Introduction to Watersheds

Have you ever passed a sign welcoming you to a watershed and wondered what it means? Here are some basic facts about watersheds to help you out.
Introduction to Watersheds - Articles

Updated: August 8, 2017

Introduction to Watersheds

What is a watershed?

A watershed is the land area that drains into a stream or other body of water. Gravity pulls water downhill from rain storms, snow melt, and even groundwater supplies until reaches the lowest point, where bodies of water are found. The boundaries of a watershed are determined by the shape of the land and generally connect all of the highest points around the body of water.

How big is a watershed?

Some watersheds are very small (less than one acre) and drain to tiny unnamed streams, like one that may flow through your backyard. Other watersheds, like the Susquehanna River watershed, are very large and may cover thousands of square miles. The tiny watersheds collectively make up the larger watersheds, just like many properties make up a neighborhood, and many neighborhoods make up a town. Any place where you stand can be part of many watersheds of varying sizes.

How many watersheds are there in Pennsylvania?

Thousands of small watersheds drain into progressively larger watersheds in Pennsylvania. Pennsylvania has six major watersheds, also known as basins. These six basin include the Ohio River, Susquehanna River, Delaware River, Potomac River, Lake Erie, and the Genesee River.

Where does the water flowing in a watershed come from?

Water in a stream during dry weather comes from groundwater, found beneath the surface and slowly emerging from springs and seeps into the stream. During a rainstorm, water running over the ground's surface or through shallow soil will contribute to the increasing amount of stream water. In the winter, snow melt also contributes to the surface water moving into a stream.

What is groundwater?

Groundwater is water stored that is underneath the ground's surface in layers of rock. The water may be stored in the pores of the rock or in cracks and fractures in the rock. Contrary to popular belief, groundwater does not usually exist in large underground lakes or streams.

How do watersheds affect the water quality of a stream?

The natural characteristics of stream water are mostly determined by the soils and bedrock geology of the watershed. Watersheds with limestone geology tend to have streams with a higher pH and more dissolved minerals. Watersheds with sandstone or shale geology usually have a lower pH and few dissolved minerals. Land use and people's activities within the watershed will further affect the stream water quality. As rain water moves across the surface of the land, it will pick up potential pollutants and carry them into the stream. These pollutants, called non-point source pollutants can include things like lawn and agricultural fertilizers, domestic animal waste, leaked or spilled automotive fluids, loose soil from construction or farming, and more.

Protecting the land in Pennsylvania is critical to maintain water quality and quantity for both human use and aquatic life. All of the land in Pennsylvania drains to our water bodies. No matter where you live in Pennsylvania, you are part of many different watersheds. Each stream, pond, lake, and groundwater aquifer has a watershed that captures and delivers water to it.