Designing and assessing the effectiveness of Denil fishways using hydraulic modeling-based approaches

Abstract

Man-made, instream structures can pose barriers to fish movement. Fish move about river systems to reach habitats associated with various stages of their life histories. If access to required habitat is blocked, it can cause detrimental effects to fish populations. Removing barriers to fish movement is often socio-economically infeasible so fishways are used to promote fish passage around barriers. Denil fishways consist of a chute for water to flow through and baffles to slow water velocities; they are a relatively cheap solution for promoting upstream fish passage over low-head barriers. The Big Hole River basin is home to the last fluvial population of Arctic Grayling in the continental United States. Per an agreement between landowners and several government organizations, Denil fishways were installed at irrigation diversions in the Big Hole Valley to provide fish volitional routes to navigate irrigation diversions. Eleven Denil fishways at irrigation diversions were evaluated for their effectiveness at passing grayling by using hydraulic modeling techniques coupled with biologic data. Hydrologic data was applied to hydraulic models to estimate water surface elevations about the Denils over time. A passage index was developed which inferred passage efficiency of the fishway based on depths at the upstream and downstream ends and assigning a 'passage condition.' Passage windows were developed which describe times when the fishways functioned to 'allow', 'limit', or 'prevent' upstream passage. Across all sites fishways were predicted to 'allow' passage 6.4% of the time, 'limit' passage 17.2% of the time, and 'prevent' passage 10.3% of the time. The modelled depth combination at fishways was 'out of range' of the passage index 66.1% of the time. A hydraulic design process was proposed with the goal of designing Denil fishways at pin-and-plank irrigation diversions to promote upstream passage at low flows. Design criteria were established, explained, and presented. One-dimensional hydraulic modeling techniques for diversions and fishways was presented and used to determine design parameter values that optimize fish passage efficiency over a broad range of instream flows. We attempted to develop a novel method of assessing Denil structures using hydraulic models; our method is useful to managers because the effectiveness of fishways was assessed by considering how they functioned over a range of instream flows and at times associated with fish movement.