In 1730, the Dutch cartographer and meteorological observer Nicolaas Samuel Cruquius constructed a spectacular map of the river Merwede. Cruquius’s map is celebrated as one of the earliest to use lines of equal depth—or indeed any type of contour lines. So far, however, the secondary literature has paid no attention to why Cruquius created these lines or to the knowledge involved in his innovation. This essay makes three related points. First, Cruquius intentionally used lines representing equal depth in an entirely (...) new way: whereas earlier maps had used similar lines to represent a river’s navigability, for Cruquius they visualized river flow. Second, different technical and epistemic practices grounded the making of the map: Cruquius built on his expertise in surveying but also on practices from both natural history and mathematical physics, with which he was familiar through contacts at Leiden University and the Royal Society of London. Third, Cruquius’s case does not fit with the way work cutting across different fields of expertise is usually conceptualized. Cruquius did not just straddle different fields. Instead, he gave new meaning to lines of equal depth by integrating specific practices from different fields of expertise and omitting others; through this integration, Cruquius created a way to visualize water flows and gave shape to possible new ways of acting on the river and its environment. (shrink)