This paper retraces the history of flood risk infrastructure projects (1850–1980) in the Grésivaudan Valley, located immediately upstream of Grenoble (France). It analyses the persistent gap between the modernist paradigm embedded in flood protection projects and the concrete hydraulic infrastructure built along the river and at confluences, questioning their unexpected effects. In this article, we demonstrate that in spite of their apparent fixity, flood protection infrastructures are constantly reshaped within hydrosocial territories. To support this argument, we analyse socio-material fracturing arising from the implementation of flood infrastructure projects. Four autonomisation processes that produce these socio-material fractures are studied: a) a competition between imaginaries at stake; b) a situation of legal pluralism denied by the State; c) an agency of sediments; and d) a conflict between the everyday practices of beneficiaries and planned practices. By reconstructing these processes, we open the black box of the hydrosocial construction and materialisation of hydraulic infrastructure, and contribute to the development of the concept of socio-material fractures.