The lake in Rustrel

Remember to bring along your swimming costume? No? Well that's a good thing, as you're not allowed to swim there (although there is a pool at Rustrel). This man-made lake is a collinear reservoir fed by the Fenouils ravine which collects rainwater from the surrounding hills. It is constructed on grey clay.
Do you know why?

A. Clay can be used to feed fish. B. Clay is easy to dig out. C. Clay is an impermeable rock