Medium confidence only because while I cannot be certain, I suspect what they are referring to here is what is more correctly known as 'backwater effect', where the level of a river rises because the flow is too great to pass through a narrowing in the water course (bridge abutments, fouling with logs, etc.). The same thing can happen if there is heavy rainfall in a catchment causing the level of a river to rise and flow back up tributaries whose catchment is not otherwise affected by the rain. The same can occur if high tides or strong wind cause a rise in sea level, making it impossible for rivers to flow out to sea at their normal level.

Remous: Le remous est la surélévation générale du plan d’eau en amont d’un pont. Il est dû au rétrécissement local causé par l’ouvrage, que entraîne l’accroissement de la vitesse du courant nécessaire à la conservation du débit.
https://ouvresdegenie.wordpress.com/glossaire/

Remous being often taken to mean 'eddies, turbulence', it is best qualified as remous amont for clarity. AKA remous d'exhaussement.