The Nez Perce Soil and Water Conservation District (District) contains four major river systems; Clearwater, Snake, Salmon and the Palouse. Each of these river systems contains many watersheds. The District utilizes watersheds as one of the major planning units for conservation practices.
A watershed is defined as the topographic boundary where water flows in two different directions and all waters drain to a common outlet. Watersheds capture precipitation, store water and determine water release into the streams. Reference: Salmonid Habitat Restoration Planning Resource.
While the stream may be visible above ground, the drainage system includes a larger landscape scale. The watershed is considered an area of land that drains water, sediment and other materials to a common outlet. Land use practices greatly influence a watershed’s function. Land use practices such as land clearing, changing vegetation type, road development and maintenance, housing developments and water diversions may impact stream conditions even when that land use practice is not adjacent to a stream. Land uses practices or activities which change soil infiltration rates, vegetation cover type, water quantity, and rate of flow may have significant impacts on the watershed at downstream locations.
The District promotes watershed level planning to ensure that one activity within a watershed does not detrimentally impact the downstream environment.
Watershed boundaries within the District are determined by using a topographic map and following the highest ridgeline around the stream and meet at the lowest point where the water leaves (or flows out) of the watershed. This is often a confluence with another major stream or river. The watershed size may be small or large. The District chose the watershed boundaries in figure 1.
A common watershed naming protocol has been defined by the United States Geological Survey (USGS). The naming protocol is defined at a regional scale then further broken down into smaller watershed units for planning purposes. The USGS system divides the US into 21 regions, 222 subregions, 352 accounting units and 2,149 cataloging units. The eight digit unit hydrologic unit code (HUC) is the smallest unit or cataloging unit. This smallest unit is commonly referred to as the 4th Field HUC or “Sub Basin”. The District further breaks down the watershed size to facilitate local planning efforts.
Figure 1. Watershed locations in Nez Perce SWCD
Federal Interagency Stream Restoration Working Group (FISWRG). 1998. "Stream Corridor Restoration: Principles, Processes, and Practices." GPO Item No. 0120-A; SuDocs No. A 57.6/2:EN3/PT.653. View on-line document.