How Transferable are River Corridor Exchange Metrics Across Geologies and Scales and What are the Associated Uncertainties of Studying Representative Reaches?

Abstract:

The influence of ecological and biogeochemical processes occurring in river corridors on downstream ecosystems and water users are controlled, at least in part, by exchange fluxes and transit times. However, studies of exchange and transit times in these systems are often site-specific and only cover small portions of the entire river corridor. Advances in the field will require information on how to best (1) transfer findings from specific reaches to other sites, and (2) methods to quantify associated uncertainties in studying smaller reach lengths. In this study, we construct 2-D models of river corridor exchange for more than 30 field site surveys across multiple stream orders and three lithologic units in the H.J. Andrews Experimental Forest, simulating exchange flux and hyporheic transit time distributions as key exchange metrics. We first compare between-site variation, assessing how transit times vary across lithologic units, drainage area, and fluvial geomorphology. Then, we assess the relative uncertainty associated with studying smaller reach lengths to a larger stream segment by comparing transit time distributions and exchange flux using varying window sizes of 10 m, 20 m, 50 m, 100 m, 200 m and 20 wetted channel widths. Comparison of the smaller reaches to the larger segment can be used to recommend appropriate reach lengths to study and model river corridor exchange and how representative those studies are across multiple sites. This study helps advance our ability to make predictions in unstudied river corridors and provides an analysis of the uncertainty in those studies.