This article deals with the work of the European Commission of the Danube (ECD) during the first two decades of its activity in the aftermath of the Crimean War. It focuses on the early stage formation of international organizations in the mid-nineteenth century when river commissions were the first organizations that issued supranational regulations and had their own bureaucracies. In this context, I argue that the ECD became a testing ground for new types of inter-imperial cooperation. First, the ECD became a site where hydraulic expertise from all over Europe was gathered and analysed. As a consequence, this exchange among the representatives of different empires and of different sub-fields of expertise generated new technical knowledge and made the ECD a space for cross-imperial knowledge production. Second, in 1865, the ECD adopted a Public Act that codified navigation rules in the Danube Delta. These regulations were among the first upholding a supranational settlement. Furthermore, the document exemplifies how such a supranational agreement was implemented through a joint imperial intervention against the authority of the Ottoman Empire, the only territorial power.