<p>This article is a transnational history of anthropogenic alterations to the St. Clair waterscape shared by the United States and Canada. Since the mid-nineteenth century, the three connected waters with the appellation “St. Clair” – river, delta, and lake – have been repeatedly engineered to facilitate deep-draft shipping as well as alter water levels in the Laurentian Great Lakes. After dredging and channel creation began in the 1840s, new canals were dug through the St. Clair Delta – the world’s largest freshwater delta. In the early twentieth century, there was a great deal more dredging and channelization, as well as mining of the riverbed for sand and gravel. Two major channelization projects took place in the 1930s and 1960s. Since then, maintenance dredging has continuously kept the shipping channels at sufficient depth. Dredged material was repeatedly formed into new infrastructure, such as islands, dikes, and confined disposal units. All this had severe consequences for the St. Clair’s aquatic ecology and lowered the levels of Lake Huron and Lake Michigan. At various times engineers and officials therefore sought to modify the St. Clair’s waters in ways that could influence not only navigation but also the levels of these two Great Lakes. To do so, experts relied on hydraulic scale models to plan control works such as submerged weirs or sills, and this study explores their approach to “disguised design”. My analysis of modelling and hydraulic processes reveals how the St. Clair was transformed into a hybrid envirotechnical system that blended the natural and the artificial.</p>