ONEGA, next to Ladoga the largest lake in Europe, having according to Strelbitskiy an area of 3763 square miles, is situated in the heart of the government of Olonetz miles. Between the northern and southern divisions of the lake there is a considerable difference : while the latter has a comparatively regular outline, and contains hardly any islands, the former splits up into a number of inlets and is full of islands and submerged rocks. It is thus the northern division which brings the coast-line up to 860 miles and causes the navigation of the lake to be so dangerous that previous to 1874, when additional buoys and beacons were laid down, the loss of life from shipwreck was about eighty persons per annum. The north-western shore between Pctrozavodsk and the mouth of the river Lumbuzha consists of dark clay slates generally in horizontal strata and broken by raised parallel bands of diorite. These bands extend far into the lake and are locally known as "hogs' backs." The eastern shore (as far as the mouth of the Andoma) is for the most part alluvial, with outcroppings of red granite and in one place (the mouth of the Pyalma) diorite and dolomite. To the south-east arc sedimentary Devonian rocks, and the general level of the coast is broken by Mount Andoma and Cape Petropavlovskii (160 feet above the lake) ; to the south-west a quartzy sandstone (well known as a building and monumental stone in St Petersburg) forms a fairly bold rim. Onega lies 236 feet above the sea. Towards the centre of the southern section a considerable area is upwards of 165 feet deep, and at one place a depth of 738 feet has been reached. The most important affluents, the Vodka, the Andoma, and the Vuiterga, come from the east. The Kumsa, a northern tributary, is sometimes represented in maps as if it connected the lake with Lake Seg, but the latter drains to the White Sea, and proposals to restore by means of a canal the communication which formerly existed here between the Arctic and Baltic basins have not yet been carried out. Lake Onega remains free from ice for 209 days in the year (middle of May to second week of December). The water is at its lowest level in the beginning of March ; by June it has risen 2 feet. A considerable population is scattered along the shores of the lake, mainly occupied in the timber trade, fisheries, and mining industries. Salmon, palya (a kind of trout), burbot, pike, perchpike, and perch are among the fish caught in the lake. Steamboats were introduced in 1832.

It is to be noted that the river Onega, which after a course of about 260 miles reaches the Gulf of Onega, an inlet of the White Sea, has no connexion with Lake Onega. At the mouth of this river (on the right bank) stands the district town and port of Onega (2275), which dates from settlements made by the people of Novgorod in the 15th century, and known in history as the Ustenskaya or Ustyanskaya volost. It has a cathedral (St Michael and the Holy Trinity), erected in 1796.