Siberian River Routes were the main ways of communication in the Russian Siberia before the 1730s, when roads began to be built. The rivers also were of primary importance in the process of Russian exploration and colonization of vast Siberian territories. Since the three great Siberian rivers, Ob River, Yenisei River and Lena River all flow into the Arctic Ocean, the problem was to find parts or branches of these rivers that flow approximately east-west and find short portages between them. Since Siberia is relatively flat, portages were usually short. Because of this, and the weakness of the Siberian tribes, Russian Cossacks were able to expand from the Urals to the Pacific in only 57 years (1582-1639).