How Water Deepens and Scours Where a River Narrows
The gorging, the great hollow at the throat of a mountain narrows, and the gravel that fills below a fall
Leonardo studies what happens where a river is squeezed through a cut in the mountains: the water gorges up in the open reach, drops and scours a great hollow just past the middle of the narrows, then shallows again as it re-enters the wide part, so that flow stays equal. He adds that the pool below a waterfall fills with gravel beneath the highest point of the leap, and that water falling the full width of a river rebounds off the bed before falling back. The recto carries a funnel-shaped 'strettura' diagram and a drawing of the cascade over a rocky bed.
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Depth forms at the exit of the narrows into the wide
Such a depth is produced where the water leaves the narrows and enters the wide reach. The change of section governs where the bed is scoured.
Gorging and the great hollow in a mountain narrows
When a river is narrowed by leaving the valleys and entering the cuts of the mountains, the water surges up in the wide part and drops through the narrows, hollowing a great concavity past its middle. Re-entering the wide reach, the depth diminishes in proportion as the width grows, so the flow is kept equal.
Gravel fills the pool beneath the leap of the water
The depth falls away after the leap of the waters because the bed fills with gravel beneath the greatest elevation of that leap. Deposition, not just scour, shapes the reach below a fall.
Water striking the bed leaps up and falls back
If the fall is as wide as the river, the water that strikes the bottom will leap up again and then fall back. Leonardo notes the rebound of the impacting sheet of water.
The 'strettura' funnel diagram
At the upper right a funnel-shaped sketch shows the river squeezed through a narrow throat, with hatching marking the constricted flow; a separate drawing runs along the foot of the leaf.