Mechanistic modeling of dynamic hyporheic exchange in diverse hydromorphic settings of a large river

Large, long-term river stage fluctuations caused by snowmelt runoff and short-term fluctuations induced by upstream flow regulation at the Priest Rapids Dam drive dynamic water exchange between the river and the hyporheic zones at the Hanford Reach of the Columbia River in the northwestern United States. High-resolution three-dimensional subsurface flow and transport simulations in diverse hydromorphic settings are used to study the hyporheic exchange flows and residence time distributions. The model covers 51 square kilometers of a selected river corridor with large sinuosity along the Reach and considers subsurface and riverbed heterogeneity. Spatially distributed river stage and groundwater level data are used to drive the model at hourly resolution. Results show that normalized to river discharge, both the size of the hyporheic zone (defined as the domain containing > 98% of river water) and mean residence time in each hydromorphic unit (HU) follow exponential functions of river discharge with distinct coefficients. Size fraction of hyporheic zone in each HU linearly changes with streamflow, e.g., linearly decreases with streamflow in riffles and increases in bars on the riverbed. Riverbed heterogeneity changes this size fraction, but not significantly in the simulated settings. These results show promise for development of information needed for reduced-order modeling at larger spatial scales, such as parameters for multirate mass exchange models to represent residence timescales in rivers at reach to watershed scales and to guide the selection of locations for field studies.