How the water turbines works?

A turbine used under a waterfall simply catches water as it falls and uses the weight of the water to turn it. A turbine that operating due to a flow of water has fan blades on it. As water pushes against the angled blades, the component of force perpendicular to the blade turns the turbine about its axle.

Once you have a method of rotating an axle, it is connected to a generator, which is a combination of rotating coils inside a magnetic field which generates eletrical potential.

Water turbines convert the gravitational energy of a falling water flow into kinetic energy, and then into electrical energy if a generator is connected to the turbine axis. The shape of turbines is optimized to get the maximum from this conversion and depend on the fall height and on the flow amount.

weight of water * acceleration of water = water pressure applied on turbine blade. Turbine blade is free to rotate & we get energy of water converted to rotating energy of turbine.