Use of Hydraulic Modeling to Assess Passage Flow Connectivity for Salmon in Streams

The maintenance of hydrologic connectivity in river networks has become an important principle for guiding management and conservation planning for threatened salmon populations, yet our understanding of how fish movement is impaired by spatial and temporal variation in connectivity remains limited. In this study, a two-dimensional hydraulic modeling approach is introduced to evaluate flow connectivity in relation to passage requirements of adult steelhead trout (Oncorhynchus mykiss) in coastal California streams. High-resolution topographic data of stream reaches with distinct channel morphology were collected by terrestrial LiDAR surveys and linked with water surface measurements to calibrate hydraulic model simulations. Quantitative metrics of longitudinal flow connectivity were then developed to assess fish passage suitability in relation to stream discharge. The specific objectives of the study were to (i) test the applicability of a 2D hydrodynamic model for predicting water depths in small, salmon-bearing streams, (ii) demonstrate the use of model simulation results to quantify passage flow connectivity, considering both spatial and temporal variation in hydraulic conditions, and (iii) compare the outcomes and assumptions of alternative methods commonly employed in salmon passage flow assessments.

The flow model reproduced observed spatial patterns in depth and velocity and differences between modeled and measured values were within the typical range of error reported for 2D models. The hydraulic model simulations for three study reaches indicated that minimum discharges of 0.9, 1.1, and 1.3 m³/s were required to maintain complete connectivity of suitable passage depths throughout the stream channel. Measured flow data from the 2008-09 winter season and simulated long-term records indicated that suitable passage flows occur with relatively low frequency and duration at all sites, suggesting that environmental flow protections for fish passage are warranted. A regional formula used by the State of California provided conservative estimates of passage flow requirements, while an approach based on riffle crest water depths substantially underestimated minimum flow needs for adult steelhead trout. The hydraulic modeling approach employed in this study may be particularly useful for testing the assumptions behind alternative environmental-flow assessment methods and for evaluating habitat-flow relationships in stream reaches of importance, such as critical habitat for threatened species.