It is impossible for a river that receives other rivers into itself to maintain itself in a given straightness.

The bends made by the banks of rivers are cancelled out in the great floods of rivers, because the maximum current drives the water in a straight course; but as it diminishes it resumes the winding course, which goes deflecting from one bank to the other. And this diminishing of the water hollows out the banks of the rivers; but in this lessened depth the water does not have a motion of even course, because the greater current leaps from one concavity to the other of the opposite banks, and the sides of the water that border the bank are of very short course.

The rounded swellings that the islands of the gravel-beds have, formed by the elbows of the bank, derive from the greatest eddies of the rivers, which extend with their revolutions between the concavity and the convexity set opposite one another in the banks of the rivers; and from here are born the small channels interposed between the shoals of the rivers and their banks, placed facing the concavities of the banks of those rivers, etc.

The entries of rivers into rivers generate the first sinuosities of rivers.
Rivers always occupy the lowest parts of the valley.

The windings of rivers in the plains are born from the rivers that pour into them. If the winding river is entirely removed from its ancient bed and put into a straight canal, it is necessary that of the rivers which pour into it from two sides, on one side they lengthen as much as they diminish on the opposite side; and what gains in length loses in speed, which speed is transferred into the shortened river.

Make the lesser rivers enter at acute angles into their greatest rivers. And the usefulness of this is that the course of the greater river bends the inlet of the lesser river and does not let it strike against the opposite bank.
But if the lesser river has its flood at the time when the greatest river is low in its waters, then the impact of the lesser river breaks the opposite bank of that greatest river.

a — b — c

Always the greatest arc of the whole river of a valley generates its convexity toward the lowest part of the width of the valley. The sinuosities of rivers are the greater the nearer they are to the entry of the lesser river into the greater river. The waves of earth made by the bank of rivers continually shift their position, and the first is reborn when the last is consumed.