From the Document: "In the second half of the 19th century, the federal government pursued a policy of confining Indian tribes to reservations. These reservations were either a portion of a tribe's aboriginal land or an area of land taken out of the public domain and set aside for a tribe. The federal statutes and treaties reserving such land for Indian reservations typically did not address the water needs of these reservations, a fact that has given rise to questions and disputes regarding Indian reserved water rights. Dating to a 1908 Supreme Court ruling, courts generally have held that many tribes have a reserved right to water sufficient to fulfill the purpose of their reservations and that this right took effect on the date the reservations were established. This means that, in the context of a state water law system of prior appropriations, which is common in many U.S. western states, many tribes have water rights senior to those of non-Indian users with water rights and access established subsequent to the Indian reservations' creation. Although many Indian tribes hold senior water rights through their reservations, the quantification of these rights is undetermined in many cases."
CRS Report for Congress, R44148
Congressional Research Service: https://crsreports.congress.gov/