MONITORING THE EFFECTS OF RIVER REALIGNMENT ON THE UPPER COLORADO RIVER, ROCKY MOUNTAIN NATIONAL PARK

A 2003 debris flow introduced 36,000 m³ of sediment into a high-elevation wetland on the Upper Colorado River in Rocky Mountain National Park (RMNP). As the first step in a larger restoration plan, RMNP staff realigned a ~100 m reach of the Colorado River into its historic thalweg in September 2015. Pre- and post-restoration measurements are compared to monitor the hydro-geomorphic response of the realignment within the channel-wetland system. Flow is bifurcated at the head of the wetland, and the channel realignment redistributed 60-70% of discharge from the western to eastern channels, which has implications for surface water-groundwater exchange. We paired a salt tracer test with electrical resistivity (ER) imaging and surface conductivity measurements in a novel, non-invasive manner to monitor changes in near-channel hyporheic exchange throughout the 500 m long wetland. A sodium chloride tracer was injected during base flow and ER was continuously monitored across the realigned channel for 24 hours following the injection. Surface conductivity was measured during the same period to calculate a mass balance throughout the system. Pre-alignment ER results highlight near-surface groundwater exchanges adjacent to the eastern channel. We expect repeat ER will further highlight changes in hyporheic exchange pathways due to channel flow redistribution. Surface conductivity breakthrough curves indicate a successful salt tracer test through the wetland, with 68% mass recovery at the wetland outlet prior to channel realignment. We expect an increase in mass recovery following channel realignment, indicating faster transport through the wetland and a decrease in net hyporheic exchange. Results from this research will inform large-scale restoration planning scheduled for 2017. This research also fills a knowledge gap in the literature, where quantitative evaluations of restoration outcomes are lacking.