Watersheds 101

What is a watershed?

A watershed (sometimes called a drainage basin or simply “basin”) is an area of land in which all water that falls on that land drains to a common waterway or destination. The topography of the landscape (including hills, valleys, swales, and other land features) determine watershed boundaries, because all water naturally flows downhill. Water can travel as stormwater, groundwater, or along streams to reach this common destination.

In its simplest form, a watershed can be thought of as a bathtub. All shower water (think: rain) that lands in the tub (think: the land) flows to one place- the drain (think: common destination). Any objects you place in the tub become part of the tub’s watershed, because shower water that lands on those objects also flow to the drain. In the landscape, these objects include buildings, cars, trees, people, and anything else that rain may fall on. Any water that lands outside the bathtub would not be in the tub’s “watershed.”

How big is a watershed?

Watersheds come in many different sizes, and it all depends on the size you are interested in. A puddle in your backyard technically has its own watershed. All of Blair County is part of the Chesapeake Bay Watershed. Its also part of the Susquehanna River Watershed. Most of it is in the Juniata River Watershed. Some of the County flows into the Little Juniata River, and those areas are part of that watershed.

Your local stream has its own watershed made up of all the land that drains into it. The Plum Creek watershed in Blair County, for example, encompasses 17.4 square miles. The Susquehanna River also has its own watershed, made up of all the land and smaller rivers that drain into it. The Susquehanna River Basin drains 27,510 square miles. The Chesapeake Bay watershed, which includes the Plum Creek watershed, the Susquehanna River Basin, and many others, covers 64,000 square miles! Watersheds can be smaller, larger, or anywhere in between these sizes depending on the body of water they flow into.

Since the Susquehanna River Basin is one of many that drains into the Chesapeake Bay, the Susquehanna River Basin is called a “sub-watershed” of the Chesapeake Bay. Likewise, the Juniata River watershed (which includes most of Blair County) has many sub-watersheds of its own. You can use the interactive mapping tool on the homepage of this website to find out which sub-watershed you live in.

Why is it important to understand how watersheds work?

No matter where you live, you live in a watershed. Many watersheds are sub-basins of larger watersheds, which means that they are connected. How can water that falls in Blair County be connected to water bodies far away?

A river carries water away from its source, often over vast distances. Therefore, rivers affect everything “downstream,” or further down the river, from them. Water that enters a stream in Blair County will flow into the Juniata River, which will flow into the Susquehanna River, which will eventually flow into the Chesapeake Bay. This also means that water flowing off of your yard or driveway flows into a storm drain, which flows to the local creek, which flows into a branch of a river, and eventually impacts everyone downstream from you.

The steps taken to protect clean water on your property will also help keep water clean for your downstream neighbors and the communities downstream from you. Your upstream neighbors can also help clean the water that runs through your community.

A healthy watershed benefits everyone: watersheds provide us with drinking water, food, recreation, irrigation, and valuable wildlife habitat. However, protecting a watershed can be complicated, because watersheds cross municipal, county, and state lines. The Juniata River watershed encompasses 12 counties and 200 municipalities, while the Chesapeake Bay watershed crosses 6 states. Keeping a watershed healthy therefore requires the efforts of everyone who lives and works there- residents, officials, developers, and businesses alike. Visit the Roles page to find your role in the community and learn what you can do to protect your local watershed.

How does stormwater impact watersheds?

Since water bodies are connected to everything “downstream” of them, stormwater that falls anywhere in the Chesapeake Bay watershed eventually reaches the Chesapeake Bay. The fertilizers, pet waste, trash, oil, pesticides, etc. that are picked up during a rainfall in Blair County join up with the stormwater from the rest of the watershed and directly influence the health of the bay. The bay’s water quality, habitats, and wildlife populations have suffered as a result. Small actions taken by people all over the watershed to manage their stormwater will add up, resulting in cleaner local waterways and a healthier, productive Chesapeake Bay.