It isn't rising fast enough either.

It isn't rising nearly so fast now as it was before it got over the bank." "Why, how do you know that, Sam?
You haven't been to look." "No, but I know it, nevertheless, simply because I know that water, left to itself, will find its level." "I don't see what that's got to do with it," said Tom.
"Perhaps not, but it has something to do with it for all that," replied Sam; "and I can make you see how, too." He paused, to think the matter over and determine how to present it to Tom's comprehension.
"You see," he then resumed, "that the river inside its banks is about four hundred yards wide.

When it rises above the banks, however, it spreads out over the level ground, and becomes, in some places, many miles wide, averaging a mile at least in width.

Now there is only a certain amount of water coming into the river every hour.

The rain has stopped, but the soil is full of water, and so there is about as much running into the river now as there was while the rain lasted.