Abstract: Impermeable guidance structures (or guide walls) are used to improve passage efficiency of out-migrating anadromous fish species. Their purpose is to guide the fish to a bypass (i.e. a sluice gate, weir, or pipe) allowing the fish to circumvent the turbine intakes and safely pass downstream. This paper details a series of experiments (nine total) performed at the USGS Conte Anadromous Fish Research Center located in Turners Falls, MA and a series of computational fluid dynamics models (24 total). The goal of the analysis was to measure the velocity components (x, y, and z) immediately upstream of the guide wall under a wide range of guide wall depths and angles to flow for a specified flow rate typical of a power canal. Multiple metrics are developed that attempt to determine whether or not a fish will 1) encounter swim speeds greater than that of the fish is capable, 2) follow a negative rheotactic behavior and swim below the guide wall, and/or 3) be entrained by the downward flow and pass below the guide wall. The presentation will discuss the history of guide walls in fish passage, introduce both the physical and computational model set-ups, and present the findings of the study.