What is a watershed?

Freshwater streams and rivers are the lifeblood of our natural world.

The land area above and around any waterway or body of water, and draining into it, is called a "watershed". A watershed can be smaller or larger, encompassing a single waterway, or several converging waterways. Thus one watershed can nest within another, with one small stream's watershed a part of a larger regional watershed containing many such smaller units.

The zone along freshwater streams and rivers - which biologists call the "riparian corridor" - is among the richest for all forms of life, as the stream carries water essential to habitat for any living organisms, from microscopic plankton to the largest mammals. Vegetation grows most densely in the riparian zone. Animals nest here, seek shelter here, drink and eat here, and use these corridors to move and migrate across the earth.

What we have learned - and are still learning - is that everything we do above and around our waterways affects the water and the organisms living in and around it, and passing through it. To effectively protect our water, we need to focus on protecting the whole watershed.

Watersheds and the water cycle

Watersheds are the setting for the essential ecological process of the water cycle. Trees and other vegetation capture, slow, and filter rainwater as it falls to the ground. Some of this precipitation moves across the surface to drain into a nearby creek; some percolates downwards through the soil to replenish underground water tables and springs; some is evapo-transpired by trees right back into the air, replenishing and stabilizing atmospheric moisture. Riparian vegetation helps to filter pollutants carried by surface water and groundwater, while absorbing some nutrients and holding soil in place to prevent erosion and sedimentation into streambeds. This vegetation moderates flood "pulses" by slowing drainage and reducing the volume of surface waters.

Streams are replenished by the groundwater and surface-water inputs. Streamwaters flow from their small headwater reaches, down through hills, meandering where they can, providing spawning grounds for fish and life to a host of organisms that support the entire food chain. Smaller streams join up into larger streams or rivers, and enter bays through marshes and estuaries, where more pollutant filtering takes place, birds nest and feed, and fish fingerlings find shelter as they grow into their adult phases. Whatever remains in the waters here will find its way into the bay waters, and then to our open oceans. Constant evaporation brings water back to the atmosphere, where the cycle starts again.

Thus our watersheds with their creeks and streams, from the smallest tributaries to the largest rivers, form the basis for the health of our bays and oceans - and of our water quality and habitat overall.