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Hyporheic Exchange Flow (HEF) is driven by head gradients defined by the hydro-topography of the stream surface, which is controlled by the particular geomorphology of the stream. We quantified the impact of large wood, which typically exerts a strong control on stream morphology, on HEF in a second order, low gradient (~1%), coastal Alaskan stream. Experimental removal of large wood (> 10 cm diameter) from Bambi Creek, a second order stream on Chichagof Island, AK demonstrated that the wood-forced pool-riffle morphology transitions to that of alternating pool-riffle sequences, and led to an increase in sediment stored in the reach (Smith et al., 1993). We constructed a series of five groundwater flow models (MODFLOW) spanning 16 years to examine the response of HEF, and we used particle tracking (MODPATH) to examine changes to the flow paths and residence times. Changes in the pattern of hyporheic upwelling and downwelling generally correspond to changes in the local hydro-topography over the five study periods and generally corresponded to locations of large wood removal. Reach total hyporheic exchange decreased from 0.96 to 0.87 L/s during loss of sediment storage, then increased to 1.22 L/s and again to 1.90 L/s for periods when sediment storage increased in the reach. The turnover length of hyporheic exchange for baseflow conditions decreased from 4.5 km to 2.3 km. The area of the streambed contributing to hyporheic flux increased over the study period, as well as the amount of hyporheic flow across meander bends. The median residence time of the hyporheic flowpaths increased from 8 to 11 hours by 4-years post removal then again to 14 hours by 16-years post-removal. Changes in the residence time probability and cumulative density functions for the models show an increase in late time portion of these functions at 4 years and 16 years post removal. The change in channel morphology from wood-forced pool-riffle to that of alternating pool-riffle sequences apparently increased the amount of hyporheic exchange. |
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