Water law is two different systems wearing the same name, and which one applies depends almost entirely on which side of the hundredth meridian the land sits on. This article explains riparian rights in the water-rich East, prior appropriation in the arid West, and the separate and largely state-controlled world of groundwater. It then turns to the federal overlay: the Clean Water Act's permitting regime for discharges and for filling wetlands, the decades-long fight over what counts as a "water of the United States," and the practical consequences of the Supreme Court's 2023 decision narrowing that definition. Separate parts cover conservation easements as both a preservation tool and a tax deduction, the drainage disputes that pit neighbor against neighbor, and the constitutional limits on what a permitting agency can demand in exchange for approval. Examples run throughout, and the recurring lesson is that water problems are almost always solved earlier and cheaper than owners expect.