This article analyses the ethno-politics of water in Argentina at the high point of European immigration, the first three decades of the twentieth century. Focusing on the drying of the Guanacache wetlands, located in the wine-producing region of Cuyo, we show how national and provincial ideologies based on ‘whitening’ and ‘civilisation’ shaped policies that favoured European immigrants at the expense of autochthonous populations in the geographic and social struggle for irrigation water. A large-scale redistribution of water resources drove the indigenisation of indigenous and criollo populations and the desertification of their land.