From the seventeenth to the twentieth century, rivers played a key role in the colonization of Hokkaido, a northern island in the Japanese archipelago. The Kushiro River, in eastern Hokkaido, was transformed into infrastructure, a process which shaped the institutions, strategies, and practices of territorial control during the transition from the Tokugawa shogunate (1603–1868) to Imperial Japan (1868–1947). Trade between local Ainu communities and the shogunate's vassals contributed to a river-based territoriality. Later in the 1800s, as the island became territory of the modern state, the river was further converted into infrastructure through settler colonialism, industrial development, land reclamation, and the dispossession of indigenous communities. This transformation empowered the state to probe territories, exert control over labor, and access natural resources. Drawing on research on the political ecology of rivers, this paper focuses on two hydrosocial functions that emerged during the process of reworking river basins into legible and governable spaces: the transportation conduit and the water delivery system. The river's transition from a living system to infrastructure coincided with and furthered the establishment of colonial settlements and the expansion of the Japanese state's imperial reach.