Overdrafted: Colorado River and Coachella Valley

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Colorado River Watershed from headwaters to mouth. Image by Shannon

Colorado River Watershed from Headwaters to Mouth. Image by Shannon.

The last thing a young Colorado River expects is to be overdrafted as gravity drags it down from the Continental Divide at La Poudre Pass Lake in Rocky Mountain National Park and through Grand Junction, picking up flow from tributary waters along the way.

At the northern Arizona border, however, the Glen Canyon Dam at Lake Powell, then the Hoover Dam at Lake Mead impounds the river, and the challenging role of providing fresh water to an arid land begins. By the time the Colorado reaches the delta it once helped build, and freedom, it can barely run.

The tragedy of the Colorado River is not rare. The United Nations reports that over 1.4 billion people currently live in overdrafted river basins. This means that the use of water exceeds the minimum recharge levels; overdrafting leads to the desiccation of rivers and depletion of groundwater. The latter impact also leads to geotechnical problems such as land subsidence.

Colorado: A Managed River

Numerous compacts, federal laws, court decisions and decrees, contracts, and regulatory guidelines collectively known as “The Law of the River” govern the management and operation of the Colorado River. In 1922, on the suggestion of Secretary of Commerce Herbert Hoover, the Colorado River Compact divided the basin into an upper and lower half, with each having the right to develop and use 7.5 million acre-feet (maf) of river water annually.

Lake Powell is the major storage facility for the Upper Basin states (Colorado, Utah, Wyoming, and New Mexico). When full it can hold 24.3 maf. The Compact specifies that the Upper Basin states are to provide a minimum annual flow of 7,500,000 acre feet (9.3 km3) to the Lower Basin states (Arizona, Nevada, and California).

Monitoring and Forecasting Colorado River Volume

The Colorado Basin River Forecast Center uses models to forecast inflow volumes to LakePowell. The unregulated inflow forecast for the water year 2013, for instance, is 41% of average based on the period 1981-2010. The forecasts are adjusted monthly based on new data. The declining volumes in the reservoir can be seen by the water lines on the sides. Forecasts for 2014 of an increase to 78% of average, based on the period 1981-2010, while encouraging, cannot in themselves repair the overdrafting currently taking place.

Low Levels in Lake Powell Can Be Seen  from High Water Ring. Image by PRA

You can see the low levels in Lake Powell by the high water ring. Image by PRA

Where managers suspect subsidence from overdrafting, they often apply Synthetic Aperture Radar (SAR) time-series analysis to produce interferograms. These are photographic records made by an apparatus for recording optical interference phenomena.

Colorado River and River Desiccation

As reported by Jennifer Pitt in the National Geographic, “Since 1960, when the gates were closed on the newly built Glen Canyon Dam (Lake Powell Colorado River has only rarely flowed to the sea, and the river’s delta (has) started to fade.

The upper part of the Colorado River hydrologic system is snowmelt-driven. In the lower basin, legally demarcated as the area below the Lees Ferry river-gauging station, it is heavy-gauged and diverted to turn desert into green before reaching the Gulf of California.

Along its route, the river supplies water for 30 million people and climbing, and for thousands of acres of farmland. At the same time, the water in the drainage system has dropped from 17.5 million acre-feet in 1922 to a present level of about 14.7 million acre-feet per year. Increased pressure on this river in a time of climate change and less water input can be laid at the feet of the farmers and ranchers who use it inefficiently for irrigation. Add to that the pressure from the cities of Phoenix, Tucson, Las Vegas, Los Angeles, San Bernardino and San Diego which pipe in water from long distances so its citizens can use double the global average of water per person.

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