Concerns regarding the decreasing population of the American eel (reported by Castonguay et al. 1994; Haro et al. 2000) have led to design restrictions for hydropower facilities in the Eastern United States. However, the effects of these restrictions on eel passage and their impacts on power generation have not been fully researched. The goal of this study was to evaluate design parameters for bar racks that have potential to prevent entrainment of silver American eels, but also have minimal impacts on power generation. Hydraulic and biological assessments were used to determine the role of bar spacing, rack angle, and approach velocity on head loss across bar racks and the effects of bar spacing and approach velocity on eel bypass efficiency. The hydraulic assessments included computational fluid dynamics (CFD) analyses and laboratory experiments conducted in a re-circulatory flume at Alden Research Laboratory (Alden) in Holden MA. The flume allowed for determination of head losses across bar racks placed at angles of 45 and 90 degrees to the flow direction, with bar spacings of 0.75, 1.0 and 1.5 inches (19, 25 and 38 mm) and approach velocities of 1.5, 2.0 and 2.5 ft/sec (0.46, 0.61 and 0.76 m/s). Biological assessment, supported by funding from the Electric Power Research Institute (EPRI), used the same flume and included experiments with a 90 degree rack angle, bar spacings of 0.75 and 1.0 inches (19 and 25 mm), and approach velocities of 1.5 and 2.0 ft/sec (0.46 and 0.61 m/s). Bypass efficiencies, defined by the percentage of eels moving through the bypass, were evaluated for eels using three 2-hour replicate trials with nighttime releases of 30 eels per trial. Eel behavior in the vicinity of the racks was observed to the extent possible using a DIDSON acoustic camera. Experiments for the 90 degree configuration showed that the guidance efficiencies for the 0.75 inch (19 mm) spacing were greater than those for the 1.0 inch (25 mm) spacing, while the head losses for the 0.75 inch (19 mm) spacing exceeded the head losses for the 1.0 inch (25 mm) spacing by more than 10 percent. Linear regression analysis indicated that 53 percent of the variations in head width are explained by changes in the length of the eel. Results of the hydraulic evaluations were used to develop a new head loss equation that has a correlation coefficient of 98.6 percent. The results of the hydraulic and biological assessments provide a basis for quantifying the impacts of bar rack design on hydropower operation and downstream passage for American eels.