Ob (ôp) [key], river, c.2,300 mi (3,700 km) long, W Siberian Russia. With the Irtysh River, its chief tributary, it is c.3,460 mi (5,600 km) long and is the world's fourth longest river. Formed by the junction of the Biya and Katun rivers (both of which rise in the Altai range) SW of Biysk, the upper Ob flows NW, then NE past Barnaul and Novosibirsk through the W Siberian lowlands to be joined by the Tom River. The middle Ob flows northwest through the swampy forests in the Tomsk and Narym regions and then is joined by the Chulym, Ket, and Irtysh rivers. The lower Ob consists of the Great Ob and the Small Ob and flows N, then E into Ob Bay, an estuary and shallow arm (c.500 mi/800 km long and 35–50 mi/56–80 km wide) of the Arctic Ocean between the Yamal and Gyda peninsulas. The width of the Ob increases downstream to c.25 mi (40 km) near its mouth. The valley of the middle Ob is subject to flooding each spring as the thaw occurs in the upper Ob basin before the ice in the lower course of the Ob has melted. Although frozen from five to six months of the year, the Ob is an important trade and transport route; Novosibirsk, Barnaul, Kamen-na-Obi, and Mogochin are the chief ports. There is a large hydroelectric power station at Novosibirsk. The largest oil and gas deposits in Russia are found in the basin of the middle and lower Ob. Severe pollution in the lower Ob has damaged the river's formerly famous fisheries.