Its all about how the land was settled, in the east, water rights are retained with the property following and English or French system (the French had long lots that all had access to a river or waterfront, sometimes called ribbon lots). The west was settled by the Spanish, and first rights decided the water...it was so dry settlement depended on water, so basically the first person to dig a well or put in irrigation systems owned all the water rights to that system....it usually worked out to be the church or monastery or other hacienda that established the first settlement, but this system carried down through the legal land transfer rights. If you want a really lucrative, crazy, but interesting career, get into water rights law in the western US. Just look into who owns the Ogallala Aquifer....
http://www.scientificamerican.com/article/the-ogallala-aquifer/