Abstract
This study examined mountain-front stream recharge to a karst aquifer in the Denver Basin (USA) from 2016 to 2023, a period of minimal water use, using a combination of existing data (geologic maps, borehole drilling and logging records, geophysical surveys, and hydrochemical stream and groundwater data) and new monitoring efforts (stream gauging, near-stream drilling, and water level (WL) instrumentation at stream gauges and monitoring wells). Stream and karst aquifer WLs were jointly analyzed in the context of all data. In a northern subwatershed, stream and aquifer WLs followed annual snowmelt cycles by rising in spring and falling in summer. Where the streambed intersects the aquifer, the stream recharges the aquifer through fractures and conduits at sinks, as supported by drilling data and similarity of hydrochemical composition and electrical conductivity of stream water at a sink and groundwater in wells located > 1 km downstream. Moreover, during a 2017–2022 drought that reduced streamflow, the karst aquifer received less recharge and its WLs declined. In two southern subwatersheds, recharge to the confined portion of the karst aquifer likely comes from both stream losses and subsurface inflow, whereas recharge to an unconfined portion of the aquifer could not be determined from the available data. Despite regional precipitation data suggesting that the drought ended in 2023, below-average mountain precipitation that year may have caused continued streamflow and aquifer depletion at the mountain front, suggesting the reliance of the Denver Basin water supply on mountain snowmelt runoff.