The folks at the U.S. Bureau of Reclamation have crunched the numbers on how the latest 2010 Colorado River runoff forecast will affect the river’s major storage reservoirs, and the news is not good for Lake Mead. At this point, according to the Bureau of Reclamation, there is just a one in five chance that there will be sufficient extra water upstream in Lake Powell to provide the extra supplies that would be needed to raise Mead from its near-record low levels.

The Arizona Republic’s indispensable water guy, Shawn McKinnon, explains:

Water levels at Powell aren’t expected to rise far enough during the spring runoff season to allow the government to release extra water down the Colorado into Mead, Bureau of Reclamation hydrologists said Wednesday. That will leave Mead dangerously close — within just over two feet — to the level used to trigger shortages.

Hoover Dam

Located east of Las Vegas, Nev., Mead is the holding pond for water to be used by farmers and cities in Arizona, Nevada and California. But there’s not so much water in it right now – the volume of water stored in the reservoir is roughly equal to the lowest that it’s been at this point in the year since they first filled it in the 1930s. (As of Sunday, Mead held 11.2 million acre feet of water, just 43 percent of capacity. It dropped to 11.1 maf in late December/early January 1964.) Between the amount of water consumed by those three states and the annual evaporation from the surface of the desert lake, Mead’s only hope to rise in the coming year would be a bounty of snow in the mountains that feed the Colorado.

The reason is the complex set of operating rules that govern what the Bureau calls “equalization” – the process by which excess water from Lake Powell upstream is released to help raise Mead’s levels and keep the two reservoirs roughly in balance. The Powell-Mead juggling act is needed to meet the terms of the Colorado River Compact, which splits the river’s water between upper and lower basin states. To meet the Compact’s water-sharing rules, the Bureau releases 8.23 maf from Powell each year. The problem is that all of that is used in the Lower Basin and Mexico and then some, thanks to the aforementioned evaporation.

Under the rules, Mead has a shot at some extra water. Under operating criteria adopted in 2007, if Powell’s surface looks like it will reach 3,642 feet by the end of the September 2010, that signifies “extra” water, and “equalization” – taking a little out of a fuller Powell and sending it downstream to an emptier Mead – would happen.

But at this point, according to numbers out today, there is just a 21 percent chance of that happening. That is down from the Bureau’s estimate a month ago that there was a 36 percent chance of triggering “equalization” rules in 2010.

A final decision on whether extra water will be released will be made in early April.

(picture courtesy U.S. Bureau of Reclamation)