During the twentieth century, new realms of expertise arose alongside the state’s legal mandate to find, measure, and allocate water rights. Abstract water knowledge embodied by legal, hydrological, engineering, and historical experts were produced as a consequence of redefining the human-water relationship. This legion of new water experts was centrally focused on materializing the new private property rights embedded in water as a substance. The costs of this process were enormous, as shown by the ethnographic evidence presented. The consequences are less clear. The new forms of water knowledge and expertise are now vital to state and federal agencies, as they are to local water users in the courts.