk in the Colorado River Compact, which, in 1922, set the terms for how water would be allocated among seven states in the Colorado River Basin and Mexico. Allocations were based upon the average flow of the Colorado River over a ten-year period. Unfortunately, the time period that was used later proved to be one of unusually high river flow, leading to a 15-20 percent overestimation of how much water could be sustainably be drawn from the reservoir.
Water from Lake Mead serves 8 million people in Arizona, Nevada and California, and irrigates more than one million acres of farmland.
