Lake Lacha Lake Lacha Lake Lacha () is a freshwater lake, located in the south of Kargopolsky District of Arkhangelsk Oblast in Russia, south of the town of Kargopol. It is the largest lake in Arkhangelsk Oblast, with a surface area of and a basin area of . Lake Lacha is the source of the Onega River, one of the major waterways of the White Sea basin. The etymological origin of the name is unclear. The river basin of Lake Lacha includes the southern and western parts of Kargopolsky District of Arkhangelsk Oblast, as well as north-west of Vologda Oblast. In particular,
Lake Onega the last Ice Age, about 12,000 years ago: In the Paleozoic Era (400–300 million years ago) the entire territory of the modern basin of the lake was covered with a shelf sea lying near the ancient, near-equatoric Baltic continent. Sediments at that time – sandstone, sand, clay and limestone – form a layer covering the Baltic Shield which consists of granite, gneiss and greenstone. The retreat of the Ice Age glaciers formed the Littorina Sea. Its level was first higher than at present, but it gradually lowered, thereby decreasing the sea area and forming several lakes in the Baltic region.