Lake Onega is relatively cold, with temperatures below for half of the year and average summer temperatures of about . The catchment area of drains into the lake via 58 rivers and more than 110 tributaries, including the Shuya, Suna, Vodla, Vytegra and Andoma. The only outgoing River Svir, which marks the southern boundary of Karelia, runs from the southwestern shore of Lake Onega to Lake Ladoga and continues as the Neva River to the Gulf of Finland. The White Sea–Baltic Canal runs through the lake from the White Sea to the Baltic Sea. The Volga–Baltic Waterway connects Onega Lake with the
Arkhangelsk Oblast of the Northern Dvina and the Onega. The railroad construction gave the momentum to the population and exploitation of these areas. A branch from Konosha eastwards to Kotlas and further to Vorkuta was constructed in the 1940s to facilitate the transport of coal from the Komi Republic. From Kotlas, another branch continues south to Kirov. A branch from Obozersky to the west, to Onega and further to Belomorsk, was built during World War II to secure the transport of goods from the harbour of Murmansk to central Russia. A piece of railroad between Arkhangelsk and Karpogory was also built in