Compliance with the Water Framework Directive (WFD) of the European Union requires water managers to establish environmental flow regimes (EFR) in rivers across the continent. Few water agencies have examined the economic and social welfare impacts of implementing environmental flow requirements. We present the approach used by the Catalan Water Agency to calculate an EFR and estimate the economic implications of its implementation in the Ter River (Catalonia, NE Spain), altered by weirs for hydro-electric production. We analyze various implementation strategies and their associated economic costs and benefits, concluding that the restoration of environmental flows in the Ter River has reasonable costs and is likely to be a socially desirable policy with economic benefits exceeding costs. This paper provides an example of how a water agency can generate policy-relevant information on the social welfare impacts of implementing environmental flow policies as mandated by the WFD.