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What is a Watershed?

A watershed is the area drained by a stream and its tributaries. Watersheds range in size from under a square mile to hundreds of thousands of square miles (the Chesapeake Bay watershed, for example, has an area of 66,388 square miles). Any rain that falls within a watershed will eventually drain from the bottom of the basin through the main stream channel. Because watersheds may incorporate broad or diverse land areas, they are better units for analyzing environmental problems than are political jurisdictions. As a case in point, a study of water pollution in the Maryland portion of the Chesapeake Bay watershed would have to take into account pollution sources in the upper Chesapeake Bay watershed of New York and Pennsylvania.

A watershed is more than a collection of streams and adjacent land areas--it is a natural resource system in which humans and other organisms interact with the land and its associated resources for sustenance, shelter, and security. The physical condition of a watershed, therefore, directly affects the health and well being of natural and social systems within its divides and indirectly affects those systems beyond its divides.

A watershed is the area drained by a stream and all of its tributaries. Any rain that falls within the watershed will pass through the main stream channel.

Diagram of a Watershed
Figure 1. Diagram of a Watershed

A divide separates each basin from the surrounding drainage basins. Divides follow ridges and hill tops. If a raindrop falls on one side of a divide, it will flow down one side of the hill, and into one drainage basin. If the raindrop falls on the other side of a divide, it will flow into a different drainage. Watersheds are composed of many smaller drainage basins. In Figure 2, a sub-watershed has been drawn for every tributary.

Figure 2. Sub-Watershed

Stream Order

The streams occupying a drainage basin form a hierarchical network of channels that, in humid areas, hold increasingly larger volumes of water as you move toward the mouth of the basin. A stream's order is its rank, or relative position, within the network. A first-order stream is a channel with no tributaries. That is, a channel at its upper reaches and near its source. A second-order stream is a channel fed by at least two first-order tributaries. The joining of two-second order streams forms a third-order stream. Stream ranking continues in this manner until the highest-ordered channel is reached. First and second-order streams are located in the headwater areas of watersheds and typically convey small volumes of water. These lower-order streams are vulnerable to pollution because they are unable to dilute contaminants, assimilate much organic waste, or remove sediments deposited on channel bottoms.

Strahler Stream Order Classification
Figure 3. Strahler Stream Order Classification
(Source: Marsh, 1998, p. 169)

Sub-Watersheds and Watershed Order

A large watershed--like the Chesapeake Bay watershed--is composed of numerous sub-watersheds that are drained by tributary streams and rivers. The Bay's sub-watersheds are shown on the following map. The largest is the watershed of the Susquehanna River, which drains parts of New York, Pennsylvania, and Maryland before entering the bay at Havre de Grace, Maryland. These sub-watersheds, in turn, are composed of the smaller watersheds of streams draining into their main channels.

Chesapeake Bay Sub-Watersheds
Figure 4. Chesapeake Bay Sub-Watersheds
(Source: http://www.chesapeakebay.net/)

Watersheds, like streams, are ranked according to order. A first-order watershed is drained by a first-order stream, whereas the main channel of a second-order watershed is a second-order stream, and so on for each higher-ordered watershed. A large watershed, therefore, is a nested hierarchy of numerous lower-ordered basins or sub-watersheds.

Nested Watersheds
Figure 5. Nested Watersheds
(Source: Marsh, 1998, p. 170)

Basin Versus Nonbasin Drainage

Generally, from 15 to 20 percent of runoff within a large watershed drains directly into its highest-order stream or water body without first flowing through lower-ordered basins. Areas that drain directly into higher-order water bodies are called nonbasin drainage areas and include lands adjacent to river, bay and ocean shorelines. In Maryland, nonbasin bay drainage areas are subject to regulation under Maryland's Critical Areas Act. This act restricts development within 1,000 feet of waters under tidal influence, and mandates 100-foot wide vegetated buffers between tidal waters and adjacent land-uses to help control surface runoff and thus control water pollution.

Nested Watersheds
Figure 6. Nonbasin and Basin Drainage Areas
(Source: Marsh, 1998, p. 170)

Stream Functions, Watersheds, and Urbanization

Stream systems have many functions. They provide habitats for aquatic organisms, are important components of terrestrial ecosystems, and function to convey runoff and sediment loads out of their watersheds. A stream's load consists of three kinds of materials: dissolved materials carried in solution, fine particles held in suspension, and heavier or coarser materials pushed or bounced along the channel bottom. Over time, a stream becomes graded. That is, a balance or equilibrium is reached among channel slope (gradient), channel characteristics, available discharge, and load. Stream banks and channels are relatively stable under graded conditions. This balance is upset, however, by changes to the land cover and surface characteristics of the watershed. The urbanization of watersheds increases the imperviousness of land surfaces, alters the density of channels, and diverts much of the surface drainage to underground storm sewers. This, in turn, dramatically changes the volume of water and the amount and type of material that streams in urbanized watersheds convey. Urbanization also alters the physical configuration and stability of stream channels, reducing their value as wildlife habitats. The sections that follow will address the environmental impacts of increased watershed imperviousness as a result of urban and suburban development.


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