Abstract: The Alden turbine was developed through industry-U.S. Department of Energy collaborative funding to provide safe turbine passage for fish at hydropower projects. The turbine was designed to minimize the probability and consequence of blade strike and keep pressure changes and shear forces within limits tolerable by fish. Suitability for retrofit was not a design criterion, and application of the Alden fish friendly turbine has historically focused on new powerhouse development in order to provide flexibility in arranging the larger Alden unit. After evaluating typical Francis and Kaplan turbine geometries and completing a basic Alden turbine retrofit study, it was found that matching the pin circle diameter of the Alden turbine distributor to that of the existing turbine will lead to minimal physical reconstruction but a relatively low power output. Increasing the size of the Alden turbine will provide an improvement to the power output but also increase the needed physical reconstruction between the existing and proposed systems. An evaluation of energy generation was completed for a hypothetical, site-specific retrofit. Energy generation given the existing Alden unit design without a fish bypass flow was 1.6% less than the base case including a fish bypass flow. A re-designed Alden unit more suitable for the site head was predicted to yield a generation loss of only 0.7%. Improvements to fish passage survival with the Alden turbine retrofit are length dependent, yielding a predicted 23% improvement (i.e., an increase from 79% to 97% predicted survival) for 300mm fish.