LIMITED RIPARIAN AQUIFER RECHARGE DUE TO DROUGHT AND DAM MANAGEMENT ALONG THE DOLORES RIVER, COLORADO

Here we document the combined impact of record low snowmelt runoff, small dam releases, and weak monsoons of the 2012 water-year on riparian aquifers along the Dolores River in SW Colorado. Flows (approximately 2600 Cubic Feet per Second (CFS)) are typically released from McPhee Reservoir into the Dolores River to simulate snow melt high flows. During 2012, flows of 35 to 70 CFS were released due to the low snowpack in alpine regions. In this study, transects of piezometers were set up in 2010, at three study sites along the Dolores River; Upper Dolores Wildlife Area, Lone Dome Wildlife Area, and Big Gypsum Valley. Tipping rain gage stations were added in 2011, and in 2012 the installation of low cost stream gages provided onsite stream flow data. 2012 piezometer data show a significant decrease in groundwater levels compared to 2010-2011, due to low stream flows and evapotranspiration. Because of exceptionally low dam releases, and record high temperatures, piezometers located shoreward in the point bar system went dry by early June of 2012 for the first time in three years, showing no sign of recharge through the monsoon season regardless of duration or magnitude of rainfall events. Soil moisture probes located at 10 cm and 50 cm below the ground surface show water absorption into soil during longer duration precipitation events at 10 cm, but not 50 cm. This is attributed to low hydraulic conductivity and evapotranspiration. Throughout 2012 and this entire study, piezometer data show stream flow and associated bank recharge as the dominant influence of riparian aquifer recharge. This is highlighted by the fact that piezometers farther than 15m from the stream showed no response from monsoon rain events in 2012, regardless of their duration or magnitude. If the drought of 2012 is an indicator of future climate variability, the health of riparian ecosystems which rely on ground water, will require careful water management.