What is a watershed?
Watersheds are defined by the USGS as the land area that drains water to a particular stream, river, or lake. It is a land feature that can be identified by tracing a line along the highest elevations between two areas on a map, often a ridge. Large watersheds, like the Mississippi River basin contain thousands of smaller watersheds.
There are several terms related to watersheds. The USGS has created a nationwide identification and naming schema known as the National Hydrography Dataset (NHD). The NHD identifies different drainage areas based on their size. The schema begins with a region, then a sub-region, basin, sub-basin, watershed, and finally a sub-watershed. These areas are used by different federal, state, and local governments and organizations to assist in protecting our waters. They use these different areas to help determine where to focus study efforts and provide funding. By identifying these different drainage areas, organization at different levels are able to cooperate and work together to help protect our waters.
For example, the watershed for a large stream, like the Trinity River, is generally broken up into several smaller watersheds that are collections of catchments. The areas of land draining to small tributary streams are often called a catchment. The Texas Parks and Wildlife Department has prepared this map to show the Trinity watershed. The map illustrates its head waters in the north and west, until it empties in the Gulf of Mexico, just east of Houston. Remember that the map to the left shows some of the smaller watersheds that make up the entire Trinity Watershed.
Why use watersheds?
Using a watershed management approach provides a more comprehensive and holistic method for protecting water quality than addressing problems one at a time. This method also streamlines resource management to more effectively use what limited resources are available for protecting water quality in a watershed.
In the early years of the field of Ecology, scientists tried to determine what the unit of scale should be. How large of an area they use when conducting studies. Watersheds were one of the first areas used as the unit of scale. Today, watersheds are used by different planning and conservation organizations to monitor and improve water quality in our lakes and streams. Denton County is committed to the watershed approach is working with other governments and organizations to help keep our waters clean and healthy.
Rural vs Urban
Different watersheds behave differently. The type of soils underground, the type of land cover above, the rate of elevation changes within the watershed, all effect how the watershed will behave. The type of human activity also has a significant impact on the way a watershed will behave.
There are some properties that all watersheds share. When it rains in a watershed, the runoff from the storm follows a route based on the topography, elevation changes, of the land regardless of the land surface. However, an urban watershed will almost always produce more runoff at a faster rate than a rural watershed. This is because urban watersheds generally have more roads, parking lots, house tops, and other impervious surfaces that do not allow the rain to soak into the soil. The types of pollutants that can be found in the watersheds will be different as well. Regardless of the type of watershed, the easiest way to fight pollution is to prevent it from entering our lakes and streams.