What is a Watershed?

we all live in a watershed!

A watershed is a land area that collects and channels rainfall and snowmelt into creeks and streams. This water then combines with other rivers and streams that progressively drains into a larger water area.

Waterways within a watershed all feed into a main body of water which could be a river, lake or stream. The beginning of a water source is called the headwaters. The area where the headwaters progressively join other water sources is called the confluence, and the endpoint of the waterways that open into the main body of water is called the mouth.

While some watersheds are relatively small, others encompass thousands of square miles and may contain streams, rivers, lakes, reservoirs and underlying groundwater that are hundreds of miles inland.

Water from hundreds (and often thousands) of creeks and streams flow from higher ground into rivers that eventually reach a larger waterbody. As the water flows, it often picks up pollutants, which may have negative effects on the ecology of the watershed and ultimately on the reservoir, lake or ocean where it ends up.

When rain falls on dry ground, it can soak into or infiltrate the ground. This groundwater remains in the soil where it will eventually seep into the nearest stream. Some water infiltrates much deeper, such as an underground reservoir called aquifers. In other areas where the soil contains a lot of hard clay, very little water may infiltrate, and it quickly runs off to lower ground instead.

Rain and snowmelt from watersheds travel many routes to a lake or sea. During periods of heavy rain and snowfall, water may run onto and off of impermeable surfaces such as parking lots, roads, buildings and other structures because it has nowhere else to go. These surfaces act as "fast lanes" that transport the water directly into storm drains. The excess water volume can quickly overwhelm streams and rivers, causing them to overflow and possibly result in floods.

How is a Watershed Important?

Watersheds directly affect water quality and the communities around them. A freshwater ecosystem is not an isolated body of water but linked to all the other watersheds that they are a part of, and one watershed ecosystem can greatly affect another.

Urban development often involves the removal of trees and native plants, the rearranging of topography and altering naturally-formed drainage networks. Instead of being absorbed by soil and plants, having the time to seep deeply into the ground or naturally flowing into a stream, rainwater is sent into a “fast lane” off of roads and buildings.

Fast lanes and stormwater runoff carries the threat of land erosion and habitat loss. Water flowing through a stream naturally picks up dirt along the way. However, if fast-moving water picks up enough soil over time, this can result in severe river bank erosion and possible failure.

As water runs over roofs and roads, it also picks up toxins. This runoff can contain heavy metals, oils, pesticides and fertilizers which become harmful water pollution that travels long distances in our streams, rivers and lakes.