While we’ve all been obsessing over the elevation of Lake Mead, there’s a second looming lake elevation problem that could really complicate Colorado River management and increases the risk of a 2016 Arizona shortage declaration beyond the current estimates. Depending on how things play out over the next couple of months, this second problem could leave Lake Mead 15 feet or more lower by the end of next year than the current forecasts would suggest.

All eyes right now are on the 1,075 feet Lake Mead elevation level that is the magic threshold.  Here’s the second number to think about: 3,575 elevation at Lake Powell. If Powell crosses that threshold at the wrong time, it would trigger a big cut in deliveries to Lake Mead later this year and leave the reservoir outside Las Vegas (where I’m currently sitting typing this) a lot emptier than current estimates suggest.

The official numbers, reported yesterday by Tony Davis, put the probability of Lake Mead hitting the shortage criteria, a surface elevation of 1,075 feet above sea level, by Jan. 1, at a one in three chance. Here’s the slide from one of the presentations done earlier this week on the latest model runs:

Lower Colorado River Basin shortage risk

But the risk has gone up since that calculation was made.

The reason is buried in the complex Colorado River operating rules, a time bomb that could detonate and send Lake Mead to substantially lower levels in the next year. It’s this language from the 2007 operating guidelines (pdf):

In Water Years when the projected January 1 Lake Powell elevation is below 3,575 feet and at or above 3,525 feet, the Secretary shall release 7.48 maf from Lake Powell in the Water Year if the projected January 1 elevation of Lake Mead is at or above 1,025 feet.

That’s why 3,575 matters.

The 33 percent figure above is based on the April 1 runoff forecast. But things have gotten worse since then. Based on the April 1 forecast, Powell would be just a hair above the magic 3,575 next Jan. 1. With less runoff now expected to come into the Upper Basin’s big storage reservoir, the risk of that 3,575 trigger is bigger than it was when the above table was generated. That hard to read gobbledygook paragraph quoted above, translated, means that if come August, the forecast calls for Lake Powell to be below that threshold, releases into Lake Mead from upstream would be reduced beginning Oct. 1. That, in turn, pushes Mead far closer to a 1,075 shortage declaration.

A 7.48 million acre feet release from Powell to Mead in the next year, instead of the usual 8.23 maf or an even more optimistic 9 maf in the Bureau of Reclamation’s most recent 24-month study (pdf) means a difference of 10 to 15 feet in elevation in Lake Mead. And the cut in deliveries to Lake Mead would start Oct. 1, meaning Mead would start dropping this year.