What is a watershed?

A watershed is defined as the geographic land area from which all water drains into a common outlet. This outlet may be a stream, lake, underlying aquifer, estuary, wetland, or ocean.

A watershed is bounded by the highest points around a particular stream or set of streams. The high points separating one watershed from another are called divides. The area where one watershed ends at a particular crest or ridge is where another watershed begins on the opposite side. A helpful way to picture a watershed is to imagine it as a kind of funnel, which channels the water to a stream or a single point - for instance where a stream intersects another body of water (another stream, lake, or the ocean).

Watersheds are also referred to as drainage areas, catchments or drainage basins. At the smallest scale- for instance that of a tiny brook; the drainage basin may be composed of a very small area and be the same as its watershed. On a larger scale, drainage basins may encompass several watersheds in a region, all draining to a single waterway. This larger scale description is the more accepted use of the term drainage basin.

Chattanooga's watersheds

Chattanooga and its surrounding areas make up the Lower Tennessee River watershed. This is encompassed by the Upper Tennessee River Basin (UTRB), which extends from Chattanooga towards the northeast to the Virginia and upper east Tennessee border. The Upper Tennessee River Basin covers most of southeast Tennessee. All the water within the UTRB drains into the Tennessee River and therefore flows through Chattanooga. Consider what this means; all the water that drains from between some of the highest points of the Cumberland Plateau and the highest points along the Blue ridge Mountains of East Tennessee passes through Chattanooga via the Tennessee River!

The Lower Tennessee Watershed itself is comprised of several smaller watersheds, often identified with the name of the stream that flows into the Tennessee River. Chattanooga has eight watersheds and one area referred to as the combined sewer area. There is the North Chickamauga Creek watershed, the South Chickamauga Creek Watershed, the Chattanooga Creek Watershed, Lookout Creek Watershed, Citico Creek Watershed, Mountain Creek Watershed, Tennessee River Watershed, and Wolftever Creek watershed.

Above are the Upper and Lower Tennessee River basins. On a larger scale below, there are the watersheds of the United States, including ours: the Tennessee River watershed which is a part of the Mississippi River watershed.

Our impact on watersheds

It is very important to realize that every single point on the surface of the Earth is in a watershed. Thus, all terrestrial human activities take place within watersheds. These activities can have effects on watersheds that range from minor to major, and from beneficial to catastrophic.

As human activities spread, so does our impact on watersheds. The more buildings and roads that become established, the more land area is covered by impervious surfaces. Impervious surfaces are surfaces such as concrete, asphalt, or any other material that rain water cannot get through or soak into.

In a natural setting, rainwater soaks into the ground or soil to can be used by plants and trees, or goes into water bodies. When the ground is covered by impervious surfaces the rain can't get through. When there is a surplus of rain, what ground is available to soak it up isn't enough. This means the overflow of rain off these surfaces can be dispersed in an unnatural way. This overflow of rain water is called stormwater runoff. In an urban setting, stormwater runoff goes into storm drains. Storm drains are the drains you see on a curb or road. They can be plain grates, or sometimes have emblems stating "Drains to creek" or "No dumping, drains to river."

Storm drains

These storm drains are for rainwater only- and the water that goes in them is not treated, but goes directly to the nearest water body. The impermeable surfaces collect pollutants from human activity such as oils and grease from vehicles, trash, and heavy metals. When it rains, these pollutants are carried off of the surfaces and roads and end up in storm drains which consequently means they end up in our natural water bodies.

 So where does the water go?

Once stormwater enters the storm drain, it goes through conveyance systems called MS4's, or municipal separate storm sewer systems. These are a separate system from sewer lines, which come from houses and businesses and are treated at the Moccasin Bend Waste Water Treatment Plant. It is important to be aware of the activities that can cause pollutants to impair our streams. Something simple such as pressure washing your home or washing your car can result in pollutants entering a storm drain.

 Point and non-point source pollution

 There are two classifications of pollutants that affect our water bodies. Pollutants such as pesticides, herbicides, detergents, and fecal matter that are carried by stormwater runoff into creeks and streams are considered non-point source pollution, since they come from various places and the source cannot be quickly identified. If the source was easily identifiable, such as a pipe discharging or smoke stack, they would be considered point source pollution. The two main pollutants currently affecting Chattanooga's watersheds are E. coli and suspended solids such as sediment and industrial waste products.

                Point Source Pollution                                 Non-point Source Pollution