There is only one location befitting a celebration of France for this repas en plein air. The French know how to do rivers. Practically every single bridge you cross, whether you are on an autoroute, route nationale or D road proudly names the river it traverses. Pride about, and knowledge of, local rivers and tributaries is embedded in the French psyche. They even distinguish between fleuves, which they flow into the sea (or into a desert or lake), and rivières which flow into another river. I know a lovely spot on the Garonne near Saint-Martory. Let’s head there.
But you can find the cache here: