A meander-scale field investigation of linkages between fluvial geomorphology and patterns in hyporheic exchange

Although physically based numerical models suggest that lateral hyporheic exchange is greatly affected by channel and floodplain geomorphology, many models regularly simplify fluvial conditions, including flood plain sedimentology. Simplifications are rooted in a paucity of available hydrogeomorphic data and of field observations at scales large enough to capture effects of buried intra-meander geomorphic structures, such as former channels or point bars, on hyporheic exchange. To better represent fluvial geomorphology in hyporheic models, we explore how the interaction between hydraulic geometry and floodplain sedimentology affects preferential flow paths of hyporheic water. The study site includes a 150-m long, sinuous reach of the East River, near Crested Butte, Colorado. Subsurface intra-meander geomorphic structures, such as buried alternate bars and channels are identified using ground-penetrating radar (GPR) and are corroborated by point sediment descriptions collected during piezometer installment. The hydraulic geometry of the river is constructed by field measurements of stream bathymetry, discharge, and slope. Hyporheic residence times and flow paths are measured using Rhodamine Water Tracer (RWT) tests, and breakthrough curves are constructed in-stream as well as intra-meander by sampling an established piezometer network. This combined approach allows for the identification of major controls that subsurface deposits may exert on hyporheic exchange and more efficient integration of subsurface heterogeneities in hyporheic models.