In "The Fall of the Roman Empire: A New History" (p. 198), Peter Heather reckons nearly 100,000 people crossed the Danube with Radagaisius (including women and children), and that probably just as many Vandals, Alans, and Suevi crossed the Rhine a few months later.
How would they have crossed these rivers? Would they have boarded barges, or would they have built a pontoon bridge? Would they have crossed in small groups or all at once?
I gather from what Heather writes (p.199) that the Roman frontier garrison troops (the limitanei) wouldn't have had enough men to be able to stop such massive influx of people. How might they have behaved in such a circumstance? Would they have died to the last man, trying to prevent the enemy from crossing, or would they have strategically withdrawn into their towers?
The traditional date for the crossing was December 31st, with many historians thinking the Rhine was frozen. So they just walked across, if that was the case. But the evidence isn't especially clear. The garrisons, which had been depleted, were overwhelmed by the numbers crossing.