| The hyporheic zone is a saturated band of sediment that surrounds the river and forms a linkage between the river and the aquifer. It is a rich ecotone where benthic, hyporheic, and groundwater species temporarily or permanently reside. The exchange between surface and subsurface water, known as hyporheic exchange, is a crucial mechanism for delivering nutrients, oxygen and other solutes to the sediment, and for washing away waste products.; Hyporheic exchange has been extensively studied in sand-bed channels having two-dimensional dune-ripple morphologies, but few studies have examined hyporheic flow in pool-riffle channels, which are characterized by coarser sediment, steeper slopes, and three-dimensional bed forms that strongly influence surface flow. These channels are particularly important habitat for salmonids, which incubate their offspring within the hyporheic zone.; Hyporheic exchange in gravel pool-riffle channels was assessed with a pumping transport model that accounts for spatial variations in total near-bed pressure resulting from flow over three-dimensional bedforms. A series of laboratory experiments were conducted to test the model across a range of discharges and bedform amplitudes. Results show good agreement between predicted and observed hyporheic exchange, indicating that the major mechanism for exchange in pool-riffle channels is bedform-induced advection.; The model was used to evaluate the relative importance of some key factors in hyporheic flow: river discharge, depth of alluvium, bedform amplitude, hydraulic conductivity of the sediment and mean groundwater flow velocity. Results demonstrate that sediment permeability, bedform amplitude, and flow regime are important factors, while depth of alluvium is significant when it is less than the hyporheic depth.; Finally, the model was used to examine the effect of redd formation on hyporheic processes. When salmon build their redds (nests) they alter sediment size and conductivity, and create macro-scale bed topography. Simulations comparing hyporheic exchange in a pool-riffle channel with and without redds show that redd formation changes not only the river hydraulics, but also the local areas of upwelling and downwelling, enhancing the flow velocity and dissolved oxygen content through the egg pocket and, thereby, potentially enhancing the survival of salmonids incubating within the streambed gravels. |
