Water Resources Inventory Area 1 Watershed Management

Introduction to Watershed Boundaries


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Watershed boundaries are defined by topographic divides and delineate areas where surface-water runoff drains into a common surface-water body, such as a lake or section of a stream. Watersheds may be identified at many different scales. For example, the entire Nooksack River System with all its tributaries is one large watershed in WRIA 1 (Streams, Canals, and Lakes in the WRIA 1 Study Area). Many smaller watersheds (also referred to as subbasins) can be identified within this large watershed, such as the South Fork of the Nooksack River, Fishtrap Creek, and Anderson Creek. Each of those smaller watersheds can again be subdivided into smaller subbasins. The level of watershed delineation depends on the purpose.

For the water-quantity evaluation of WRIA 1, watersheds are delineated so streamflow discharge can be estimated at the outlet of each delineated watershed (also referred to as a nodal point) for different times of the year, for different climatic conditions, and for different land use practices. These estimates consider both surface- and ground-water contributions to streamflow discharge. The watershed delineations shown on this web page represent an initial identification of the nodal points. During the next study phase, when streamflow discharge is actually estimated, additional nodal points (and thus watershed delineations) may be added.

It is important to consider that while watershed boundaries identify surface-water runoff divides, they often do not represent ground-water flow divides. Ground water generally follows the topography in its flow direction, just as surface-water runoff does. However, the movement of water in the subsurface is more complex and ground-water flow directions may not coincide with surface-water flow directions.

Watershed boundaries are selected on the basis of topographic information. Traditionally, watershed boundaries have been identified by manually drawing them onto paper topographic maps. The person who draws the lines makes the judgment of where a divide is located in a particular location based on topographic and other information on the map. If the map is incorrect, or if the analyst makes a mistake, the watershed boundaries may be in error. More recent methods use computer programs to delineate watersheds. Topographic contours have been transformed into a grid of numbers (called a Digital Elevation Model, or DEM), where each number represents the land-surface elevation interpolated from the original topographic map. Computer programs delineate watersheds on the basis of DEMs.

The accuracy of topographic maps is plus or minus one half of one contour interval. In the U.S. part of the WRIA 1 study area, contour intervals range from five feet in the lowlands to 40 feet in the uplands. In the Canadian part of the WRIA 1 study area, contour intervals are 20 meters (65.6 feet).

Delineating watersheds in the steep terrain of the uplands is usually unambiguous. This is not true in the flat areas of the lowlands. Delineating watersheds in flat terrain is very difficult by any method (manual or computer methods using topographic maps or field checking) and watersheds may have to be delineated at a larger scale in areas where surface-water runoff directions cannot be determined with certainty.