In simple terms, a watershed is an area of land that encompasses all the water that flows from the tops of the mountains and into a drainage area, such as a lake or the ocean. Water either flows over the surface of the land in streams and rivers or seeps into the ground to flow underground and recharge groundwater aquifers.

Water flows originating at the mountain tops define the watershed’s upper boundaries. At their base, watershed boundaries are defined by the drainage area they flow into (usually the ocean but it can also be a lake) and include all water sources that drain into this area.

Groundwater aquifers are part of the watershed area, however their boundaries may not coincide directly with the surface water flows. Established watersheds are separated from one another by the height of the land; these are known as "drainage divides".

These boundaries do not typically align with political boundaries, which makes for a complicated and complex structure when it comes to watershed management.

Watersheds are nested systems of large and small streams and aquifers that ebb and flow, combine and separate through-out the watershed, essentially nourishing and connecting ecosystems within.

The geology of the watershed, combined with several other characteristics such as the nature of the soils, types of vegetation within the watershed, and land-use features, make each watershed unique.

When watersheds are healthy they supply water, habitat, and food sources for native plants and animals. By moving sediment downstream they cycle nutrients and convert them into forms that living organisms can use. They even influence air quality by absorbing pollutants. This is why it’s vitally important that we keep our watersheds healthy.