The geography of the former watermills on the River Thames demonstrates the state of contingency upon which the liquid materiality of landscape is founded. Watermills were introduced into England during the Roman occupation and came to have an important role in the socioecology of the Thames, and an important place in its medieval and early‐modern landscape. The pattern of mills established on the river by the eleventh century remained stable for some hundreds of years because of the locational constraints enforced by a framework of capital investment and legal rights. However, the built form of the mills caused conflict in the use of the river between milling and navigation, and this conflict was worsened by the growth of traffic in the early‐modern period. By the late eighteenth century the ownership and operation of mills was perceived a significant element in the crisis affecting the Thames. That crisis was only resolved in the nineteenth century with the marginalisation of milling in the socioecology of the river and therefore in its water landscape.