Delineating Watersheds—A First Step Toward Effective Management

A watershed is an area of land that drains to an associated water resource such as a wetland, river or lake. Depending on the size of the area and the topography, a watershed can contain numerous tributaries, such as streams and ditches, and ponding areas such as detention structures, natural ponds and wetlands.

Rainwater and snowmelt that does not evaporate or infiltrate into the soil runs off into a nearby tributary or ponding area, and then flows to the main wetland, river or lake within the watershed. It is through this linkage that the upper portions of a watershed can effect downstream areas. Thus, the quality of a wetland, stream or lake often reflects the land use and other activities being conducted in the upstream areas.

Because the relationship of cause and effect can extend for distances throughout the entire watershed, it is important to address environmental management issues from a watershed perspective.

Stormwater Management

One such environmental issue which challenges many local and municipal officials today is stormwater management. Stormwater is created when rain falls on impervious surfaces and runs off the surface. This runoff can have negative effects on water quality and quantity within the watershed.

Water Quality Concerns: As development and urbanization increase within the watershed, various pollutants (e.g., nutrients, bacteria, pesticides, heavy metals, sediments, organic matter, trash) will be deposited. Urban surfaces also act as heat collectors. When stormwater runs off the urban landscape, it carries the pollutants with it into the receiving waters. Also, the temperature of the stormwater increases as it passes over these surfaces. As a result, the quality of the water resource tends to degrade within the immediate vicinity of the urbanized area, as well as in areas further downstream. The degradation of water quality can cause immediate (acute) and long term (chronic) negative effects on aquatic and terrestrial plant and animal species and their habitat, while also degrading drinking water supplies, and limiting other uses of the water resource such as swimming and recreation. In addition to these water quality concerns from increased urbanization, water quality also degrades when stream banks erode due to changes in water quantity. The sediment from stream bank erosion can alter stream bed characteristics, thereby affecting benthic habitat, and reducing light transmission.

Water Quantity Concerns: When agricultural and natural areas become urbanized, vegetation (grasses, trees, etc) and base soils are replaced with impermeable materials such as asphalt and concrete. Stormwater that once soaked into the soil now runs off in greater quantities and at higher velocities to the smaller tributaries and ponding areas, which in turn convey the water to the wetland, river or lake of that watershed. Without proper stormwater management, urbanization affects the water quantity, velocity, peak flow rates, and flow frequency in the watershed's tributaries and river system. This can cause flooding, erosion and scouring problems downstream.

Stormwater Management from a Watershed Perspective

Effective stormwater management to protect and preserve nearby and downstream water resources can be conducted on a watershed level. To begin this process, one of the primary tools required is a map of the watershed.

Such a map can be used to determine the cumulative effects of land development or land use changes on both water quantity and water quality. After a watershed is delineated, useful determinations can be made that can aid in the land use planning process such as:

• Visualizing the effects that a future development may have on downstream areas, as well as the influence of additional or existing developments upstream.
• Measuring the acreage of the watershed to predict soil loss and estimate pre-development and post-development stormwater runoff volumes.
• Measuring the acreage of proposed projects to estimate pollutant loads.
• Determining appropriate sites for regional stormwater detention facilities to control peak volume runoff and sedimentation.

In addition, watershed maps can:

• Show natural resource assets like woodlands, prime farmland, and habitat areas.
• Target areas susceptible to soil loss and erosion.
• Identify wetland areas and potential flood zones.

Watershed Delineation: A common method of locating and delineating the boundaries of watersheds is to use topographic maps following the basic principle that water runs downhill. A topographic map represents the physical features of the land such as hills, valleys, basins, ridges, and channels. The mapping technique used is based on elevation datum (usually mean sea level) and contour intervals commonly of ten feet.

Watershed Delineation Steps

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

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

3. Examine the brown lines on the topographic map that are near the watercourse. These are referred to as "contour" lines. They connect all points of equal elevation above or below a known reference elevation.

The dark brown contour lines (thick lines) will have a number associated with them, indicating the elevation. The light brown contour lines (thin lines) are usually mapped at 10-ft intervals, and the dark brown (thick) lines normally are mapped at 50-ft intervals. To determine the final elevation of a location, simply add or subtract the appropriate contour interval for every light brown (thin) line, or the appropriate interval for every dark brown (thick) line.

Contour lines spaced far apart indicate that the landscape is more level and gently sloping. Contour lines spaced very close together indicate dramatic changes (rise or fall) in elevation over a short horizontal distance.

4. Check the slope of the landscape by locating two adjacent contour lines and determine their respective elevations. The slope is calculated as the change in elevation divided by the distance.

A depressed area (valley, ravine, swale) is represented by a series of contour lines "pointing" toward the highest elevation.

A higher area (ridge, hill) is represented by a series of contour lines "pointing" towards the lowest elevation.

5. Determine the direction of drainage in the area of the water body by drawing arrows perpendicular to a series of contour lines that decrease in elevation. Stormwater runoff seeks the path of least resistance as it travels down the slope. The "path" is the shortest distance between contours, hence a perpendicular route.

6. Mark the break points surrounding the water body. The "break points" are the highest elevations where half of the runoff drains toward one body of water, and the other half drains toward 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.

Editor's Note: This article, edited to Water Online's and Public Works Online's Journalistic style, was prepared originally by the Watershed Management Unit-Water Division of the U. S. Environmental Protection Agency's Region V, the U.S. Department of Agriculture Soil Conservation Service (Illinois), the U.S. Fish and Wildlife Service Chicago Metro Wetlands Office, and Terrene Institute. Tetra Tech, Inc. assisted with the publication of the paper. For more information or copies of this and other articles in the series, contact Terrene Institute at 1717 K Street, Washington, D.C. 20006; Tel. 202-833-8317.