I ask because I had always assumed it was the Vikings (or Varangians, I guess) in the 800s and 900s who first sailed down the Volga, the Dnieper, etc. to trade with the Middle East and Byzantine Empire, but I came across this in Michael Pye's The Edge of the World:
Tribes who poured into Eastern Europe in the sixth century blocked the old trading links between Scandinavia and Byzantium, the river routes that tracked across what is now Russia
which suggests the Varangians were just reopening well-established trade links that had been cut off for a couple centuries.
... which suggests the Varangians were just reopening well-established trade links that had been cut off for a couple centuries.
I'd probably phrase it 'intensifying'. Researchers generally agree that there was a large-scale economic depression—perhaps even global and climate-caused, complete with plagues and famine—during the 600s. Things began to rebound during the 700s, especially in the old economic powerhouses of the Eastern Mediterranean. It seems like commerce connecting north from these areas first began to flourish via the Danube, but later shifted toward the Russian riverways. This goes into the period where the Varangians dominated trade.
There were lots of factors in this eastward shift, but I think the economic strength of the new caliphate—which straddled the junctures of Africa and Asia as well as the Mediterranean and the Indian Ocean—was probably the biggest factor. Traders from the north realized they'd have better luck pushing east through Russia to the Caliphate rather than south toward the Byzantine Empire.
There's obviously deep literature that goes behind these kinds of sweeping generalizations, and with deep literature comes intensive debate. But for a general overview that holds up pretty well, I'd recommend Peter Frankopan's Silk Roads (2015).