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What is a Watershed?
Source: USGS
Watersheds are composed of land areas where all the tributarties, precipitation, and mountain snow eventually flow to a common outlet.
As surface water makes its journey via various streams, water may evaporate (or freeze), seep into groundwater aquifers, or be consumed for human use. Snowmelt is a major source of water for the western United States, and it is common to see river discharges peaking during the spring months as warmer climates erode snowpacks.
Hydrologic Units
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. 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.
| Name | Level | Digits | Avg Sq Miles |
| Region | 1 | 2 | 177,560 |
| Subregion | 2 | 4 | 16,800 |
| Basin | 3 | 6 | 10,596 |
| Watershed | 4 | 8 | 700 |