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 the 1970s and is expected to become part of the Belkomur project — a railway line connecting Arkhangelsk via the Komi Republic with the Perm Krai and the Ural mountains. Almost the entire rail network belongs to the Northern Railway, which west of Onega connects to the Oktyabrskaya Railway. There is also a railway line from Severoonezhsk west to Yangory (an extension of the line from Puksa to Navolok), which belongs to the Department of Corrections. A big number of narrow gauge railways have been built in the 1950s and 1960s to facilitate the transport of timber, but since then