EFFECTS OF RESERVOIR OPERATIONS ON CHANNEL MORPHOLOGY IN BROWNS PARK

Despite a two-fold increase in the mean annual flood released from Flaming Gorge Dam, widespread bank erosion has not been observed along the downstream Green River. Increased flood magnitude since 2012 was implemented for environmental management purposes, and there has been vocal landowner concern about the geomorphic impact of these floods, especially on bank erosion of farm fields. In this study, we quantify downstream channel adjustment on the Green River in response to the closure of Flaming Gorge Dam and subsequent dam reoperations intended to recover endemic endangered fish in the system. We used repeat aerial images to calculate planform channel adjustment following Flaming Gorge Dam closure and for different periods of dam operations. Our air photo analysis uses a sophisticated three-fold uncertainty quantification method, combining co-registration error with interpretation and digitization uncertainty, resulting in a probabilistic range of channel change. Preliminary results quantifying channel change along a 25 km section of Browns Park show an overall trend of channel narrowing since dam closure, but the narrowing rate has varied during different periods of dam operations. Immediately after dam closure, operations were intended to maximize hydropower, and the channel narrowed at a rate of 0.52 (±0.87) meters/year. Since the 2000s, dam operations have been modified to target critical habitats for endangered fish. These modifications have included reintroducing a small annual flood release and base flows targeted for nursery habitat. We found that narrowing increased to a rate of 1.67 (±0.57) meters/year and propose lower summer base flows may have caused more channel narrowing. More recently, there has been a 2-fold increase in the annual flood release, raising concerns about widespread bank erosion by local landowners. However, our analysis indicated that larger annual floods had not caused widespread bank erosion and that channel narrowing continues. Narrowing rates under these recent environmental flows have decreased to 0.57 (±0.51) meters/year. These results show how reservoir operations can influence the trajectory of the channel response, and such knowledge can help balance the biological and anthropogenic uses of the Green River.