As I understand it, there are two basic types of hydro power.
One depends on the weight of the water.
The other on the impact of the water.
The old post card style depends on the weight.
High volume is needed, even at low pressure or inertia.
That is why you will sometimes see wheels as much as ten feet wide to catch a large volume of low inertia water.
They are catching more weight.
The smaller wheels, "turbines," use the force of impact to spin.
This design goes beyond just water.
Steam, compressed air, anything you can force through a nozzle, will work.
They need the high head, even if at low volume, for the pressure.
You need the pressure to create the impact.