In Northern California, the water system/ecosystem crisis in the Sacramento-San Joaquin Delta understandably gets top billing. But the state’s water supply picture is also affected by Hoover Dam straddling the Nevada-Arizona border and the body of water behind it, Lake Mead. Or, perhaps more accurately put, the lack of water behind Hoover Dam.

Last fall the lake dipped to its lowest level in 75 years, a stunning visual metaphor to the limits of water supply in the West. But what will happen this year?

As the winter has unfolded, two very different scenarios are emerging. Either Lake Mead will dip this fall to new record low levels, bringing three western states (including California) closer to shortage conditions. Or the lake will dramatically rebound and hold 30 feet more water – roughly a trillion more gallons of supply compared to the dry scenario. A single modest storm between now and the end of March – or the lack of such a storm – may prove to be the difference.

Why? Three-hundred and fifty miles north of Lake Mead is the Colorado’s other giant body of water, Lake Powell. The two lakes at present are quite the contrast. Lake Powell (capacity 24 million acre-feet) is 57 percent full. Lake Mead (the nation’s largest reservoir, capable of holding 28.5 million acre-feet) is 42 percent full. Lake Powell is the key water reserve for the Colorado River Basin’s upper states (Colorado, New Mexico, Utah and Wyoming).

Lake Mead is a key lifeline for Nevada, Arizona and California. It’s a big deal for seven western states when Lake Powell has to release additional supplies to help replenish Lake Mead. And it has never happened since the water interests in these states agreed in 2007 how to share in a potential shortage and to “equalize” the lake levels.

If the rest of the winter is average to wet, Lake Powell will have to release additional supplies that will flow through the Grand Canyon and help to fill Lake Mead. If the weather trends dry, Lake Powell may only have to release a baseline supply (8.2 million acre-feet). The federal Bureau of Reclamation will make the decision in April based on the snowpack levels in the Rocky Mountains and the “equalization” formula in that 2007 agreement.

This could be like a too-close-to-call election that hinges on the counting of those final absentee ballots. In the case of the Colorado River, Mother Nature, and what she delivers or does not in the weeks ahead, will determine whether the Lower Basin is very close to shortage conditions or gets a temporary reprieve.