Water is found everywhere on Earth and is one of the most basic and common substances. It is the only natural substance that exists in all three physical states: solid, liquid, and gas. While most water is found in the oceans there is constant exchange of water between the oceans, atmosphere, and land. These complex exchanges are described by the hydrologic cycle. This document focuses on precipitation and surface water flows to and in streams and rivers and the management of excess precipitation runoff to comply with the federal Clean Water Act (CWA).