EO Study: Drought Lowers Lake Mead

  Drought Lowers Lake Mead
 

The western half of the United States has received far less rain and snow in the past several years than normal. This sustained drought has caused many changes easily detected by NASA’s Earth-observation satellites: dramatic decreases in soil moisture, shrinking snow pack in the Rockies, withering vegetation, more frequent and severe forest fires, and falling water levels in major reservoirs throughout the West.

 

 

 
 

The image above, acquired by the Landsat satellite, shows the shoreline of Lake Mead in May 2000. Place your cursor over the image to see the shoreline in May 2003. Dramatic changes are quite evident in the three-year span between these images. Water levels in the lake between these images dropped 18 meters (60 feet). Water management officials predict a further drop of between 4.5 to 6 meters (15-20 feet) this year, if weather and water use predictions are correct.

 

In the space of just three years, water levels in Lake Mead have fallen more than sixty feet due to sustained drought. Move your cursor over the image to compare the shoreline in 2000 to 2003. (Image by Jesse Allen, based on data provided by the Landsat 7 Science Team)

 

Photograph of Low Lake Level

Lake Mead is one of the most important water resources in the West. Created in the 1930s, it ensures a steady water supply for Arizona, Nevada, California, and northern Mexico by holding back the flow of the Colorado River behind the Hoover Dam. It is one of the largest water reservoirs in the world. When full, the lake contains roughly the same amount of water as would have otherwise flowed through the Colorado River over a two-year period: roughly 36 trillion liters (9.3 trillion gallons). As of May 2003, Lake Mead was at roughly two-thirds of this capacity.

Lake Mead lake is divided into four major basins: (from west to east) the Boulder, Virgin, and Gregg Basins, plus the Overton Arm which extends northward from the Virgin Basin to the confluence of the lake with the Virgin and Muddy Rivers. Each of these basins shows signs of the water level changes in the reservoir.

At the northern end of the Overton Arm, for example, the Muddy and Virgin Rivers flow into the lake. Water level changes in this area have brought back to view the foundations and road grid of the flooded town of St. Thomas, left when the town was demolished to make way for the new lake. At the southern end of the lake, where the Hoover Dam holds back the Colorado River, water level drops have changed the shoreline. The Boulder Basin now has a series of islands that were beneath the water in 2000, including a pair adding to the cluster of Boulder, Little Boulder, and Rock Islands. Black Island has become a part of the northern shoreline. Saddle Island was formerly only connected to the shore by the water pipes which are fed by the water intakes on the island; now the shoreline extends well beyond the peninsula under the pipes.

 

The thick white band ringing Lake Mead’s shoreline shows the drop in water levels. The near-vertical walls of Boulder Canyon are just upstream of Hoover Dam. (Photograph courtesy National Park Service, Lake Mead National Recreation Area)

 

Image Pair of Boulder Basin

 

Lake Mead water supports diverse uses in the West. It irrigates about a million acres of farmland in the United States, much of it in southern California’s Imperial Valley, and another half million acres in northern Mexico as part of international water agreements. The water released through the dam can be used to generate electricity for about 500,000 homes and to lift water up over the Sierra Nevada Mountains to irrigate southern California, but power generation is secondary to agricultural and municipal water demands in places like southern California, central Arizona, and southern Nevada.

Ninety percent of southern Nevada’s water comes from Lake Mead, with releases regulated by the Southern Nevada Water Authority. When the water levels in the lake are declining and expected to cross below an elevation of 1,145 feet, the Water Authority declares a Drought Watch. Once the water is below 1,145 feet, the watch is shifted to a Drought Alert—the status in May 2003, when the latter Landsat image was acquired. If the lake top drops below 1,125 feet, a Drought Emergency goes into effect. Each of these water-level alert states triggers various water restrictions and practices in the area, from restrictions on watering gardens, washing cars, running fountains in civic parks and public places, to increases in the rates charged for water to encourage conservation. The water elevation last dropped below 1,125 feet in 1965.

 

In the Boulder Basin of Lake Mead, the lower water level has connected former islands like Saddle Island to the shoreline. The National Park Service has also moved marinas to the new shoreline or new sites, such as moving Las Vegas Bay marina to Hemenway Harbor. (Image by Jesse Allen, based on data provided by the Landsat 7 Science Team)

Graph of Annual Lake Mead Water Levels

 

Ninety-six percent of the water flowing into Lake Mead comes from snowmelt that drains into the Colorado River. Early spring melt sends water surging through the river and into Lake Powell. Water released from Lake Powell then flows through the Grand Canyon and into Lake Mead. The lake level is typically highest in early spring and lowest in mid- to late summer when irrigation needs have spiked and drawn down the water levels.

 

Since Hoover Dam was completed in the 1930s, Lake Mead has suffered three major droughts. In 1955 the elevation of the lake surface dropped to almost 1100 feet, but recovered to normal levels within a year. In 1964 the level dropped below 1100 feet, and took a decade to recover. (Graph by Robert Simmon, based on data provided by the U.S. Bureau of Reclamation)

Graph of Monthly Lake Mead Water Levels

 

Water flows out of Lake Mead at roughly the same rate from year to year (though irrigation demand fluctuates with rainfall). Inflow depends on release from Lake Powell. In years with much less snow, as has been the case in the past several years, Lake Powell releases only the minimum flow rate required by law. This inflow from Lake Powell is less than the outflow from Lake Mead, and water levels in Lake Mead will continue to decrease until the next heavy snow season. A Drought Emergency might well be declared before next spring’s influx of snowmelt.

The retreat in the water level in Lake Mead has forced changes in the recreational services within the park; for example, National Park Service Rangers have moved the Las Vegas Bay Marina and Lake Mead Ferry Service to Hemenway Harbor to keep them functional and in the water. Marinas elsewhere around the lake’s edge have had to migrate down the shoreline. It is estimated that each twenty feet in water-level change costs the National Park Service around $6 million. Rangers are also rediscovering roads and boat ramps built during the severe dry spell in the mid-1960s when the water levels were last this low. In the forty years since, the locations of these features were forgotten after they were submerged.

The change in the water level in the lake, while dramatic, is not yet cause for great concern. Boats on the lake have a new set of navigational hazards to face, many of them are unnamed reefs and other obstacles which were previously in deep enough water not to pose a collision risk. But the low levels of the lake are not unprecedented, as the lake levels underwent a similar decrease in the 1960s. Sufficient water remains to meet irrigation and residential needs. Lake Mead is still a huge lake with enough water for the many recreational opportunities for the eight to ten million people who visit it each year. Fluctuations in the lake level are a natural part of its operations. Nevertheless, the sustained dry spell will need to end before the lake’s level returns to more historically normal elevations. Unfortunately for water planners in the region, it is difficult to predict how long this might take. During a drought in the late 1950s, water levels returned to normal within a year. However, it took the lake almost ten years to recover from the record low levels of 1965. Water managers are keeping a careful eye on climate predictions to tell how soon the lake may recover. Meanwhile, satellites like Landsat will be watching for changes in water level that signal the progress of Lake Mead’s recovery.

 

Lake Mead’s water level varies on a regular basis throughout the year. Melting snow raises the lake in the spring, while evaporation and siphoning of irrigation water lowers the level in summer. Since 2000, meltwater has raised the lake’s surface only one or two feet each spring, while summer water use has lowerd the level about 20 feet each year. (Graph by Robert Simmon, based on data provided by the U.S. Bureau of Reclamation)