The movement of river water: floods, meanders, and surface flow
Why low water spares the banks at flood, meandering rivers run slower, and surface water holds its path
Three drawings of winding, S-shaped watercourses accompany Leonardo's propositions on the movement of rivers. He observes from experience of the Po that low water runs in transverse courses and undermines the banks at their foundations, causing great ruin, but joins and follows the main flood when the river is high, sparing the banks. Further notes hold that a river with a more meandering path runs more slowly than a straight one of equal size, and that surface water keeps its straight course even over cross-waves passing beneath it.
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Low water follows the flood and spares the banks
The lesser sum of water obeys the great floods, changing course to unite with them and losing force to excavate the banks. In the Po, low water runs transversely, striking the banks at their foundations and hollowing under them to cause great ruin; but at full level it leaves that course, drawn along by the greater sum, and no longer damages the banks.
Meandering versus straight rivers and their speed
For rivers of equal origin, equal plains, equal bank-widths, and the same outlet, the one with the more meandering path moves more slowly. As many times as the length of the straight river enters into the meandering of the other, so much more rapid is its course.
Surface water keeps its path over submerged waves
Water flowing on the surface covers the waves passing below it, and even when those are crosswise it does not deviate from its original straight path, because the surface flow bordering the air is more powerful than the enclosed contrary flow beneath. The proof is seen in light things that scud over the water.