CHANNEL REALIGNMENT IMPACTS ON FLOW AND AEROBIC ACTIVITY IN THE HYPORHEIC ZONE

Nationwide, over a billion dollars is spent annually on stream restoration projects (Bernhardt et al., 2005) for a variety of reasons including erosion reduction, nutrient retention, and habitat creation, particularly for charismatic fish species such as trout. A commonly employed method of restoration is channel realignment. However, few studies have been conducted to understand the impacts of channel realignment on basic functional processes like hyporheic exchange (Hester and Gooseff, 2010; Becker et al., 2013). This study seeks to fill that gap through field measurements of hyporheic exchange in channel reaches that have and have not undergone channel realignment using the reactive resazurin-resorufin tracer system on four streams in northeastern Minnesota. Previous studies on the impact of channel realignment used conservative tracers and 1D transient storage models. They interpreted faster exchange rates between the main channel and transient storage in realigned reaches to be the result of reduced exchange with the hyporheic zone (Mason et al., 2012; Becker et al., 2013; Dallow, 2020). However, conservative tracers and 1D transient storage models do not differentiate between surface storage and subsurface storage because there is no information specifically gathered to indicate that water has entered the subsurface (Mason et al., 2012; Becker et al., 2013). This is why this study will utilize the reactive resazurin-resorufin tracer system. Resazurin is a weakly fluorescent dye that converts to the strongly fluorescent compound resorufin in the presence of aerobic respiration by the removal of an oxygen (Haggerty et al., 2008). It has been shown that the conversion mostly occurs in stream bed sediments, typically the top 5 cm, and not the water column (Haggerty et al., 2008; Knapp et al., 2017). Thus, water carrying resorufin is labeled as having exchanged with the hyporheic zone. Using 1D transient storage models that include this labeled water, this study will compare the differences in the hyporheic exchange parameters between the realigned and non-realigned reaches. Based on the previous research with conservative tracers and modelling studies (Pryshlak et al., 2015), it is expected that hyporheic exchange will be reduced in the restored reaches.