<P> The Ob's entire main course is within Russia, though its tributaries extend into Kazakhstan, China and Mongolia . The river splits into more than one arm, especially after joining the large Irtysh tributary at about 69 ° E. (Originating in China, the Irtysh is actually longer than the Ob from source to the point of their confluence .) From the source of the Irtysh to the mouth of the Ob, the river flow is the longest in Russia at 4,248 kilometers (2,640 mi). Other noteworthy tributaries are: from the east, the Tom, Chulym, Ket, Tym and Vakh rivers; and, from the west and south, the Vasyugan, Irtysh (with the Ishim and Tobol rivers), and Sosva Rivers . </P> <P> The Ob zigzags west and north until it reaches 55 ° N, where it curves round to the northwest, and again north, wheeling finally eastwards into the Gulf of Ob, a 1,000 - kilometre - long (620 - mile) bay of the Kara Sea, separating the Yamal Peninsula from the Gydan Peninsula . </P> <P> The combined Ob - Irtysh system, the fourth - longest river system of Asia (after Yenisei, and China's Yangzi and Yellow rivers), is 5,410 kilometres (3,360 mi) long, and the area of its basin 2,990,000 square kilometres (1,150,000 sq mi). The river basin of the Ob consists mostly of steppe, taiga, swamps, tundra, and semi-desert topography . The floodplains of the Ob are characterized by many tributaries and lakes . The Ob is ice - bound at southern Barnaul from early in November to near the end of April, and at northern Salekhard, 150 km (93 mi) above its mouth, from the end of October to the beginning of June . The Ob River crosses several climatic zones . The upper Ob valley, in the south, grows grapes, melons and watermelons, whereas the lower reaches of the Ob are Arctic tundra . The most comfortable climate for the rest on the Ob are Biysk, Barnaul, and Novosibirsk . </P> <P> The Ob provides irrigation, drinking water, hydroelectric energy, and fishing (the river hosts more than 50 species of fish). There are several hydroelectric power plants along the Ob river, the largest being Novosibirskaya GES rated at 460 MW . </P>

What mountain system serves as the beginning of the irtysh and ob rivers