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The USGS divides the USA into hydrologic regions (1 code or 2 digit HUC), subregions (2 code or 4 digit HUC), accounting units (3 code or 6 digit HUC) and cataloging units (4 code or 8 digit HUC). The last (smallest) divisions, also known as CUs. Finer divisions are used for some large-scale (small area) projects. These are becoming increasing used in water-oriented reporting as they provide more natural boundaries at several scales. The ESG at Montana State University has implemented a HUC-based GIS for the National Wild Fish Health Survey. Much of it should be useful to any water-oriented project. Outlying areas of the USA are not covered, but all parts of the Conterminous USA are included. Over 32,000 files are now on-line as part of this project.

Lead Topics

HUC Lists by Region

  1. New England Region
  2. Mid-Atlantic Region
  3. South Atlantic-Gulf Region
  4. Great Lakes Region
  5. Ohio Region
  6. Tennessee Region
  7. Upper Mississippi Region
  8. Lower Mississippi Region
  9. Souris-Red-Rainy Region
  10. Missouri Region
  11. Arkansas-White-Red Region
  12. Texas-Gulf Region
  13. Rio Grande Region
  14. Upper Colorado Region
  15. Lower Colorado Region
  16. Great Basin Region
  17. Pacific Northwest Region
  18. California Region
There is also a large list for all these regions combined. If searching by name, note that HUC names are not unique. A Graphical Locater result page should help determine the correct HUC. Our HUC pages can also be reached by following the HUC link on any Graphical Locater result page. Some good starting points are the HUC maps for the: We use percentile rankings at several places to help interpret the output. Percentiles are most familiar as "class standing". These are given for the 2111 units of the conterminous USA (% in USA =) and for the current region (% in Region =). In case of ties, the value reported is from the top of the block. This is most important for units which lack certain features (numerous ties at the low end).

The ESG's HUC pages start with navigation links to all of the larger units (if any). Region, subregion and accounting unit maps can be reached from the HUC page for any cataloging unit in that area. This is the only way to reach the pages for the subregions and accounting units. Accounting unit pages are only available when the area is different from the subregion (no 00 code units). The number of cataloging units in the region is given to help interpret the percentile rankings. The pages for the cataloging units have 6 sections, the larger units have only the Graphical Locater maps.

I. Graphical Locater Maps

For each HUC, there are 3 maps available; "roads", "water only" and "8 Digit HUCs". The base data are the same as Graphical Locater. The cataloging unit maps always use 1:100,000 scale data and the larger units use 1:2,000,000 scale data, except for a few, small units that use the 1:100,000 scale data. The scale varies widely from unit to unit, so these maps all have a scale bar. For the cataloging units, "the 8 Digit HUCs" maps shows all neighboring units. Water bodies outside the unit are red and they are outlined only. For the larger units, these maps only show the cataloging units within the larger unit. There is a page that gives the color scheme for Graphical Locater Maps.

The HUC maps provide alternatives to the standard 1:100,000 (1° by 0.5°) Graphical Locater maps. The HUC maps are generally superior to the original cell maps due to refined procedures. They avoid the disruptive artificial seams, at least for drainage oriented projects. The base map for all HUCs is available for on-line mapping, such as the "Place point in HUC" function. All Graphical Locater maps are "hot" everywhere, which means you can select areas outside the limits of the HUC. This is useful to identify and switch to neighboring HUCs.

The HUC boundaries are key to all that follows so these need some further discussion. Three versions of HUC boundaries, from the USGS were all too small scale for this project. These were used as a guide and then the boundaries were corrected to conform with the 1:100,000 data (an operation that I do not want to think about anymore). Three areas in California (180201, 1803 and 1804) were not always divided by hydrological boundaries by the USGS. Some of these were restructured to be more useful for fish oriented projects. We disconnected the HUC boundaries from coastlines so that the HUC boundary moves through the open ocean to capture relevant land areas. This creates far fewer polygons and it allows the boundaries to be better used at multiple scales. For some units (ocean bays and the Great Lakes, which here includes the Canadian portion also), some islands are included in both the water unit and the bordering land unit. Unit identification favors the water unit. The boundaries are best evaluated on the Cataloging Unit "8 Digit HUCs" map.

The area of the HUC is based on the Graphical Locater map for "water only". The area of the black background, the scale bar and outside water bodies (darkest blue) are all excluded. The percentile ranking of the area helps put it into perspective. The area from the USGS Water-Quality Monitoring Networks file, HUC_NAME:TXT is given for comparison. The largest differences occur in the restructured California units and in coastal units. In any case, what-you-see in the Graphical Locater "water only" map is what-you-get for area. Please report any significant errors, a couple (in my area) have already been found and fixed.

II. Flow Connections

This is based on our own analysis of the main flow connections between units as seen in the finished Graphical Locater maps. The page should be self-explanatory.

III. Named places in this watershed

These are based on the GNIS and Graphical Locater's HUC identification procedure. A small buffer area was used to include points that were very near the HUC boundary. These are marked with a "?". The Named places are linked back to Graphical Locater to provide more information or to "Place point in HUC." Note that while a single named place may occur in 2 or 3 neighboring HUCs (due to the buffer area), the "Place point in HUC" function will only place the point in the single best HUC (that is the one identified on the Graphical Locater result page).

IV. Elevation Analysis

The elevation analysis is based on elevations interpolated from the 1-degree DEMs on a 50 m grid that fell within the HUC. The sample size is given and it is normally very large. The mean and standard deviation with their percentile rankings should be straightforward. The coefficient of variation is simply the standard deviation expressed as a percentage of the mean. The vertical relief is the difference between the highest and the lowest elevations. This will generally be a low estimate, as the grided DEM will tend to be a little low for peaks and a little high for valleys. The table and graph of the elevations percentiles are for the elevations in the current cataloging unit. Because of the regular DEM grid, these translate directly into percent of the HUC area. The lowest, highest and median elevations are labeled in the graph.

V. Map Line Analysis

This is based on the TIGER/LINE data in vector format. The number of points in each line was expanded to keep all 2-point segments smaller that the current pixel size. These segments were then assigned to the best HUC and the total lengths were accumulated by line category. The percentile rankings are for the density of the feature, not the raw total length. The percent intermittent water is calculated by length and combines streams and shoreline.

VI. Other sites

We provide links to sites that provide a page with uniform access for all USA HUCs.


As with Graphical Locater most of the base data for this project came from the U.S. Geological Survey. This project was funded by the U.S. Fish and Wildlife Service to help support the National Wild Fish Health Survey.
1999 JUNE 03, last updated on 2003 JAN 24 D.L. Gustafson