The river that flows through Paris and continues to Le Havre should really be called the Yonne. At Montéreau-Fault-Yonne, where the Seine and Yonne meet, the flow of the Yonne is greater than that of the Seine, which is therefore a tributary and should end there.
I've been through Montéreau-Fault-Yonne three times & been rained on every time. Miserable hole.