We will know in a little more than a month whether Lake Mead will get a flush of “bonus water” this year, and if so how much, thanks to the better-than-average snow pack in the Upper Colorado River Basin.

Right now, things look good. Current snow levels, as measured by the network of federally funded snow sensors around the mountains, are 22 percent about average on the tributaries that feed into the Colorado River above Lake Powell (numbers via the Colorado Basin River Forecast Center). With a bit more than two days left in the month, both Mead and Powell are close to the optimistic side of the probability distribution in the most recent US Bureau of Reclamation “24-month study” (pdf).

Hoover Dam cutaway, courtesy USBR

A lot is at stake. The current forecast calls for enough inflow into Powell to meet the criteria of “equalization”, which means letting more water out of Powell to “equalize” levels between it and its downstream sibling, Lake Mead, behind Hoover Dam. Without equalization, Lake Mead would drop close to the level that triggers a shortage declaration in the lower basin. The US Bureau of Reclamation makes the equalization decision based on a formula that takes into account the levels of both reservoirs. The bottom line is that the forecast calls for Powell to be high enough at the end of the 2011 that all of the extra water flowing into the system this year will be passed straight through, down the Colorado River through the Grand Canyon and into Lake Mead.

As a result, despite the big snow pack, Lake Powell is forecast to end the current water year at pretty much exactly the same level it was at a year earlier – 3633 feet above sea level, 14.2 million acre feet in storage. Lake Mead, on the other hand, will have risen 21 feet, to 1105 feet above sea level, or 11.9 maf in storage.

So what’s wrong with this picture?

Let’s follow the numbers. Lake Mead gets 3.2 million acre feet of “bonus water” from Lake Powell – water delivered above and beyond the Colorado River Compact’s required 8.23 maf annual delivery, essentially all the extra water nature delivers in this above-average year. But its end-of-year storage will be just 1.85 maf above last Sept. 30. This is because the states of the lower basin use, year in and year out, more water than nature and minimum compact requirements deliver.

Depending on above-average winters does not a sustainable water policy make.