The tidal currents, or races, or roost (as some of them are called locally, from the Icelandic) off many of the isles run with enormous velocity, and whirlpools are of frequent occurrence, and strong enough at times to prove a source of danger to small craft.
Sometimes enormous rocks have fallen from the mountains and spread over the river-bed causing huge whirlpools.
But just before reaching the Uaupes there is a long series of reefs, over which it violently flows in cataracts, rapids and whirlpools.
In the fifth and last section from Old Moldova to Orsova and the Iron Gates the river is enclosed by mountains and rocky banks, and the obstacles to navigation are rocks and whirlpools.
I wrote a lot of very lurid, melodramatic stories about people being trapped by the tide and swept away by whirlpools.
