RIVERS. When I was writing my guide to the A26 I told my husband that the Marne was the longest river in France. He replied, ‘ No it isn’t.’ I replied, ‘ Yes it is, I have just read it on a French website.’ Thus began the debate. We found out that the French have two words for rivers, ‘rivières’ and ‘fleuves’. Rivers are the tributaries that flow into ‘fleuves’, which flow into the sea. The Marne is the ‘riviere’ that flows into the Seine which is a ‘fleuve’.