Lake Balaton was formed mainly by tectonic forces 12,000-20,000 years ago. Prior to the opening of Sio-canal in 1863, its water level was 3 m higher and its surface was about twice larger than at the present. With its surface area of 593 km2, Lake Balaton is the largest lake in Central Europe, but its mean depth is only 3.2 m.

The main inflow, the Zala River, empties into the southwestern end of the lake, while the Sio-canal drains the water from the eastern basin into the River Danube.

The lake is covered by ice in winter. In summer the average water temperature is 23deg C. The strong waves swirl up much sediments, rendering the transparency low.

The major ions of the water are Ca2+, Mg2+ and HCO3-. The pH is 8.4, rising to higher values during intensive primary production. Oxygen deficiency is formed only temporarily in the western part of the lake in calm summer periods with algal blooms.

The distribution of macrophytes is restricted by strong waves to a relatively narrow belt. Only 3 percent of the lake surface is covered by reeds, and even less by submerged macrophytes. The major primary producers are phytoplankton. Zooplankton is not abundant. Zoobenthos represents an important food for the fish. The annual commercial fish catch is 1200 tons.

The southern shore of the lake consists of sandy beach, while on the northern shore there are mountains of volcanic origin with old ruins on their tops and vineyards on their slopes. The picturesque landscape and the water ideal for swimming and other water sports attract 2 million tourists annually.

The sewage discharge from rapidly developing towns in the watershed, the growing use of fertilizers in agriculture and large animal farms increased the nutrient loading to the lake in the last decades. A rapid eutrophication became apparent by increased production and biomass of phytoplankton. Blooms of blue-green algae are frequent in the most polluted western part of the lake.

A eutrophication control program has been formulated, based on intensive scientific researches. Most of the municipal sewage is now diverted from recreational areas. Phosphorus removal was introduced at other sewage treatment plants. A reservoir was constructed to retain the nutrients carried by the Zala River. Pollution due to liquid manure was reduced. Construction of more reservoirs on major tributaries of the lake and a soil protection program are in progress (Q).