Snowpack—which essentially serves as a water tower for the western United States—produces vital meltwater that flows off the mountains each spring. Like a time-release capsule, snowpack refills streams and reservoirs and waters crops and cities through the dry summer in this largely semiarid region.

But the snowpack is becoming more like a snow gap, as temperatures in the Cascades and Sierra Nevadabecome too warm for the snow that replenishes the ecosystem each winter. Temperatures in the West are rising, and winter storms—which have been in infrequent for years—are bringing more rain and less snow.

As a result, the water tower of stored mountain snow—which typically provides one-third of California's water—is no longer refilling each winter, leaving a gap in summer water supplies.