A hydrologic unit is a topographically defined area of land, the boundaries of which are (usually) ridge tops. The
United States is divided into successively smaller hydrologic units which are assigned hierarchal hydrologic unit codes
(HUCs).
In Missouri, hydrologic units are mapped at the 2 digit, 4 digit, 6 digit, 8 digit, 11 digit, 12-digit and 14 digit HUCs.
More digits describe smaller areas of land. For instance, the Missouri River basin is assigned a two digit code
of “10”. The Little Chariton River basin is assigned an eight digit code of “10280203”.
The first two digits indicate the Little Chariton River basin is included in the Missouri River basin.
Although hydrologic units are commonly referred to as watersheds, the two are not synonymous. A watershed is usually thought
of as all of the land that water flows over before reaching a specific point (i.e. everything uphill from the dam).
A hydrologic unit may include only a portion of the land that drains to the hydrologic unit outlet within its boundary.
A hydrologic unit may also possess more than one outlet. The following graphic illustrates this difference.
Figure 1 – Hinkson Creek Watershed. Hinkson Creek watershed includes two 12-digit hydrologic units, 103001020907
in brown, 103001020906 in green. In the southeast, Grindstone Creek watershed is only a portion of a hydrologic unit.
The lower hydrologic unit (brown) receives water from the upper hydrologic unit (green), meaning it is only part of a watershed.
HUCs and watersheds are important when managing environmental risk because the health of the watershed is dependent on what
happens everywhere in the watershed. A farmer 5 miles away in the same watershed as your land may impact your water quality
environment more than a neighbor 1/4 mile away in a different watershed.
The Missouri Watershed Information Network (MoWin) Homepage
provides watershed information and management tools. Within the site you can find watershed-related information
by county under "Watershed Assistance" and local contacts for your particular watershed location under "
Watershed Management and Planning Assistance."
Here are some additional sites with watershed information that might be of interest.
Local contact information for your watershed can be found at:
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