What is a Watershed?
Originally published May 19, 2021
In simple terms...
A watershed is an area of a land where all the water within its border will eventually drain into a common outlet. Let’s dive into that on a small scale first, with an example we’re all familiar with.
If you live in a place that gets a lot of rain, you likely have a slanted roof on your home. There are two sides to the roof, but some rain falls down one side and some down the other. Although both sides have their own gutter, the water eventually winds up channeling into one common drain. Some rain water may fall down one side and some rain down the other, but in the end all of the water collect in the same place. This is exactly how a watershed works; any water that exists within the a single watershed will eventually meet together in one final outlet.
The big and small of it
Although your home is likely not its own watershed (with possibly the exception of Frank Lloyd Wright's Fallingwater house), everyone does live within a watershed. Oregon boasts around 90 watersheds across the state. The healthy rivers, streams, forests, and wetlands within a watershed help keep our homes and neighborhoods safe and livable. They help prevent flooding and protect a city’s infrastructure. And, they provide a place for fish and other wildlife to live and thrive. Watersheds may be small or large, but topography and gravity is ultimately what determines the borders of them. Much like the peak of a roof determines which gutter rainwater falls, ridgelines do the same thing. This doesn’t just happen with rainfall, though. Existing water features such as creeks and rivers, as well as groundwater underneath the surface, will also eventually drain to the same place as the rainwater will. Sometimes, this end point may be a lake or a basin. Sometimes it’s a bay, ocean, or even an aquifer. Think of all the land within a watershed as one big funnel, eventually draining into one common location.
The largest watershed in the United States is the Mississippi River watershed, which encompasses 1.15 million square miles across 31 states and two Canadian provinces, stretching from the east side of the Rockies to the Appalachian Mountains. That said, there can be many small watersheds within larger ones. For example, the Willamette watershed encompasses much of northwest Oregon, stretching from Portland all the way past Eugene and to the east towards Bend. However within that watershed there are much smaller ones too, such as the Tryon Creek watershed in Portland, which is only 6.5 square miles. The Tryon Creek watershed encompasses all the land whose water eventually ends up in the creek, but Tryon Creek isn’t the last stop in the grand scheme of things, so it can be a small fragment of a larger whole. Having multiple scales to look at watersheds is helpful for restoration and conservation efforts in local communities, as well as big picture restoration projects by large federal agencies.
