Every spring, snow begins to melt throughout the Rocky Mountains, flowing down from high peaks and into the streams and rivers that form the mighty Colorado River Basin, sustaining entire cities and ecosystems from Wyoming to Arizona. But as spring becomes summer, the melting snow slows to a trickle and, as summer turns to fall, all but stops.

Scientists have known for a long time that flow in rivers is sustained by contributions from both snowmelt runoff and groundwater.