Hydrologic Units (HUs) represent the area of the landscape that drains to a portion of the stream network. Each drainage has a unique Hydrologic Unit Code (HUC). The most current national HU dataset is the Watershed Boundary Dataset (WBD). HUCs from other sources might be out of date or differ from the WBD standards.
The WBD includes six required levels of nested HU polygons (2- to 12-digit), two optional nested levels of HU polygons (14- and 16-digit), and supporting geospatial data. 14- and 16-digit HUs only exist in a few areas of the country.
HUs and HUCs are easily found through the National Map Viewer:
- Go to The National Map Viewer (it might take a minute to fully load).
- In the green bar above the map, click on the Layers icon, which looks like a stack of paper.
- Click on the box beside "Watershed Boundary Dataset".
- Zoom in to your area of interest. As you zoom in, click on the arrow beside the Watershed Boundary Dataset box in the Layers list to toggle higher resolution layers on and off.
To view the attributes of a flowline (stream) including the Name and Reachcode, follow the above instructions to turn on the National Hydrography Dataset layer in The National Map Viewer and click on any stream to see its attributes.
The WBD is a companion to the National Hydrography Dataset (NHD), which contains information about the nation’s surface hydrography. The first 8 digits of the reachcode are equivalent to the 8-digit hydrologic unit code for the unit that the NHD feature resides within. View both WBD and NHD using The National Map Viewer or download WBD and NHD products for use with GIS software.
Learn more: Video lessons 3a-3e on using The National Map Viewer
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Use The National Map Viewer to display or create watershed maps with the National Watershed Boundary Dataset. Watersheds are shown as Hydrologic Units, each one with a unique Hydrologic Unit Code (HUC). Go to The National Map Viewer (it might take a minute to fully load). In the green bar above the map, click on the Layers icon, which looks like a stack of paper. Click on the box beside "Watershed...
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- FAQ
Can the USGS install a streamgage at my location?
The USGS can install a streamgage almost anywhere, but in most cases we do not choose when and where to do that. We typically install and operate streamgages where other entities ask and reimburse us to do so. We operate streamgages for entities such as cities, towns, counties, state agencies, and other federal agencies to assist with their missions and to provide data for general public use...
Where can I get watershed maps?
Use The National Map Viewer to display or create watershed maps with the National Watershed Boundary Dataset. Watersheds are shown as Hydrologic Units, each one with a unique Hydrologic Unit Code (HUC). Go to The National Map Viewer (it might take a minute to fully load). In the green bar above the map, click on the Layers icon, which looks like a stack of paper. Click on the box beside "Watershed...
How often are real-time streamflow data updated?
USGS real-time streamflow data are typically recorded at 15-minute intervals, stored onsite, and then transmitted to USGS offices once every hour, depending on the data relay technique used. Recording and transmission times might be more frequent during critical events (floods, for example). Data from current sites are relayed to USGS offices via satellite, telephone, and/or radio telemetry and...
What is a reach?
“Reach” can have slightly different meanings, depending on how it is used.A reach is a section of a stream or river along which similar hydrologic conditions exist, such as discharge, depth, area, and slope. It can also be the length of a stream or river (with varying conditions) between two streamgages, or a length of river for which the characteristics are well described by readings at a single...
What does the term "stream stage" mean?
Stream stage is an important concept when analyzing how much water is moving in a stream at any given moment. "Stage" is the water level above some arbitrary point in the river and is commonly measured in feet. For example, on a normal day when no rain has fallen for a while, a river might have a stage of 2 feet. If a big storm hits, the river stage could rise to 15 or 20 feet, sometimes very...
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