EVOLVING HYPORHEIC EXCHANGE FLUX DURING BASEFLOW RECESSION: USING HIGH-RESOLUTION HEAT DATA TO QUANTITATIVELY ASSESS TEMPORAL PATTERNS

Small beaver dams create punctuated head differentials along the water surface profile and trap sediment into complex bedforms. This enhances hyporheic exchange, although the magnitude of this exchange is not uniform across the streambed, and may respond to temporal forcing mechanisms in a variety of ways. High spatial resolution fiber-optic distributed temperature sensing (DTS) may be used to collect heat data over vertical profiles in the streambed for extended periods of time. This data can be interpreted with one-dimensional flux models to quantitatively determine how vertical hyporheic flux patterns evolve over time. We installed nine custom DTS sensors with 1.4 cm vertical spatial resolution to 0.8 m depth and recorded temperature every 20 minutes for one month above two beaver dams in Wyoming, USA. The sensors were arranged spatially to cover the dominant streambed morphologies (pools, bars and glides), and at varied distance from the dam step. During the study period stream discharge dropped by 45%, and stream stage and velocity generally declined as well. We explored correlations between stream discharge and vertical hyporheic flux at every depth along the nine profiles over the month. We found significant positive correlations between falling stream discharge and temporal changes in hyporheic flux at intermediate-to-deep profile depths at the glide and bar locations close to the dams. Therefore, at these profiles hyporheic flux became shallower and weaker as stream discharge dropped. In contrast, at bar locations farther from the dams we found significant increases in shallow vertical flux with decreasing discharge, which was unexpected. The shallow hyporheic exchange at the bar locations farther from the dams was likely driven by hydraulic pumping over the bar, which may have been enhanced as the dam exerted less control over the water surface profile there and turbulence increased. Changes in flux over time were corroborated by detailed biogeochemical data collected within the streambed, which showed a transition from oxic to anoxic conditions when as flux decreased and the opposite pattern when flux increased.