How did the Mormons bexome leaders in water usage in the West?

Answer 1

They had a system that included shared use of a limited resource. In other areas of the country whoever is higher upstream has the rights unless the guy down river can prove he had a preexisting use of the water. The church leaders claimed the rights for all, and distributed accoring to need.

Answer 2

The area is naturally very dry. The pioneers could either irrigate it and grow crops, or they could starve to death. They chose the former.