How rivers and rain wear away mountains and banks
Book 9 on water: erosion of concave river-banks and mountain slopes
This text page, headed 'Book 9, of permanent well-water and of fugitive water,' sets out how running water and rain sculpt mountains and river-banks. Leonardo argues that concave banks are eaten away most at their base, where the descending current strikes hardest, and distinguishes right-angle impacts that gouge out the bed from oblique ones that sweep debris downstream beneath the arc of the reflux. He also explains how the tilt of a mountain's rock strata governs both a slope's steepness and the flow of the river born from it, and how rain wears the flanks of mountains while their summits, wearing least, remain the highest. Faint sketches of a mountainous ridge and other lines appear in the margins.
On this page
Concave banks worn most at their roots
River-banks, being concave, are worn more by the water's course in their lower parts than near the surface, because the water running against the concavity descends with the impetus of the interposed hill and, striking and rubbing, consumes the roots of the concave banks.
Right-angle versus oblique currents on a bank
Some current strikes the bank at a right angle, springs into the air, falls back, and hollows the bed, leaving the excavated material around the struck place; another arrives at an acute angle, bends toward the bottom, and carries that deposit away in its refracted motion, leaving most of it beneath the arc of the reflux.
Strata direction and rain shape the mountains
When the faces of a mountain's rock strata are turned toward the rains it absorbs much water, and the river born of that slope is abundant but not lasting. Mountains, undercut at their roots by rivers and washed on their sides by ever-swifter rains, keep their summits highest because these wear away least compared with the valleys.
Faint sketch of a mountainous ridge
A light ink sketch of a jagged ridge or mountain profile is drawn in the upper-left margin, with further faint lines lower on the sheet. No text was transcribed for these marks.