Siberian River Routes were the main ways of communication in Russian Siberia before the 1730s, when roads began to be built. The rivers were also of primary importance in the process of Russian conquest and exploration of vast Siberian territories eastwards. Since the three great Siberian rivers, the Ob, the Yenisei and the Lena all flow into the Arctic Ocean, the aim 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. Despite resistance from the Siberian tribes, Russian Cossacks were able to expand from the Urals to the Pacific in only 57 years (1582-1639). These river routes were crucial in the first years of the Siberian fur trade as the furs were easier to transport over water than land. The rivers connected the major fur gathering centers and provided for relatively quick transport between them.