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What is a watershed?

Mississippi River watershed and Lake Itasca subwatershed

A watershed is all of the land and water areas that drain toward a particular lake or river segment. Thus, a watershed, also called a drainage basin, is defined in terms of each selected lake or river.

Watersheds can be identified on different scales. Large scale watersheds are composed of smaller areas called subwatersheds. For example, the Mississippi River has an extremely large watershed, encompassing most of the central United States, including all or part of 31 states. In contrast, Lake Itasca in north central Minnesota has a relatively small watershed. As the source of the Mississippi River, Lake Itasca's drainage basin is considered a subwatershed of the entire Mississippi River basin.

The Minnesota Department of Natural Resources uses the following terminology to identify watersheds on different scales: regions, subregions, basins, subbasins, watersheds, and subwatersheds.

Topography determines watershed boundaries

A watershed can be defined as the area of land that drains to a particular point along a stream. Each stream has its own watershed. Topography is the key element affecting this area of land. The boundary of a watershed is defined by the highest elevations surrounding the stream. A drop of water falling outside of the boundary will drain to another watershed.

See basins and major watersheds in Minnesota.

Why should I care about watersheds?

Clean water matters to all of us! It's important because we depend on water for drinking, as well as for recreation and irrigation. In addition, fish and wildlife depend on clean water. In Minnesota, fishing and hunting, plus other water-based recreation, are an important part of our economy. Studies have also shown that water clarity is linked to property values: as water clarity declines, so does the value of the properties surrounding the lake.

How does all this relate to watersheds? The water quality of lakes and rivers is affected by activities upstream or upland of the waterbody within the watershed so it is important to know the geographic area encompassed by the watershed surrounding your lake. Especially important are the watershed processes that affect how water, sediment, and other materials get transported downstream. Looking at both natural processes and human influences from a watershed perspective is vital for dealing with concerns such as lakes that are unsafe for swimming or declining fish stocks.

Actions on the land, impacts in the water

Hydrologic cycle

Water moves through a watershed according to the hydrologic cycle. Water on the surface of the land can either flow downstream through the watershed, it can soak into the ground, it can evaporate into the atmosphere, or it can be taken up and used by plants or animals. Human impacts to that process can change the water quality (how clean it is), the water quantity (how much there is), or the timing (when it moves through, especially seasonally).

As an example, imagine that Hypothetical Lake is suffering from declining water clarity. Data collection shows that too much sediment is coming into the lake. To decrease the amount of sediment, resource managers must consider what is happening upstream in the river that runs through the lake. Likewise, they must also consider what land uses upstream might be allowing excess sediment to get into the river. Everything happening upstream in Hypothetical Lakes' watershed can have an impact on the lake. In most cases, however, the land uses in neighboring watersheds won't have an impact on Hypothetical Lake.

Read a watershed story – From Farmer to Shrimper: The Dead Zone

Watch the video After the Storm, highlighting three watershed case studies

What are the sources of water to lakes and streams?

Water runs over land surfaces (overland flow) and into stream channels when it rains, when snow melts, or during irrigation. Water also seeps into and through the soil and underground (groundwater flow). Groundwater does not necessarily always follow the same watershed boundaries as surface water. In fact, groundwater flowing into lakes can sometimes originate from outside the watershed. Plants intercept water on their leaves and take up water from the soil. Land uses can alter the natural flows of water. Impervious surfaces such as parking lots, roads, and rooftops increase the speed and amount of stormwater flow into lakes and rivers.

Who can I contact if I have questions or a problem related to watersheds?

Additional resources about watersheds

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