GEOMORPHIC AND SEDIMENTOLOGICAL CONTROLS ON HYPORHEIC FLOW IN AN ALPINE RIVER

The hyporheic zone of rivers, where surface water temporally enters the sediments and may mix with groundwater, can impact surface water quality, and vary with physical parameters such as bed geomorphology and subsurface geology. Complex streambed morphology and sediment characteristics in meandering rivers lead to heterogeneity in hyporheic exchange. To assess and quantify physical controls on hyporheic exchange in a meandering alpine river system, sediment samples were collected for grain size analysis in East River, Colorado (USA). Additionally, vertical hydraulic head gradient measurements were taken at eighty locations along a 200 m river meander. A ground-penetrating radar system was also used for subbottom imaging of the river channel to further understand how streambed morphology influences subsurface geology and fluid flow. We hypothesize that grain size and permeability of the riverbed control vertical fluid fluxes in the bed over smaller scales (e.g. grain to bedforms), while the meander morphology controls larger scale patterns over tens of meters. Preliminary observations show that a region of focused groundwater discharge occurs in thick, permeable alluvium on the downstream edge of the point bar. These results will improve understanding of the interplay between hyporheic flow and streambed sediment properties, further influencing porewater chemistry and overall water quality.