The Varna Lake, a lagoon formed at the mouth of the Provadijska River, is the biggest lake on the Bulgarian Black Sea coast. It is separated from the sea by a continuously enlarging sand strip 2 km in width. The southern bank of the lake is high and steep, while the northern bank is slanting. The valley bottom in which the lake is located is covered by a thick (10-30 m) mud deposit.

The lake is a tectonic unit formed by the rise of sea-level at the end of Pleistocene. The nearby district is characterized by the occurrence of many fault valleys, 30-120 m wide and favored with rich sources of water.

Anavigation channel was constructed across the sand strip in 1909, resulting in the invasion of sea water into the lake. Later on in 1976, the lake was dredged along the river course to put another new navigation route into operation, which is 12 m deep with underwater bank slope. The linkage of the lake with Varna Bay by the two channels enhanced the exchange of water by generating slow currents between the two water bodies, and caused changes in the thermal regime, water quality and ecosystem of the lake.

The lake water level is mainly determined by the change of sea level, and partly by the inflow from the Provadijska and the Devnja River. The salinity of lake water is 11-12 on an average, and the density increases downward from the surface value of 1.0076 to 1.0114 in bottom water. H2S gas is released from bottom layers under 4-5 m depth in summer, owing to decomposition processes and strong thermal stratification.

The completion of the new channel led to the establishment of a large industrial and electric power generation complex and a railway-port-ferry boat centre on the northern bank of Lake Varna. Their impacts on the lake environments are now under study, and some changes such as the increase of anthropogenic sediments have already been recorded. A large amount of silt, containing rich organic matter, pesticides and other pollutants, are supplied via the channels to the bay causing eutrophication and spoiling sand beach recreational areas (2-6, 9).