A watershed in simple terms is all the land drained by a river and its tributaries. In fact, nothing is simple about a watershed. It is a complex system of natural elements and the effects of human activities upon these elements.
The watershed is shaped by the contours of the land, its soil covering, and the patterns of water drainage. The drainage system is the movement of surface water in ditches, streams and rivers, and the collection of still water in ponds, lakes and marshes. It is also the movement of underground water as rain and snowmelt percolates through the earth's surface to move downhill or collect in gravel aquifers. Groundwater may well up to the surface as springs in low areas or stream beds. Groundwater aquifers may also be replenished by absorption from wetlands.
Water movement, quality and quantity are affected by external forces such as climate, and by vegetation that filters water flowing from the land to streams and lakes. Forests and marshes also moderate storm flows by slowing and retaining heavy seasonal rains and snowmelt.
Watershed life includes myriad small wetland and forest creatures that modify the system by their roles as grazers, predators and scavengers. They consume nutrients as they feed, and provide nutrients to other parts of the system by their death. In a fine balance, the continuous interaction between all parts of a healthy system allows streams and rivers to be healthy and self cleaning.
Human activity changes many parts of the equation. Human impacts include forest removal, changes in drainage patterns, and draining or paving of large areas where water previously was absorbed into the ground. Water quality in the rivers and wetlands is affected by contaminants entering the waterways. Unbuffered drainage channels and storm water drains allow silt, bacteria and chemicals to enter the river. Streams, previously cooled by upwelling groundwater, become warmer when silt laden streambeds do not allow the passage of spring water. Bacteria and algae grow vigorously in warm water, and beneficial water lifeforms, such as insects and fish, are not able to live in, or ameliorate the unhealthy conditions.
Groundwater aquifers are pumped steadily to provide an abundant supply of water to watershed residents, although there is not always a realization that recharge areas and wetlands at some distance may be the source of the vital water supply. As suburbs move further from the river, recharge areas may be lost to drainage or construction. Watershed residents often fail to recognize their connectivity to the watershed system, until there is a serious and obvious problem.
A watershed is the sum of all the living and non-living parts. The health of the watershed reflects directly on human health and quality of life.