A water what? A watershed, or drainage basin, is the area of land that drains to a particular body of water, such as a stream, river, lake or ocean. The topography of the landscape determines where the water flows with surrounding ridgelines defining the boundaries of the basin.
A tributary creek which joins a small stream, which is tributary of a larger river, is thus part of a series of successively smaller area but higher elevation “nested” basins. Arrows indicate the direction of waterflow into the basin.
Larger watersheds contain many smaller watersheds. For instance, each mountain stream that flows into Big Cottonwood Creek has its own watershed, and when combined these streams are all part of the larger basin that drains to Big Cottonwood Creek. In turn, the watersheds for Mill Creek, Big Cottonwood Creek, Rose Creek, etc. are all part of the greater Jordan River Watershed. At an even larger scale, the Jordan River Watershed is part of the Great Salt Lake Basin. The Great Salt Lake is a closed basin; the lake is the lowest point in the landscape and does not have an outlet. Many regional scale watersheds in the United States will ultimately drain to the ocean.
When it comes to protecting nature and controlling pollution, it’s beneficial to think in terms of watershed-scale planning. That’s because everything that happens within a watershed may ultimately affect the water quality of the stream, river or lake at the bottom of the basin. Pollutants on the ground are picked up by storm runoff and make their way into our streams and rivers.
Runoff in developed areas, where there are many more impervious (non-porous) surfaces like rooftops and pavement, will flow faster and pick up more pollutants as compared with precipitation that falls on undeveloped land. As urbanization continues, more impervious surfaces lead to ever growing volumes of runoff dumping into our streams and rivers. This results in a higher probability of erosion, degraded water quality, impacts on fish and wildlife, flooding, and property damage.
Watersheds often cross city, county, and state boundaries, posing challenges to watershed planning and protection. Stream restoration, educational outreach, use of best management practices (BMPs), and collaboration across watershed boundaries are tools used to protect watersheds.