June 21, 2016--Study finds surprising source of Colorado River water supply (Cortez Journal)

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. The groundwater is composed of rivulets of water hidden below ground —some thousands of years old — that are particularly important for sustaining a river’s flow after the spring snowmelt has subsided. Less clear, however, was exactly how much of the flow in rivers came from groundwater, a critical source of much of the West’s water supply. Now, a new study, released last month by the U.S. Geological Survey (USGS), helps quantify just how much: more than half the flow of rivers in the upper part of the Colorado River Basin is sustained by groundwater. That finding, say experts, highlights the need to better protect a resource threatened by overuse and climate change. “Because we now have numbers on this connection, we have a better understanding of the importance of groundwater as a contributor to our surface water supply, and anything impacting the groundwater system will also impact flow in rivers.” says Matthew Miller, a USGS scientist and the lead author of the study.

To determine how much of the flow in rivers came from groundwater, scientists examined streamflow data at 146 sites in Colorado, Utah, Wyoming, New Mexico, and Arizona, measuring the electrical conductivity of the water. Low conductivity meant the water had not had time to pick up ions from the ground, indicating it came from recent snowmelt. Meanwhile, higher conductivity signified the water had picked up ions as it trickled through soil and rocks below ground. Researchers then used the information to determine the percentage of water originating from snowmelt runoff and the percentage originating from groundwater and created a model that predicts where streamflow originates in the Upper Colorado River basin. On average, Miller and his team found that 56 percent of that flow comes from groundwater. To read the full article visit the Cortez Journal.