A watershed is a connected series of streams, rivers, and lakes that collects water from a specific area of land. Watersheds are important habitats for animals and plants, and offer a source of drinking and recreational water for many communities. The quality of the water in a watershed, also sometimes referred to as the health of the watershed, is important to preserve or remediate (to repair after damage). Water quality generally refers to the appearance, smell, and, above all, ability to serve as drinking water (a quality known as potability).

At the beginning of the twentieth century, most watersheds were unaffected by man-made pollution; pollution occurred only from natural sources such as animal waste. Before increased levels of man-made contamination, most watersheds were able to dilute pollutants or filter them out through surrounding wetlands (grassy areas that hold water throughout most of the year). Today, pristine and uncontaminated...