What is a Watershed?

Brady Lake stream, Holy Cross Wilderness, Colorado

The US Environmental Protection Agency defines a watershed as:

"the area of land where all of the water that is under it or drains off of it goes into the same place … Watersheds come in all shapes and sizes. They cross county, state, and national boundaries. In the continental US, there are 2,110 watersheds; including Hawaii, Alaska, and Puerto Rico, there are 2,267 watersheds."

Human Impacts

Humans have greatly modified watersheds and aquatic ecosystems with often detrimental consequences for water quality. For example, some aquatic ecosystems receive too much nutrients (such as nitrogen and phosphorus), sediments, and contaminants. Excess nutrients can cause blooms of algae, which can create numerous water quality problems, especially if the blooms include algae that produce toxins.

Excessive runoff of sediments can cause lakes and rivers to fill in, thereby decreasing water storage capacity of the ecosystem and degrading habitats for fish and other organisms. Runoff of contaminants (such as herbicides) can affect aquatic ecosystems in many ways, and cause problems with drinking water supplies.

Importance of Water Resources

Watersheds and freshwater ecosystems provide important ecosystem services to people, including:

  • clean, drinkable water
  • flood control
  • fisheries
  • recreational opportunities
  • sequestration of carbon and contaminants

The economy rests on an ample supply of clean water resources, and thus deterioration of water quality incurs a large economic cost:

The cost of degradation of inland freshwaters by harmful algal blooms related to eutrophication in the USA has been estimated at $2.2 billion. *

In Ohio alone sport fisheries have an annual economic value of $1.8B ($1B from inland waters, $0.8B from Lake Erie).

* Dodds, W., W. W. Bouska, J. L. Eitzmann, T. J. Pilger, K. L. Pitts, A. J. Riley, J. T. Schloesser, and D. J. Thornbrugh. 2009. Eutrophication of U.S. freshwaters: Analysis of potential economic damages. Environmental Science & Technology 43: 12-19.

Ohio Division of Wildlife