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WHAT IS A WATERSHED?





A watershed is the area of land that catches rain and snow that then drains or seeps into a marsh, stream, river, lake or groundwater. Homes, farms, ranches, forests, small towns, big cities, and more can make up watersheds. Some cross county, State, and even international borders. Watersheds come in all shapes and sizes. Some are millions of square miles; others are just a few acres. Just as creeks drain into rivers, watersheds are nearly always part of a larger watershed.


watershed-boundary


Delineating a Watershed
Have you ever wondered where rain goes when it falls? Stop sometime and watch a stream or river flowing by - does it end up in our drinking water? What things keep water clean in nature and in the water we drink? The land areas filter water naturally as it passes through on the surface or into ground water.

Waterbasins and watersheds are two ways that we classify water boundary units. Waterbasins are large land areas that are drained by a major river system. Watersheds are smaller land areas that are defined as the area that is drained by one stream or river. In order to collect background information on your watershed, you will probably want to get out in the field and take water quality measurements. You will first need to delineate the watershed boundaries.

1)Use a topographic map to locate the river, lake, stream, wetland, or other waterbodies of interest.

watershed-source


2) Trace the watercourse from its source to its mouth, including the tributaries. This step determines the general beginning and ending boundaries.

watershed-source2


3) The lines on the map are known as contour lines. Contour lines connect all points of equal elevation above or below a known reference elevation. The numbers beside the lines indicate the elevation. Thin lines mark 10-foot intervals in elevation, while thick lines mark 50-foot intervals in elevation.

contour-lines


Contour lines that are spaced far apart indicate that the land is gently sloped and more level. Closely spaced lines, however, indicate steep changes in elevation over a short distance.

floodplain


4) Check the slope of the landscape by locating two adjacent contour lines and determining their respective elevations. The slope is calculated as the change in elevation divided by the distance. A depressed area (valley, ravine) is represented by a series of contour lines "pointing" towards the highest elevation. A higher area (ridge, hill) is represented by a series of contour lines "pointing" towards the lowest elevation.

ridge


5) Determine the direction of drainage in the area of the waterbody by drawing arrows perpendicular to a series of contour lines that decrease in elevation. Runoff seeks the path of least resistance as it travels down a slope. The "path" is the shortest distance between contours.

directional-arrows


6) Mark the breakpoints surrounding the waterbody. The "break points" are the highest elevations where half of the runoff would drain towards one body of water, and the other half would drain towards another body of water.

7) Connect the break points with a line following the highest elevations in the area. The completed line represents the boundary of the watershed.

boundary-line


boundary-map


finished-map





Additional Information:
     WV Watersheds: http://dep.wv.gov/WWE/getinvolved/sos/Pages/Watersheds.aspx
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