Relative influences of the river channel, floodplain surface, and alluvial aquifer on simulated hydrologic residence time in a montane river floodplain

Abstract

Current methods for characterizing the influence of geomorphic structure on river processes are not well suited for study of large rivers with extensive hydrologic exchanges between the channel, floodplain surface, and alluvial aquifer. Here we applied a spatially explicit, three-dimensional hydrologic model to simulate surface and subsurface flow within the 16-km2 Nyack Floodplain on the Middle Fork Flathead River in northwest Montana, USA. We ran the model for four years and simulated nine conservative particle releases across a range of river discharges. Our objective was to evaluate the interactions of hydrologic dynamics and floodplain structure in determining the residence time of water in the system. Particle tracking simulations revealed when river discharge was below bankfull, mean residence time of surface water decreased with river discharge. During overbank flows, however, mean residence time increased with discharge because of surface water storage on the floodplain. When surface and subsurface water were considered, mean hydrologic residence time was more than an order of magnitude greater than the residence time of surface water alone because of storage in the alluvial aquifer. Residence time also decreased exponentially with river discharge, because a smaller percentage of surface water entered the subsurface at high discharges. Our analyses illustrate the importance of considering in-channel flow, floodwater storage, and surface-subsurface exchange in determining the overall residence time of water within a river segment, especially in floodplain environments.