"Watershed" is the term used to describe the geographic area of land that drains water to a shared destination. The drainage system (and the watershed) also includes the geographic area surrounding the stream system that captures precipitation, filters and stores water, and determines water release into stream systems. The stream system is the visible, aboveground portion of a larger drainage system. A watershed, therefore, is "an area of land that drains water, sediment, and dissolved materials to a common outlet" (FISWRG 1998).
Any activity that changes soil permeability, vegetation type or cover, water quality, quantity, or rate of flow at a location can change the characteristics of a stream or even the watershed at downstream locations. Land use practices such as clearing land for timber or agriculture, developing and maintaining roads, housing developments, and water diversions may have environmental consequences that greatly affect stream conditions even when the land use is not directly associated with a stream. Proper planning and adequate care in implementing projects can help ensure that one activity within a watershed does not detrimentally impact the downstream environment.
In recent years, watersheds across the United States have become a focal point for community-based environmental conservation. Through the collaborative efforts of watershed groups consisting of the people living and working within each watershed major improvements in water quality, fisheries enhancement, wildlife habitat, and overall quality of life have been accomplished. A watershed provides water for drinking, recreation, and agriculture, and is a rich source of biological diversity that includes habitat for many threatened and endangered species.
Every waterway lies within a watershed, and smaller watersheds join together to become larger watersheds. Watershed boundaries always follow the highest ridgeline around the stream channels and meet at the bottom or lowest point of the land where water flows out of the watershed. The boundary between watersheds is defined as the topographic dividing line from which water flows in two different directions. However, the scale at which the landscape is examined is relevant for identifying and defining watersheds.
A watershed may be small and represent a single tributary within a larger system, or be quite large and cover thousands of miles. Because using a common language will help with coordination and management, watersheds have been defined and named using standardized protocols. The naming conventions used by federal and state agencies are defined at a regional scale, and then these large hydrologic units are broken down into smaller watershed units for management purposes. The federal system divides the United States into a four-tiered hierarchical system, which is defined by the United States Geological Survey's (USGS) hydrologic unit codes (HUC). Based on hydrologic features, this system divides the country into:
|Level 1||21 Regions|
|Level 2||222 Subregions|
|Level 3||352 Accounting Units|
|Level 4||2,149 Cataloging Units.|
The the Cataloging Unit, which has an eight digit HUC is commonly referred to as a 4th Level or a Sub Basin. The 4th Level is generally used for local planning efforts but the scale is still quite large (mapping scale is 1:500,000, or approximately 448,000 acres for the smallest watersheds). Knowing a watershed's HUC designation may be important for understanding management issues within the region, as well as necessary for applying for watershed assistance granting opportunities (visit the United States Environmental Protection Agency's Surf Your Watershed Web site to learn more about your HUC watershed).
Over the last ten years, many federal and state agencies have realized current 8-digit hydrologic unit maps are unsatisfactory for many purposes because of inadequate bases or scales. Thus, The Natural Resources Conservation Service (NRCS) and the United States Department of Agriculture have worked with other federal and state agencies and with the Subcommittee on Spatial Water Data Federal Geographic Data Committee (FGDC) to establish a federal interagency standard covering mapping and delineation of hydrologic units that would be suitable for all agencies. In cooperation with the FGDC and the Advisory Committee on Water Information (ACWI), a new interagency guideline was written. During December of 2002, this document was presented to the FGDC for review. This document has superseded NI-170-304 as the official standard for delineation of 5th and 6th level hydrologic units.
With the interagency standard, some changes have been made to the criteria for delineation and attribution of the 5th and the 6 level. These changes include coding the 5th level as 10-digit (formerly 11-digit in NI-170-304) and 6th level as 12-digit (was 14-digit in NI-170-304). Another change is that the 3rd level will officially be called "basins" (formerly known as "cataloging units") and the 4th level will be called "sub-basins" (formerly known as "accounting units"). Additional attribute fields have also been added to the dataset. Over the last several years, a series of workshops have been held to promote this interagency effort and to resolve delineation and attribution issues.