Read it through once

'Yes. And you've got to have good fair marks from one end of the river to the other, that will help the bank tell you when there is water enough in each of these countless places--like that stump, you know. When the river first begins to rise, you can run half a dozen of the deepest of them; when it rises a foot more you can run another dozen; the next foot will add a couple of dozen, and so on: so you see you have to know your banks and marks to a dead moral certainty, and never get them mixed; for when you start through one of those cracks, there's no backing out again, as there is in the big river; you've got to go through, or stay there six months if you get caught on a falling river. There are about fifty of these cracks which you can't run at all except when the river is brim full and over the banks.'