HYPORHEIC EXCHANGE CONTROLLED BY DYNAMIC STREAM AND HILLSLOPE FLUCTUATIONS (Invited Presentation)

The relative roles of dynamic hydrologic forcing and geomorphology as controls on the timescales and magnitudes of lateral and vertical exchange along the stream-hyporheic-aquifer-hillslope continuum are unknown, but required for management of stream corridors. In this talk, we demonstrate the influence of dynamic hydrologic forcing, driven by diel fluctuations, on lateral exchange independent of geomorphic controls. We present a series of simulations of unsteady groundwater flow through a straight, planar aquifer under time-varying stream and hillslope boundary conditions. We developed a comprehensive framework relating the magnitude and timing of diel hydrologic fluctuations and valley slope to hyporheic exchange flux, flowpath geometry, and residence time. We found that diel fluctuations alone can produce hyporheic flowpath lengths and residence times that span orders of magnitude. With these results, hyporheic flowpath residence times and lengths can be predicted from the timing and magnitude of diel fluctuations and valley slope, which could be leveraged to achieve beneficial outcomes in regulated rivers. Finally, we show that diel fluctuations can produce spatial and temporal scales of hyporheic flowpaths equivalent to those driven by many well-studied geomorphic features. This comparison indicates that dynamic hydrologic and geomorphic controls must be considered together in future efforts of transferring reach-scale findings to network-scale predictions.