What is a Watershed?

“…that area of land, a bounded hydrologic system, within which all things are inextricably linked by their common water course and where, as humans settled, simple logic demanded that they become part of a community.” – John Wesley Powell, American geologist, ethnologist, explorer and government administrator

We all live in a watershed—an area of land that captures, stores, and sheds water. The water that flows from the land eventually drains to waterbodies, from which most watersheds get their names.

Montana is home to some of the world’s most interesting watersheds. For example, in Northwest Montana, the Flathead Watershed originates in Alberta, Canada in the north, flows through nearly six million acres of forests, farms, and cities, through the waters of the Flathead, Stillwater and Swan Rivers, and drains to Flathead Lake—the largest freshwater lake west of the Mississippi River. The Flathead River then serves as one of the headwater rivers of the Columbia River Basin, which drains to the Pacific Ocean in Astoria, Oregon. This international boundary-crossing watershed illustrates how upstream activities can affect downstream neighbors.

To outline a lake watershed boundary, one needs only to connect the highest points of elevation around a lake on a topographic map. Water falling within this area flows by gravity into streams and groundwater to the lake. Watershed boundaries can be quite large, so even if you can’t see a lake or river from where you are, you can be sure that you are in a watershed. Imagine a drop of rainwater landing in your yard, a few miles from a lake. It flows downhill into the roadside ditch where it travels into a culvert under the road and empties into a stream that feeds the lake. Along the way it picks up phosphorus from lawn fertilizer, motor oil from the road, and sediment from the ditch.

All of these things end up in the lake, and they all contribute to declining water quality. Now imagine what happens when you multiply this one drop by the countless number of raindrops and snowflakes that fall within your watershed each year.

The Water Cycle
Lakes are one part of the water cycle. Snowmelt and rainwater flow over the land and fill our lakes, rivers, streams and oceans. In a natural setting, water from rain and snow is cleansed and filtered by plants and soil. Some water penetrates deep into the ground to become groundwater, eventually discharging into lakes, rivers and oceans. Evaporation then starts the cycle over again. Lakes play an important role by containing, filtering and evaporating water and by recharging important underground aquifers.


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