The Ob is a river in Siberia. Together with the Lena and the Jenissei, it is one of the most important rivers of Siberia. Depending upon how its length is measured, it can be between 3.650 km and 6.210 km long. Some important cities on the banks of the river include Novosibirsk(Russia's third largest city), Barnaul, Kolpashevo, Nizhnevartovsk, Surgut, Khanty-Mansiysk, Beryozovo, Labytnangi, and Salekhard. The Irtysh is the Ob river's main tributary. Other tributaries include the Omsk, Parabel, Chaya, Tobol and Ishim.