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Roberta X poses an engineering question and I am curious enough to investigate.
What I find is an engineered canal-river intersection near the middle of the French Mediterranean coast. I read the description on Wikipedia, and I looked at the pictures, but I didn't really understand what was going on until I saw the map and this picture. The canal runs East-West (across the picture). The river runs basically North-South. French engineers put two forks in the river: one upstream of the canal that splits the river in two, and one downstream that allows the two streams to recombine. The two streams flow through either end of the structure seen in this image.
When the river is quiet, which is most of the time, boats can pass through this structure unimpeded. When the river floods and a boat wants to cross, a system of gates is brought into play to force the river to one side while the boat enters the channel on the other side. When the boat has reached the quiet center section, the gates are reversed, the river is diverted to the other side and boat can sail out the far side.