The Lake of Varna

Varna Lake. Click to enlarge. Varna Lake is a firth formation at the mouth of Provadiya river and it is the largest and deepest lake on the Bulgarian Black Sea coast. A constantly growing strip of sand reaching to 2 km in breadth divides the lake from the sea. The lake itself is elongated; its southern side is high and steep, while the northern one is rather slant. Its basin is covered with thick (10-30 m) slimy deposit. The bottom of the lake's deepest parts is comprised of black hydrogen-sulphide silt. Varna lake has a tectonic structure formed as a result of the sea-level elevation towards the end of the pleistocene. The topography is characterized by a number of valleys reaching from 30 to 120 m in width but larger water sources are absent. The lake's water supplies flow mainly from the Devnya springs through the Beloslav lake and some under-water springs in the western part. The bigger rivers in the area are the Devnya and Provadiya rivers and until correction works to Provadiya river have been carried out in 1945, both rivers flowed in at one place in the western part of Beloslav lake. Other smaller rivers also flow in the lake. At high waters some of them carry a certain amount of quartz sand thus forming beach strips at particular places. 

Until the start of XX century Varna lake water flowed to the sea through the high-water but shallow Devnya river (Varna river) which passed below the southern wall of Varna citadel. After the construction of Varna Port the river was drained and in 1906-1909 a navigation canal, was built through the sand strip dividing the lake and the sea. As a result the water level lowered with approximately 1.40 m and sea water flowed to the lake. Gradually, marshy grounds alongside the lake dried up and turned into agricultural lands. Later on in 1976 a new 12-m deep canal was constructed and the lake was dredged alongside its current. The 2 canals connecting the lake to Varna bay increased the exchange of water through slow currents between the two water basins. Those changes however have not influenced beneficially the total flow - approximately 4 times its volume per year. 

The lake water level is defined mainly by sea-level fluctuations and partially by the flow of Provadiya and Devnya rivers. Its temperature and salt content also depend to a great extent on the sea water flow. On the surface layer temperature fluctuations are very wide and reach up to 25C. The average annual temperature there reaches 14C and that of the bottom layer - approximately 8C. 

Surface water salt regime is characterized by spring decrease due to Provadiya river rise in that season. Salt increase in the summer and autumn months is caused by low river flow and lake water evaporation. At certain times in summer winds could also bring about a considerable amount of sea-water flow, but due to its higher temperature it is of lower density and spreads over the colder and heavier deep lake waters. Salt amount differs not only in vertical direction, but also in the different lake areas; that closer to the sea contains approximately 12 and the western part - approximately 7.

Water density varies in accordance with temperature degrees and salt amount. Average surface water density reaches 1.00766 kg/cub.dm and towards the bottom it increases up to 1.01139. Water transparency fluctuates from 0.5 to 7 m depending on the seasonal phytoplankton growth and flowing water muddiness. With the coming of the summer months bottom layers under 4-5m start emitting hydrogen sulphide due to decaying processes and intense thermal stratification. 

There are evidences that even in prehistoric times Varna lake sides have been inhabited by people. Various remains of ancient civilizations have been discovered here - flint tools, pile dwellings, dug-out canoes. The famous Varna chalcolith necropolis can be seen on the northern side in the region of Varna Western Industrial area.

 

Physical and Geographical Data

Area 17 sq. km
Volume 165 mln.cub.m
Average depth 9.5 m
Maximum depth 19 m
Fairway length 10.5 km
Beech strip length 25 km
Catchment basin 2680 km
Water level fluctuation amplitude 0.3 m
 

Average annual water balance, Mln. cub.m

Inflow Outflow
From Beloslav lake 117.4  Evaporation 15.0
From floods and streams 22.0 To Beloslav lake 14.7
From precipitation 9.0  Outflow to the sea 602.7
Sea-water 484.0
Total 632.4 Total 632.4

Special thanks to Esin Halid for the translation of this page.
 


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