A hydrologic unit code is a sequence of numbers or letters that identify a hydrological feature like a river, stream, snow covered area, or a watershed (also called a drainage basin). The USGS maintains a hierarchical system of hydrologic units broken up into regions, sub-regions, accounting units, and cataloging units. Each hydrological area has its own Hydrologic Unit Code (HUC). View hydrology reports by HUC code to get a closer look at river and snow conditions across the US.