Much of the area of the oblast belongs to the drainage basin of the Neva, which is the only outflow of Lake Ladoga. The Neva, which flows to the Gulf of Finland (the city of Saint Petersburg is located in its river delta) is relatively short, but its drainage basin is very large, including Lake Onega and Lake Ilmen. The Svir and the Volkhov flow from Lake Onega and Lake Ilmen, respectively, to lake Ladoga. Other major tributaries of Lake Ladoga include the Vuoksi and the Syas. Rivers in the western part of the oblast flow to the Gulf of Finland; the two biggest rivers there are the Luga and the Narva, which forms the border of Russia and Estonia. Small areas in the east of the oblast lie within the river basin of the Chagodoshcha, a tributary of the Mologa, and of the Suda, both within the Volga basin. A ridgeline in Tikhvinsky District in the eastern oblast forms part of the divide between the Baltic Sea and Caspian Sea basins.