Final answer:
The Vikings used the river system in Russia, particularly the Dnieper River, as a trading route and means of invasion and settlement, which led to the formation of the early Russian state.
Step-by-step explanation:
The geographic feature of Russia that the Vikings used to further their invasions and establish trade was the river system, specifically the Dnieper River. The Swedish Vikings, known as the Rus, used the Dnieper as a trading route to reach Byzantium and the Abbasid Caliphate, transporting goods such as furs, amber, and honey. The use of the river system facilitated the Vikings' economic interactions and eventual settlement in regions that would later be part of Russia, thus forming the early nucleus of the Russian state. The Vikings would establish fortified trading towns along these routes, construct colonies, and make advances deep into Europe. Additionally, the longships of the Vikings were crucial, as their design allowed for navigation of both riverine and sea routes, bolstering Vikings' ability as traders and conquerors.