hello
you have to call a phone number, it's a pub called "le cafe du commerce" just next to the river.
they know where to look and they will tell you.
I don't have the phone number, I'll try to find it if you want.
for the water level:
there is a kind of pavement in the river (we call that "la margelle), they will tell you: the water level is at the pavement:), 5cm below the pavement:(, 10cms above:D....
for a good level: level margelle ( the water is just on it)
go there for the description of the river:
http://www.eauxvives.org/rivieres/
the choose :les topos des rivieres, the par ordre alphabetique, then tarn. le haut tarn is the first part
the level is explain in french of course.
voila
oh one more thing: I don't think the bartender speaks english!