The Vuoksi is a river running through the northernmost part of the Karelian Isthmus from Lake Saimaa in southeastern Finland to Lake Ladoga in northwestern Russia. The river enters Lake Ladoga in three branches, an older main northern branch at Priozersk (Käkisalmi), a smaller branch a few kilometers to the north of it, and a new southern branch entering 50 kilometers (31 mi) further southeast as Burnaya River, which has become the main stream in terms of water discharge. Since 1857, the old northern distributaries drain only the lower reaches of the Vuoksi basin and are not fed by Lake Saimaa. The northern and southern branches actually belong to two separate river systems, which at times get isolated from each other in dry seasons.

The Karelian Isthmus is the approximately 45–110-kilometre-wide (30–70 mi) stretch of land situated between the Gulf of Finland and Lake Ladoga in northwestern Russia, to the north of the River Neva. Its northwestern boundary is a line from the Bay of Vyborg to the westernmost point of Lake Ladoga, Pekonlahti. If the Karelian Isthmus is defined as the entire territory of present-day Saint Petersburg and Leningrad Oblast to the north of the Neva and also a tiny part of the Republic of Karelia, the area of the isthmus is about 15,000 km2 (5,800 sq mi).