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
Onega, Russia town is also served by the Arkhangelsk–Murmansk rail line, which branches off in Obozerskaya railway station from the railroad between Moscow and Arkhangelsk and runs west to Onega and Belomorsk where it joins the railroad between Petrozavodsk and Murmansk. The railroad was built during World War II to secure the transport of goods from the harbor of Murmansk to central Russia. Onega is connected to Severodvinsk by a road. There are no all-seasonal roads on the left bank of the Onega River. The Onega is navigable downstream from the "selo" of Porog; there is regular passenger navigation. There is also