Every place on the earth is a part of a watershed. Homes, farms, ranches, forests, small towns, big cities and more are within a watershed. The concept of watershed is an important one because it pertains to everyone - no matter where someone lives, they live in a watershed. A watershed is defined as a region of interconnected rivers and streams which functions as a system for water transport. It is an area of land that drains all the streams and rainfall to a common outlet such as a reservoir, mouth of a bay, or any point along a stream channel. Watersheds have surface water (e.g., lakes, streams, reservoirs, and wetlands), and underlying ground water.
Watersheds come in all shapes and sizes, - some are millions of square mile while others are only a few acres. They cross county, state, and even international borders. A watershed is comprised of many living and non-living things. Larger watersheds contain many smaller watersheds. The area of land that drains water to the outflow point is the watershed for that outflow location. The stream-flow and the water quality of a river are affected by things, human-induced or not, happening in the land area "above", or upstream of the river-outflow point.
Water that is polluted anywhere in the watershed, even if it is hundreds of miles away, will eventually flow into the mouth of the river or stream. It is therefore essential for the water within the transport system to be kept clean, which will help to maintain a healthy environment for the plants and animals that live within the watershed. The benefits and services provided by healthy watersheds are numerous and include reduced vulnerability to invasive species, climate change, future land use changes and the protection of our drinking water. Healthy watersheds with natural land cover and soil resources also provide carbon storage capabilities, offsetting greenhouse gas emissions. Healthy watersheds provide habitat for mammals, fish, amphibians, birds, and insects, all contributors to a balanced ecosystem.