France is going through a severe shift in RE at present.
And what I have seen from failed Hydro applications here in Normandy is that their are just to many hoops to jump through to make a small RE project cost effective.
Basically the locals/community are getting there say back, as they had been trampled on by the big corporations.
However, there are ways of re-instating a hydro if it once already existed, but you will need to check the old deeds, and have a serious chat with the local Council and the Regional Council.
At present around us the French Councils are keen on long term stability, local employment, and local folk populating and staying locally.
Rivers/streams here are generally mostly controlled by associations, fishing and the like and the Regional Council will take you through that. If I dug a 50m by 50m lake, its likely that a local association would take control of my water.
Remember, France is still a Republic. YOU MUST ASK the right questions, Officialdom/Functionaries here will not volunteer information around your project.
In general, if the water appears/issues on your land, source/spring, you can do as you wish with it.