Large numbers of dams for hydroelectric power production are currently planned or under construction in many areas around the world. While positive and negative social and environmental impacts of dams are increasingly well understood, little is known about attitudes of the general public towards dams, even though benefits to wider society are often cited to legitimise their construction. In Brazil’s Upper Paraguay River Basin, more than 100 mostly small-scale hydropower dams are planned or under construction in what can be considered a regional dam construction boom. Here we analyse public preferences for strategies to manage dam impacts in the area by investigating the value base that underpins such preferences, drawing on the recently proposed Value Landscapes Approach as our theoretical framework and data from a large representative household survey (N=1067). We find that contrasting attitudes towards dams, expressed in preferences for economically or ecologically oriented water policies are informed by opposing underlying value landscapes, that is, groups of closely related fundamental, governance-related, and assigned (water) values. While such tensions between opposing values can never be fully eliminated, our research nevertheless gives insights to policy-makers seeking to minimise value conflict and to improve the political legitimacy of public decision-making on dam construction. Moreover, we find that a majority of members of the general public would prefer concentrating dam construction on some rivers while keeping others free-flowing, with direct implications for ecosystems and inland fisheries. This finding may guide policy-makers wishing to develop publicly supported water resources management strategies.