REAL RIVERS HAVE CURVES: A FIELD INVESTIGATION QUANTIFYING RIVER SINUOUSITY’S EFFECT ON HYPORHEIC EXCHANGE

Recent river-network scale models suggest that geomorphic controls on hydraulic conditions and scaling effects of streambed area and stream-order densities result in similar biogeochemical potential across stream orders and argue for comparable importance of hyporheic exchange in larger rivers in addition to smaller streams. However, field studies examining hyporheic exchange have almost exclusively focused on straight, narrow headwater streams. Although physically based numerical models suggest that lateral hyporheic exchange is greatly affected by channel planimetry, sinuosity’s significance has been inherently overlooked in field studies and subsequent models. Conflicting conclusions regarding sinuosity’s role in hyporheic exchange suggest the need for field observations of hyporheic exchange in a natural, sinuous stream. Here, we explore: 1) the quantification of stream sinuosity’s effect on lateral hyporheic exchange, which has been modeled to be greatest at meander necks; and 2) the presence or absence of increased vertical hyporheic exchange along the outside of meander bends, caused by super-elevation of the water surface. Observations are made through a comparative field study that combines tracer tests, surface- and groundwater discharge estimates, and morphological data (bathymetry, bedform mapping, bed sediment size) at two stream reaches with large sinuosity differences. The field work was conducted along the East River, near Crested Butte, Colorado which offers a fluvial setting representative of Rocky Mountain watersheds feeding the Colorado River. We explore a sub-kilometer stretch of channel containing two stream reaches with contrasting sinuosities, but very similar bedforms, streambed sediments, discharge, floodplain characteristics and vegetation. The two reaches’ proximity and the integrated field approach minimizes uncertainties related to other controls known to affect hyporheic exchange, namely surface and groundwater discharge, hydraulic conductivity, and bedforms, which are largely similar between reaches. This study offers the first set of field observations focused on sinuosity’s impact on lateral and vertical hyporheic exchange in a natural environment will better facilitate the incorporation of river planimetry into catchment-scale models.