The spillway

This is probably not technically a spillway, nor is it a dam. It could be called a weir, but that's not a word we Americans would normally use. In French, it could be called un déversoir or un seuil. In this case, a concrete structure is built across the point at which the Cher River splits around an island. The river's flow is backed up behind a bridge at Saint-Aignan to provide water power to a mill that once stood there. This spillway allows excess water to flow around the island rather than flood lands further upstream.

The spillway on the Cher at Saint-Aignan. On this day it was dry.

When the river level is low enough that all of its flow passes over the dam at the bridge, the spillway becomes dry and the water in the river's northern channel is shallow and still. When upstream rains cause the water level to rise, it flows over the spillway and rejoins the main channel downstream.