Implementation strategies and a cost/benefit comparison for compliance with an environmental flow regime in a Mediterranean river affected by hydropower Mònica Bardina a, *, Jordi Honey-Rosés b and Antoni Munné a a Agència Catalana de l’Aigua, C/Provença 204, 08036 Barcelona, Spain *Corresponding author. E-mail: mbardinam@gencat.cat b School of Community and Regional Planning, University of British Columbia, 1933 West Mall, Vancouver BC, Canada V6T 1Z2 Abstract 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. Keywords: Environmental flows; Hydropower; Water Framework Directive; Willingness to pay 1. Introduction The Water Framework Directive (WFD) (2000/60/CE) inaugurated a new legislative context for European water managers (European Commission, 2000; Hering et al., 2010). This legal framework explicitly requires European Union (EU) Member States to prioritize the recovery of water bodies, while balancing human needs (Kanakoudis & Tsitsifli, 2010). To reach these goals, the WFD requires that Member States identify and implement measures that will restore water bodies to a ‘good ecological Water Policy 18 (2016) 197–216 doi: 10.2166/wp.2015.169 © IWA Publishing 2016