This guide covers the three regimes governing where water goes: federal stormwater permitting, the national flood program, and the state common law of surface water. It develops the construction, municipal, and industrial stormwater permits in operational detail, including the plan, inspection, and recordkeeping obligations that generate most enforcement, and covers the jurisdictional contraction and expansion produced by recent decisions. It explains the flood program as a bargain between insurance availability and local floodplain regulation, works through map zones and base flood elevations, and details the map amendment and revision procedures that are the practical remedy for an owner facing a mandatory purchase requirement. It covers substantial improvement and substantial damage, the rules that turn a repair into an elevation project. It develops the three incompatible surface water rules and the reasonable use standard most States now apply, and closes with the takings framework for government-induced flooding, exaction analysis of drainage conditions, and dispute strategy.