Evaluating the effectiveness of a step-pool rock ramp fishway for Yellowstone cutthroat trout passage twelve years after construction

Abstract

Longitudinal connectivity is essential for the persistence of migratory freshwater fish species. However, barriers to fish passage, such as dams and culverts, disrupt river connectivity, leading to declines in species populations by obstructing access to vital spawning habitats. Fish passage structures play a critical role in mitigating these barriers by facilitating conditions conducive to the passage of target species. This study evaluates the effectiveness of an existing step-pool nature-like fishway structure for the passage of Yellowstone Cutthroat Trout by comparing hydraulic models using the design plans with those that use current GPS survey data. This comparison can be thought of as an assessment over time - the design plans are an estimate of the historic conditions, and the surveyed conditions represent the current case approximately twelve years later. Utilizing 2D HEC-RAS version 6.3.1 for hydraulic modeling, the models indicate that both surfaces were likely effective fish passage structures, providing suitable depths and velocities for the target species across various flow rates. However, analysis revealed degradation in the step-pool weir structures over time, likely due to factors such as hydrologic events, sedimentation, and potential ice effects. These findings have significant implications for future design considerations and provide valuable insights for the assessment of newly designed fishway structures in field applications.