What is a Watershed?

The specific land area that contributes water to a particular stream or river. Watersheds are sometimes called catchments or drainage basins. Adjacent watersheds are separated by drainage divides.

Rain falling on one side of a particular divide will flow into the river that drains that watershed. Watersheds can be defined at a variety of scales. The Mississippi River watershed encompasses roughly 1.5 million square miles, or 41% of the continental United States, and drains water from 31 states and two Canadian provinces.

As illustrated in the figure below, watersheds can be subdivided into smaller watersheds based on the drainage area of tributary streams.

For instance, the Missouri River Watershed is part of the larger Mississippi River Watershed and it could be sub-divided further based on its tributaries, such as the Platte River Watershed. In comparison to the Mississippi River, the watershed of smaller streams may only be a few square miles or even just a couple of acres. 

Mississippi Watershed

Image of the Mississippi River Watershed which drains most of the central U.S., courtesy of EPA.gov.


Why are Watersheds Important?

The health of a stream is directly related to its watershed. If a watershed is polluted, the land use altered, or the vegetation is modified, the stream will be affected.

For instance, excess pesticides and chemicals used to treat a farm crop will eventually make its way to the stream via surface flow or groundwater. This contamination may impact the streams ecology or the quality of a downstream neighbor’s water supply.

We all live in a watershed and by being aware of how our actions may impact a local stream, each of us can play a part in protecting that stream and our watershed.

Watershed Map

Southeastern Minnesota Major Watersheds.


How is Water Quality Measured?

Water quality is measured in a variety of ways with a variety of tools. Scientists, resource managers, and even private citizens all monitor stream water quality.

Each may use a different approach to determining the quality of the water in the stream based on the goals behind their monitoring efforts. Measuring water quality can be as simple as taking stream transparency and water depth measurements.

In Minnesota, and many other states, volunteers who live near streams conduct these sorts of measurements regularly throughout the year and then report their results to the Minnesota Pollution Control Agency. Other water quality parameters are also regularly measured by various people in streams, including dissolved oxygen content, nitrate concentrations, and fecal coliform levels.

For an explanation of what these parameters tell you about a streams health visit the National Sanitation Foundation.