INVESTIGATING THE THERMAL IMPACTS OF HYPORHEIC EXCHANGE IN AN ALLUVIAL ARCTIC RIVER

Understanding how climate change will affect river temperatures, and therefore Arctic carbon feedbacks and aquatic habitat suitability, necessitates an understanding of dominant heat fluxes between Arctic rivers and their surroundings. A process based river temperature model that tracks heat in the main channel and riverbed has been constructed for the Kuparuk River, Alaska USA based upon understanding of dominant heat fluxes in temperate rivers and includes heat fluxes at the air-water interface, bed conduction, and transport (downstream advection and lateral inflow). This model reproduces spatial and temporal trends in river temperatures under moderate to high flows where radiative heat fluxes at the air-water interface and lateral inflows dominate the energy budget. Under low flow conditions, however, the model substantially overestimates longitudinal gains in heat resulting in observed river temperatures that are significantly “buffered” in comparison with model predictions. These observed river temperatures under low flow conditions were accurately reproduced by incorporating hyporheic exchange into the river temperature model. To verify the magnitude and extent of hyporheic exchange and its impact on river temperature, solute tracer studies were conducted over a 1.5 km study reach within the domain of the river temperature model. Discharge, temperature, and meteorological conditions were collected concurrently to estimate heat fluxes within the model domain. By adding solute transport into the temperature model, temperature and tracer data are combined to provide multiple lines of evidence of the magnitude and significance of hyporheic exchange. Solute breakthrough curves collected from nested wells in the hyporheic zone were first used to bound the rates and vertical extent of hyporheic exchange. Next, solute breakthrough curves and temperature information from the main channel were used to estimate reach average transient storage properties (including hyporheic exchange). The data support our modeling results that suggest extensive and rapid hyporheic exchange buffers instream river temperatures from atmospheric heat fluxes that would otherwise result in substantial warming in river temperatures during low flow periods.