Lake Vortsjarv is the second largest lake of Estonia. The lake depression was formed in the pre-glacial period, but was later transformed by the action of inland ice which partly eroded the lake wall and partly filled the depression with deposits. The present lake has existed since the Middle Holocene. The basic substratum is formed by Middle Devonian deposits, which are exposed on steep banks on the eastern shore of the lake. Elsewhere the lake shore is rather low. The bottom of the depression around the lake is covered with moraine, fluvio-glacial sand and gravel, lacustrine/alluvial sand, clay and peat. About two-thirds of the lake bottom are covered with mud lying on marl. The total volume of the mud and marl amounts to 3.6x1.0E+9 million m3.

The number of main tributaries is 18, the Vaike-Emajogi River being the most important. The outflowing River Suur-Emajogi drains into Lake Peipus, the fifth widest lake (3,555 km2) in Europe. The hydrologic feature of the lake is characterized by intensive and prolonged high water level in spring, low water level in summer and winter, and a noticeable rise of water level in autumn. On an average, one year is needed for a complete change of lake water. The whole lake is prevailingly homothermal in summer, while there is persistent inverse stratification in winter. The surface water temperature remains quite even all over the lake, local differences not exceeding 2deg C. Ice cover lasts for 135 days from November to April. Ice is the thickest in March.

The lake is situated at some distance from industrial centres, and the drainage basin is mainly used for crop production and cattle breeding. About 36% of the catchment area is covered by forests.

L. Vortsjarv has been strongly eutrophied, accumulating a great amount of biogenic substances. Oxygen deficit may sometimes take place under ice cover in the southern part of the lake and near the shore. Typical features of the lake water quality are its high buffer capacity and very large seston contents. The most important factors responsible for the present state of the lake are non-purified or only unsatisfactorily treated wastewater and excessive use of fertilizers in agriculture. Low water level brings about a rise in trophic level and is therefore dangerous. In order to keep the water level within an optimum range, it is planned to build a regulating lock at the head of the outflowing Suur-Emajogi River (1, 3).