It is supposed to be a really intrusive invasive plant, but, no matter, Floating Primrose-willow (Ludwigia peploides subsp. montevidensis) is pretty to look at.
At the end of summer, yes, until in October, it shows off ist yellow flowers. It grows in water or at in places where land and water touch. Here and there it form dense mats of vegetation on the banks of the Dordogne river or even in the water where there is not much current.
Rosettes of leaves detach themselves from the mother plant and float down the river to new spots.