HOW TO MAKE a dam for small river (1,5m wide) for hidrogenerator needs ? help?

I would start with a load of riprap (class #1) from your local quarry. That should give your dam shape and support. Then I would use a smaller quarry stone to fill in the gaps and reduce the water flow. You could put down a heavy guage plastic (landscaping plastic) before you apply the smaller stone to really seal it up.