River Courses Between Their Banks in Flood and Low Water
How the volume of water governs a river's straightness, bed and banks
Mirror-script notes contrast a river carrying little water, which runs long, slow and winding between its banks, with one in full flood, which runs shorter, faster and straighter. Leonardo argues that abundant water makes river courses straighter and deeper, better able to hold the great floods that often occur without destroying neighbouring properties, and describes how a flood breaks the humps left by low water, cuts new hollows and abandons its old bed. He adds that a river bed grows more even in high flood and more irregular in low water, because the swollen, faster water fills the hollows with sand and other matter. A small marginal diagram at the right shows two curved bank-lines labelled with letters.
On this page
Volume of water sets a river's speed and length of course
The same channel behaves in two opposite ways depending on how much water it carries. A scanty flow runs long, slow and winding between its banks; a plentiful flow runs shorter and faster. This paired conclusion frames the whole page.
Floods break the banks and shift the river's bed
A flood, finding the bulges of bank left by weak low-water currents, strikes them and rebounds from the opposite shore, doing fresh damage because it will not follow the windings of the small waters. It breaks the opposing humps, spills into neighbouring fields, hollows them out and at last redirects its course, abandoning its ancient bed.
Straighter, deeper channels spare neighbouring lands
Leonardo concludes that giving rivers a continual abundance of water keeps their courses straighter, so they deepen and grow capable of holding the great floods. Comparing the short bank a b c with the longer course of the little water d b f, he reasons that a full river would run straighter and ruin its banks less, leaving riverside properties intact.
Sediment fills the bed's hollows in high flood
In great floods all the hollows of the bed fill with sand and other material, which stays because the swollen water is too fast to bend into the low-water hollows. The bed therefore becomes more even the larger the flood, and more varied the less water there is.