A set of guidelines for managing the Colorado River helped several states through a dry spell, but it’s not enough to keep key reservoirs in the American West from plummeting amid persistent drought and climate change, according to a U.S. report released Friday.

Millions of people in seven states and Mexico rely on the river for drinking water and growing crops. The 2007 guidelines were meant to lessen the blow of any future cuts in the water supply for growing areas, giving states an idea of what to expect each year and ways to manage the risks.