Environmental mitigation represents an important, recurring cost to the hydropower industry, the largest renewable power source in the United States. Environmental flows are one such expense whereby hydro operations maintain a minimum flow in the river to mitigate impacts on aquatic and riparian ecosystems. Any hydroelectric facility may have a habitat maintenance flow requirement, but facilities with assisted aquatic organism passage structures, or fishways, may be subject to additional flow requirements associated with specific species migrations. This study assesses the economic impact of meeting environmental flow requirements in terms of losses to power generation at a representative hydroelectric facility and fish lift in the Northeast.