Hydropower tailrace channels are unique and attractive locations for hydrokinetic energy harvesting due to fast currents, scheduled flow releases, proximity to existing structural and electrical infrastructures, and low risk of additional environmental impacts. However, energy-extracting devices create flow resistance, inducing a small but measurable water level increase which may diminish the available hydraulic head and reduce hydropower generation, defeating the initial value proposition. This study combines a one-dimensional momentum balance approach with the backwater equation for surface-varying open channel flow to analyze the water level increase and determine the optimal turbine siting distance that maximizes the net power production (balancing hydropower loss vs. hydrokinetic gain), as a function of the channel hydraulic conditions and the hydrokinetic turbine characteristics. Finally, using a subset of sites from the U.S. hydropower fleet, we provide a high-level estimation of the hydrokinetic potential available in tailraces in the United States and discuss two case studies. This work advocates for the adoption of hydrokinetic turbines downstream of dams as an opportunity to increase energy production at existing plants and Non-Powered Dams (NPDs) with minimal structural intervention, and, alternatively, as viable sites for large-scale field testing for hydrokinetic devices.