GROUNDWATER LEVEL INFERRED BASEFLOW IN THE ROARING FORK RIVER, COLORADO

Groundwater is vital in sustaining streamflow via baseflow, particularly during periods of low precipitation and overland flow. Baseflow, a proxy for groundwater discharge to streams, accounts for a substantial portion of streamflow. Knowledge concerning the role of groundwater and its interaction with surface water is critical in informing water resource management. Limited baseflow studies within the Upper Colorado River Basin indicate that approximately half of streamflow is accounted for by baseflow. How important is the role of groundwater in sustaining streamflow? This study uses groundwater levels to infer baseflow to the Roaring Fork River, a major tributary within the Upper Colorado River Basin.

The Roaring Fork River flows along the western margins of Colorado’s Southern Rocky Mountains physiographic province. Groundwater level data from 2000 to 2022 within the Roaring Fork watershed were obtained from the Colorado Division of Water Resources. A subset of the data, consisting of more than 150 shallow wells, was used to interpolate static groundwater level elevations. Hydraulic gradients near the Roaring Fork River were elucidated from contoured groundwater levels. Existing estimates of hydraulic conductivity were analyzed using empirical pumping test formulae. On the basis of hydraulic gradient and hydraulic conductivity, a mean annual groundwater discharge of 1.57 m³/s to the Roaring Fork River is estimated. In parallel, graphical baseflow separation using the USGS Hydrologic Toolbox was conducted, yielding a similar magnitude of baseflow. Together, this study confirms that groundwater accounts for a substantial portion, approximately 60 percent, of streamflow in the Upper Colorado River Basin. This study, for the first time we are aware of, demonstrates the potential of utilizing existing publicly available groundwater level data to supplement the study of baseflow, in consequence establishing a cost-effective method in quantifying baseflow contribution. As the groundwater monitoring network expands, the efficacy of this method will only improve.