Effects of small sediment barrier removal on geomorphic complexity and habitat diversity

Dam removals can remobilize large volumes of sediment into channels, and the effects of these sediment pulses on channels downstream remain poorly understood. With increasing frequency of barrier removal, it is critical to understand what impacts the barrier removal may have on ecological integrity. In this study, we examine how a small barrier removal on the Calapooia River (Brownsville, OR) affects ecological integrity using geomorphic complexity as a proxy for habitat diversity. These affects are characterized through changes to sediment distributions, bedform morphology and organization, and hydraulic structure. Six geomorphic complexity metrics are developed to describe these changes, and inasmuch as change to each metric reflects a specific increase or decrease in geomorphic complexity, we evaluate the benefit or hindrance of small dam removal to ecological integrity. We explore the hypothesis that, after an initial homogenization of the downstream bed, the liberation of reservoir sediments improves geomorphic complexity and habitat diversity both upstream and downstream of the forming barrier by increasing local scour and deposition as natural bar-pool morphology is restored. In addition, we applied DREAM-2, a 1-dimensional sediment transport model specifically developed for cases of dam removal, to these data to generate expectations of channel changes. The model was evaluated as a predictive tool through analytical methods comparing model outputs with field observations. Preliminary results suggest that, following a small dam removal, geomorphic complexity and habitat diversity increase, primarily downstream and as soon as one year post-removal, and the magnitude of these changes decreases with increasing distance from the former barrier.