I didn't forget. Scandinavians did travel the rivers from the Baltic to the Black Sea, and the very name Rus' is said to have meant "river traveler" or "boatman". They took over Kyiv in the 9th century, and were the original power in the Rus' empire that was wiped out by the Mongols in 1237-42. But they didn't contribute much to the modern-day East Slavic gene pool. Probably because the Mongols killed almost everybody they met. Some historians I've read say that the steppe region that is now Ukraine, and the city of Kyiv, were entirely depopulated, which would have obliterated most if not all of the Scandinavian genes.
Wow- I learned something new this evening and I thought I knew everything four decades ago. Funny how that happens almost overnight.