Don't know if this has been posted yet somewhere. Maybe this thread can be moved to "Places, France"?
The french site http://www.hydro.eaufrance.fr/ gives a lot of information about water levels (mostly flows) of almost every french river. Not all stations are updated daily, but you can always get historical info that can be helpful planning a trip on a french river.
Some knowledge of the french language will be useful, and you might need some time to find your way, but I use it a lot.
I find the "entre2" and the "QJM" views to be the most useful. The "entre2" view provides longtime averages, lowes and highs, combined with the actual daily averages. The "QJM" view provides daily and monthly averages for a given year. Useful when planning, or to look for an excuse after you have been swimming.
The "entre2" view of the Canche at Brimeux for instance shows that in January the rivers average (and thus expected?) level will be close to its top. And we'll be there in less than two weeks