Lake Ladoga is a unique natural object and the largest freshwater
lake in Europe. Almost all water flowing to the Gulf of Finland through Neva
river (74 000 106 m3) is comming from the Lake Ladoga. Furthermore, the river Neva brings three fourths of annual water entering the Gulf of Finland and together with the second largest river of the Baltic drainage area - Wisla is responsible for tow thirds of the freshwater inflow into the Baltic Sea.
The lake is the source of drinking water for St. Petersburg - one of the largest cities of the world; and is responsible for the water quality in the Gulf of Finland and, therefore, the Baltic Sea.
Natural processes in Lake Ladoga and on its shores largely depend on the size and shape of the lake basin and the watershed area (tab. 3.1.).
Table 3.1. General characteristics of Lake Ladoga
Characteristic
Numerical value
Lake area, (thousands km2)
17.9
Watershed area (thousands km2)
258.6
Maximum depth (m)
230
Mean depth (m)
46.9
Shoreline length (km)
1570
Water volume (km3)
837.9
Mean surface runoff (km3)
80.1
Retention time (years)
11
Annual water level fluctuation (m)
0.7
Mean water transparency (m)
2.3-3.9
pH
6.4-7.1
Mean concentration of nitrogen in water (mg.l-1)
650
Mean concentration of phosphorus in water (mg.l-1)
17
Mean concentration of chlorophyll "a" in water (mg.m-3)
4-6
The main islands of Lake Ladoga are Riekkalansaari (53.5
km3), Mantsinsaari (39.4 km3) and Valaam (27.8
km3). The Valaam archipelago seems to be a system of islands unique
for large freshwater lakes. It represents an intrusive body that emerged due the
border position of the lake on the Baltic shield.
The Lake Ladoga system formed where the Baltic crystalline shield and the Russian platform meet. Geologically, the lake basin can be divided into two parts northern and southern.
Throughout the Lake Ladoga drainage area the bedrock is overlain by a cover of loose quaternary formations. They are represented by complexes of glacial, aquaglacial, lacustrine-glacial, alluvial, lacustrine and bog sediments up to 75 m thick.
The topography of the Lake Ladoga watershed area is non-uniform. Its southern part, with well-developed Palaeozoic sediments, is noted for plain topography with wide-spread deposition formations and those generated by river erosion, lacustrine and marine abrasion. Absolute heights vary from 5 to 300 m. The topography of the area was formed by the last glaciation.
The south-east of the watershed is the Valdai upland with altitudes from 150 to 300 m. The south-western topography has a distinct klint 20-40 m high. Outcrops of the crystalline basement in the form of rocky ridges (selkas) are found in the Baltic shield area. A lacustrine plain now and then enclosed by face terrace benches and beach ridges stretches along the Ladoga eastern shore.
With regard to depth and bottom topography patterns Lake Ladoga
is divided into three sections separated from each other by benches (fig. 3.1.1). The first section covering the
northern part of the lake has the most complicated topography with the range
reaching 230 m, and the bottom slope - 10o. It is noted for the greatest depths
(230 m). The bottom configuration is very patchy.
The second section in the centre of the lake is occupied by the accretion plain. It mainly is made up of the Riphean sedimentary rocks. Depths here range from 50 to 100 m. The bottom shows dome- and ridge-shaped uplifts.
The third section occupies the southern part of the basin, and is shaped as a plane, mildly sloping abrasion-accumulation plain. Its depths are not over 50-60 m.
The last glaciation of the quaternary period, which development reached its maximum 10-25 millennia ago, was the decisive factor in the formation of the drainage area and the basin of the lake system. During the peak of their development glaciers covered all of the north Europe including the modern catchment area of Lake Ladoga. Ice from the central parts of the ice massive moved along several paths to the periphery modifying the topography of the underlying terrain. Thus, the Lake Ladoga system is rather young. Its formation finished and the modern appearance shaped just several millennia ago, when the area was already populated.
Geological subdivisions
of the Lake Ladoga basin:
(1) The northern part
- is situated in the southern margin of the Baltic shield and has Precambrian rocks underlying thin quaternary deposits. In this part the most ancient rocks (granites and gneisses) aged up to 2 billion years come out to the surface.
(2) The southern part
- Comprised mainly of terrigenous and carbonaceous sediments of the Russian platform cover with a notable angular unconformity on the underlying rock of the folded basement.
