Water that falls on land has to go somewhere, and three separate bodies of law govern where. This article explains the federal stormwater permit program that reaches construction sites, municipalities, and industry, and why the permit is a paperwork obligation that generates real enforcement. It covers the flood program — how maps are made, what a base flood elevation means, why the mandatory purchase requirement is what makes maps matter to ordinary people, and how to get a property removed from a mapped floodplain. It then explains the neighbor law of surface water, where three incompatible common law rules produce opposite results on identical facts, and why most States have converged on a reasonableness standard that decides nothing in advance. It closes with the takings questions raised when a government project floods private land, and the practical reality that drainage disputes are engineering disputes with a legal caption.