The Lake Zurich consists of two parts, a longer one with a length of about 28 km and a smaller one with a length of about 12 km. The lake lies in a valley formed by a glacier in the last ice period. With the state capital, the city of Zurich at the end of the lake, hilly shore areas offer favorable sites to live. Owing to the sewage discharge from this area, the lake became eutrophic at the beginning of this century. With the construction of sewage treatment plants, which include mechanical, biological and chemical steps in the form of phosphorus flocculation and flocculation filtration for all the wastewater in the catchment area, the loading of organic material and phosphorus compounds has been reduced, and the trophic state has tended toward mesotrophic. For one hundred years Lake Zurich water has been used as an important drinking water resource for Zurich and for various villages in its surroundings. The lake has a great importance also as a large recreation area.