The article proposes an approach from the legal geography to understand the social construction of water markets in the semi-arid basin of the Elqui river, from the second half of the 20th century. Using qualitative methodologies (ethnography, interviews, archi-ves), it reveals the jointly spatial and legal dimension of the formation of water markets. The technical water objects installed since the second half of the 20th century attract new investors and transform the political economy of the valley. Since then, the users have used technical water objects and have instrumentalized or produced law locally to buy, sell or rent water rights, forming five water markets.