Why Are Some Rocky Mountain Lakes Ephemeral?

Abstract

AbstractSmall mountain lakes function as temporary storage basins for rain and snowmelt‐derived water. Many small lakes lose water seasonally, but questions remain about the processes involved and effects on watershed hydrology. Evaporation and groundwater outflow from lakes may influence baseflow in streams, hydrologic connections among lakes, and water fluxes from a watershed. To evaluate the role of small mountain lakes in watershed hydrology and the dominant pathways of water loss, we studied the water balances of four shallow, closed‐basin, subalpine lakes in southern Wyoming that lose up to 99% of their volumes between early summer and late fall. We tested the performance of seven evaporation models, compared observed rates of water loss with simulations of evaporation and drainage, and conducted geophysical surveys to evaluate the hydrologic environment between lakes. Our results show that groundwater outflow, rather than evaporation, can dominate water loss and cause closed‐basin mountain lakes to be ephemeral. Groundwater fluxes may contribute to varied rates and timing of water loss from the lakes. Evaporation accounted for 14% of water loss in a lake that overlays thin (<0.5 m) sediments and fractured bedrock and 83% in a lake underlain by >3 m of sediments and clay. Gradual recharge of groundwater (<18,000 m3·km−2·day−1) from each study lake likely helps sustain baseflow in streams once snowmelt has subsided. Total water loss from closed‐basin, subalpine lakes may therefore help to maintain baseflow of rivers in late summer, but their impact varies based on geological context and snowmelt availability.