Flow regulation alters alpine river thermal regimes

Impacts of anthropogenic flow regulation on the thermal regimes of alpine river systems are poorly understood. This is surprising given the importance of water temperature for river ecosystems and the widespread regulation of mountain rivers across the world. This study examined water temperature dynamics year-round between July 2008 and September 2009 in the Eisboden river system, central Austrian Alps. Water temperature data were examined alongside hydroclimatological data to infer the key processes driving thermal variability from diurnal to inter-annual scales. As expected, interactions between meteorology and water source controlled year-round thermal heterogeneity. However, water entering the proglacial river from a hydropower storage reservoir caused significant increases in water temperature during both late summer and early winter, resulting in a marked longitudinal thermal discontinuity. The timing and duration of flows discharged from reservoirs, and thus effects on river thermal regimes, differed considerably from previous studies of subalpine hydropeaking. Furthermore, thermal responses to flow regulation extended laterally to some groundwater tributaries even where there was no upstream surface connectivity, suggesting significant hyporheic flow or conduction of heat through coarse alluvium. River water temperature continued to be altered even after reservoir releases had ceased due to the removal of winter snow cover and recharged groundwater sources. Together, these insights into the thermal variability have broad implications for conservation and management of alpine river systems because water temperature is a key variable influencing aquatic ecosystems, and because anthropogenic pressures on alpine environments are expected to grow in the future.