Stream-aquifer interactions and hyporheic exchange in gaining and losing sinuous streams

The irregular planform morphology of rivers leads to formation of hyporheic zones along its banks. This study investigates how hyporheic exchange in stream banks, whose planform is idealized as sinusoidal, is affected by net gains from and net losses of water to the adjacent aquifer. These effects are studied via numerical modeling of groundwater flow adjacent to sinuous channels across a broad range of sinuosity and gain/loss magnitude. Hyporheic zone areas and fluxes both decrease exponentially with increasing magnitude of net gain or loss relative to the case where the stream has no net flux of water (neutral). Residence time through the hyporheic zone also decreases with gain/loss magnitude. The hyporheic zones become constrained near the apex of bends, indicating that these areas could be hot spots for mixing and biogeochemical processing. Hyporheic zones in channels with smaller sinuosity are more prone to hyporheic flux and area reduction while very sinuous channels are able to maintain a hyporheic zone even under largely losing or gaining conditions. Equations fitted to the suite of simulation results allow for prediction of hyporheic flux, area, and residence time on the basis of aquifer hydraulic conductivity, channel sinuosity, and the ratio of along-valley and across-valley mean head gradients.