This paper deals with the geoenvironmental politics of early-modern Venice as a case study of geological agency that enlightens the entanglements of geo-history and human history. It focuses on a controversy that was sparked by Galileo’s pupil Benedetto Castelli, as he claimed that his mathematical treatment of running waters could solve all of the most urgent problems linked to the management of the Lagoon of Venice. From an epistemological viewpoint, the controversy is relevant as a case of clashing ‘styles of thought’, as it constituted a disciplinary conflict that pitted Galileian physico-mathematical abstraction (which resulted from the isolation of a set of quantifiable data) against ‘geological’ concreteness (a form of comprehensive knowledge which aimed to cope with systemic complexity). Castelli was not able to convince the Venetian authorities that his method could solve the main problems relative to the conservation of the lagoon at a time when its depth and navigability were worryingly diminishing. While the Venetian authorities invested in diverting rivers away from the lagoon to reduce sediment supply, Castelli argued, to the contrary, that it was precisely the diversion of the rivers that caused shoaling because of the loss of the great quantity of water discharged by the rivers, which he accurately calculated. His analytical approach was dismissive of the comprehensive knowledge and complex methods that Venetian water experts and engineers had developed towards a systemic understanding of the hydrogeology and the environment of the lagoon with the active involvement of citizens and fishermen in the assessment of the state of the waters.