Abstract: From the High Middle Ages on, the coastal wetlands of the North Sea area have been intensively reclaimed and settled. In order to enable intensive agricultural production in these areas, a complex drainage and flood control system was gradually installed, one that demanded a permanent investment of huge amounts of capital and labour. As the maintenance of the water control system was vital for the coastal agro-system, the long-term evolution of investments is an important, yet rarely used, indicator of the economic, social and environmental fortunes of the coastlands. Based on new and very early serial data on water control funding in late medieval Flanders, this article argues that long-term fluctuations in the funding of drainage and flood control were first and foremost related to structural changes within the coastal economy, where an overall decline of investment levels ran parallel to the fourteenth-century crisis of the peasant smallholding economy in this region. Exogenous pressures on the other hand, such as storm surges, only provoked a short-term disruption of investments.