What is a Watershed? - Definition, Facts & Protection

Instructor: Laura Nappi
Have you ever wondered what happens to rain after it hits the ground? In this lesson, we will trace the amazing journey a raindrop takes after it falls on land. You will learn a definition of a watershed and explore examples of how watersheds are protected.

Exploring the Journey of a Raindrop

Imagine you were able to hop on and ride a rain drop. Let's explore the journey a raindrop takes once it hits land. First, the rain drop crashes into the ground and connects with other raindrops. Some raindrops quickly soak into the ground. The raindrops not soaked up race downhill toward a nearby river. As the raindrops roll over plants, their speed slows down. Some of the raindrops are gobbled up by these thirsty plants. The raindrops not gobbled up by plants, will continue moving downhill and finally reach a river. In the river, they will join other raindrops that also traveled downhill. Soon after entering the river, the raindrops will enter the crashing waves of the ocean.


This area of land raindrops roll over is called a watershed. A watershed, also known as a drainage basin, is an area of land where all water drains to a central point like a lake, river, or stream. When rain sweeps over a surface, it will eventually make its way to that central point. The speed the water drains to the central point depends on various factors such as type of soil, amount of plant life, and the steepness of the terrain. The boundary of a watershed is drawn by the natural landscape, such as hills or mountain ridges. Gravity pushes water from the higher mountains toward the lower river.

Mountain ridges form the boundary of a watershed


There are different sizes of watersheds. Let's start big. In the United States, the Great Continental Divide separates the Western states from the Eastern states. This divide is also known as the Rocky Mountains.

Watersheds of North America
Watersheds of NA

The direction water flows off the divide depends on which side of the mountain the rain hits. Water falling on the Western side eventually reaches the Pacific Ocean. On the contrary, water falling on the Eastern side eventually reaches the Atlantic Ocean. Rain might first soak into the ground and slowly reach groundwater. Overtime, groundwater will seep out feeding a stream. This stream will then flow into a river. And this river will eventually reach the ocean.

Water in these containers each flow to a different ocean

The Pacific Ocean watershed is a enormous area of land. There are smaller watersheds nested inside of this larger watershed. These smaller watersheds are like puzzle pieces that are all connected forming the larger Pacific Ocean Watershed. For example, nested in the Pacific Ocean Watershed is the Columbia River Watershed and the Sacramento River Watershed. Notice that often watersheds are named after the major river that water drains to.

Columbia River Watershed comprises of numerous states and part of Canada, Photo by Kmusser
columbia river

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