The water has been adjudicated to many water users, some of who are native but the tribe has now been awarded rights to water that feeds the Milk River and Milk River water. Water Rights are a very complicated issue, A farmer/rancher uses water from the Milk to irrigate his crops and lets say he uses/pays for 1000 acre feet of water and 700 acre feet of water actually soaks in his ground to be used by the plants. the other 300 runs off into the Milk from his fields and then gets resold to a downstream user. The downstream user is counting on his allocation of lets say a 1000 acre feet of water, The irrigation district in charge of water allocation has to make sure he is going to get his 1000 acre feet of water so they controll who gets the water and how much, If you own a water right you have rights to how ever much water your water right gives you, You have the right to that water, This has been divided up a century ago and we really have allocated more water than what is available except some of the water is not used or is used more than once. If you want to really get involved in water rights take a look at the bean lake water rights history. many farmers/rancehrs downstream are depending on water that originates in Montana and feeds the milk that feeds the Missouri that feeds the Mississippee, all the while maintaining enough water for navigation in the rivers, all of the water has been spoken for, and used many times I would guess the last water rights issued are going to be null and void.