RESPONSE OF INTRA-MEANDER HYPORHEIC EXCHANGE TO FLOODING AND PERMEABILITY CHANGE IN A LOSING ARTIFICIAL STREAM

Changes in groundwater-surface water interactions between a meandering stream and its adjacent banks were observed on multiple time scales via head, temperature, and permeability measurements at the Outdoor StreamLab, an artificial but full-scale channel-floodplain system at the Saint Anthony’s Falls Laboratory, Minnesota, U.S.A. The steady-state head distribution under normal stream discharge shows that stream water flows into the point bar aquifer from both the upstream and downstream sides, indicating that the stream is losing. Flooding initially creates a higher head gradient between the stream and point bar aquifer thus increasing hyporheic exchange. Exchange then decreases to initial levels until steady state is re-established within several hours. Vertical temperature profiles in the aquifer during flooding show downwelling of surface water into the aquifer, consistent with a losing stream. On a longer time scale, pneumatic slug tests of the same piezometers conducted in the summers of 2008 and 2009 suggest that permeability has both increased and decreased in certain sections of the aquifer. Correspondingly, this has increased or decreased the magnitude of hyporheic exchange in different areas of the point bar. These findings demonstrate the transient nature of hyporheic exchange under field conditions and offer insight into the evolution of shallow alluvial aquifers. In particular, recognizing the temporal variability of permeability is relevant to stream re-meandering efforts, aquifers formed by avulsing streams and rivers, and groundwater-surface water models, which typically assume constant permeability though time.