Lake Constance is the second largest (to Lake Geneva) European pre-alpine lake both by area and volume, and the largest lake in Germany. The lake is surrounded by the provinces of Baden.Wurttemberg and Bayern of West Germany in the northern and north-eastern coast and by the territory of Switzerland and Austria in the southern coast. The lake is divided into two parts, the deep "upper lake" and the shallow "lower lake". The Rhein River originated from the Alps flows into the lake at its southeast end and flows out from the west end of the lake. The lake plays a major role as a natural reservoir to make clear the milky turbid inflowing water derived from the glaciers.

The lake basin was formed by the erosive forces of the Rhine glacier which excavated a 500 m deep valley during its last extension, about 30,000 years before the present. At the end of this glaciation, 15,000 years ago, the lake area was more than twice its present cover, including large parts of the Rhine Valley and both Lake Walen and Lake Zurich. The catchment area is 20 times the lake area, reaching to the Alpine rift in Switzerland, Austria and Italy. The annual water budget is about 12 km3, resulting in a water residence time of 4 to 5 years. The main tributary is the Alpenrhein contributing 75% of the total inflow. Both water budget and lake level reflect the seasonality of the Alps which retain snow during the winter. The lake level is therefore maximum in early summer and minimum during the winter.

In its natural state, Lake Constance was a typical oligotrophic pre-alpine lake with low concentrations of nutrients, low densities of phytoplankton, high water transparency and high hypolimnic oxygen concentrations. With an increase in human population and fundamental changes in the economy and human social behaviour, the trophic state of the lake was changed over the past four decades. At present, the 1.5 million people living in the catchment area, local industry and agriculture together discharge sewage water equivalent to 3.2 million inhabitants. Until the early 1970's, the major part of this sewage entered the lake without any treatment, resulting in a strong increase of the P-load, additionally enhanced by increasing inputs from agricultural sources and from precipitation. At present, 75% of the sewage phosphorus is chemically precipitated in treatment plants. Further reduction of the P-load is expected after completion of the sewage purification programme in large treatment plants. Depending on the remaining P-load, new lower equilibrium concentrations will be reached in the lake. Thus, this provides an example of a successful restoration programme beyond national borders (Q, 1).