What you throw in the trash, pour down the drain, and put on the ground can pollute the water. One quart of oil contaminates 250,000 gallons of water. About 250 million people are using the same water resources that 4 million people did 200 years ago. It is the land area from which water, sediment, and dissolved materials drain to a common watercourse or body of water. Land and water are linked directly by the water cycle.
Water comes to the watershed in seasonal cycles, principally as rain or snow.
Some precipitation infiltrates the soil and percolates through permeable rock into groundwater storage and recharge areas called aquifers . Natural ground water discharge is the main contributor to streamflow during dry summer and fall months. Water quality is largely determined by the soils and vegetation in a surrounding watershed. Accordingly, human activities have pronounced impacts on watershed quality. Wetlands are natural recycling systems that cleanse nutrients and heavy metals from runoff water before emptying into lakes and streams.