The role of water management and river morphology on stranding pool formation

Abstract

Fish can be stranded in pools during flow recessions that may occur naturally or as a result of dam operations. Past studies have recommended dam operation guidelines that only consider water surface elevation (WSE) recession rate to reduce fish stranding potential. Here, we holistically consider the role of flow, WSE recession rate, and river-floodplain topography under unregulated and regulated flow regimes to predict the potential of stranding pool formation. The proposed methodology leverages advances in two-dimensional (2D) surface hydraulic (HD) modeling and high-resolution topobathymetric LiDAR surveys. We applied this method to the South Fork Boise River (SFBR), Idaho, USA. We developed a 2D HD model supported by meter-resolution topobathymetric data to simulate channel and floodplain hydraulics and stranding pool formation for different flow scenarios.Comparison between modeled and fieldsurveyed stranding pool locations shows good match, supporting the use of this method to predict stranding pool potential. Results show that potential for stranding pool formation (number of pools and their surface area) increases with flow for the same flow recession rate. Contrary to expectations, our model predicts that unregulated flows have a considerably higher number of potential fish stranding pools than for regulated flows in the SFBR. The reach-average WSE recession rates were higher for unregulated (0.07 to 0.09 cm/h) than for regulated (0.02 to 0.04 cm/h) flow based on daily mean flows. Our study has shown the importance of river and floodplain morphology and hydraulics to predict fish stranding pool locations and analyze impacts of flow regulations on fish stranding. This approach may provide helpful information to identify potential fish stranding pools and mitigation and to manage for fish stranding issues in regulated and unregulated river systems.