Lake Onega - Topography and Hydrography

Topography and Hydrography

Lake Onega has a surface area of 9,700 km² without islands and a volume of 280 km³; its length is about 245 km and width about 90 km. It is the second largest lake in Europe, and the 18th largest lake by area in the world. Its southern banks are mostly low and continuous, whereas northern banks are rocky and rugged. They contain numerous elongated bays shaping the lake into a giant crayfish. In the northern part lies a large Zaonezhye Peninsula (Russian: Заонежье); south of it, is the Big Klimenetsky (Russian: Большой Клименецкий) island. To the west of them lies a deep (deeper than 100 m) Big Onego (Russian: Большое Онего) area with its Kondopozhskaya (Russian: Кондопожская губа, depth up to 78 meters), Ilem-Gorskaya (42 m), Lizhemskoy (82 m) and Unitskoy (44 m) bays. To the south-west of the Big Onega lies Petrozavodsk Onego (Russian: Петрозаводское Онего) with its large Petrozavodsk and small Yalguba and Pinguba bays. To the east of Zaonezhye there is a bay, northern part of which is called Povenetsky Bay and the southern part is Zaonezhsky Bay. There, deep sections alternate with banks and islands which split the bay into several parts. The southernmost part of them, Small Onega, is 40–50 meters deep. All shores there are stony.

The average depth of the lake is 31 meters, and the deepest place (127 meters) is located in the northern part. The average depth is 50–60 meters in the center and rises to 20–30 meters in the southern part. The bottom has a very uneven profile, it is covered with silt, and contains numerous trenches of various size and shape in the northern part. The trenches are separated by large shallow banks. Such bottom structure is favorable for fish, and the banks are used for commercial fishing.

The water level is stabilized by the Verhnesvirskaya hydropower plant and varies by only 0.9–1.5 meters over the year. It rises due to the spring flood which lasts 1.5–2 months. The highest water level is in June–August and the lowest is in March–April. Rivers bring 15.6 km³ of water per year to the lake, that is up to 74% of the water balance; the rest is provided by precipitation. Most of the lake water (84% or 17.6 km³/year) outflows via a single River Svir, and the remaining 16% evaporates from the lake surface. There were frequent storms more characteristic of a sea than a lake; waves of 2–3 meters are not uncommon and may even reach 5 meters. The lake freezes near the coast and bays in late November–December and around mid-January in its center. Sawing starts in April in the tributaries and reaches the lake in May. Water in the deep parts is clear, with the visibility up to 7–8 m. In the bays, the visibility may decrease to about a meter. The water is fresh, with the salt content of 35 mg/L. This is relatively low for a lake and is about 1.5 times lower than in another large lake of the area, Ladoga Lake. Maximal surface water temperature is 20–24 °С on the lake and even 24–27 °С in bays. The deep waters are much colder, from 2–2.5 °С in winter to 4–6 °С in summer. Weather is relatively cold, with temperatures below 0 °C for half of the year and average summer temperatures about 16 °C.