Although allocation of water resources is generally a matter of state law, the federal government may also allocate water rights. Under the Supreme Court's 1908 Winters v. United States decision, when Congress creates an Indian reservation, the water necessary to fulfill the reservation's purposes is reserved implicitly. Although this doctrine appears to be fairly straightforward, the scarcity of sufficient water in more arid parts of the country has resulted in contentious debate over the requirements of Winters and its impact on competing water rights. Because federal reserved water rights under the Winters doctrine often have not been quantified but are generally senior to other water rights, the rights of more junior water users can be affected significantly, when these federal rights are exercised. This report provides background on Indian reserved water rights under Winters . It analyzes the scope of these rights, including the water sources that may be used to fulfill the rights and the quantification standards courts commonly use to clarify the rights. The report also examines the effect of the McCarran Amendment, through which Congress authorized state courts to adjudicate Indian reserved water rights. In Winters , the Supreme Court examined tribal rights to water associated with the Fort Belknap Reservation located in what would later become Montana. The Fort Belknap Reservation was created by an agreement in 1888 between tribal parties and the U.S. government. At the time, the government had a policy seeking to transform Native Americans from "a nomadic and uncivilized people … to become a pastoral and civilized people" by providing them lands to develop for such purposes. By 1905, the area experienced water shortages that ultimately resulted in the Winters lawsuit being filed to enforce tribal rights to water against non-Indian water users who had been diverting water from the region. In announcing its decision, the Court explained that the lands provided under the agreement for the purpose of developing an agrarian society "were arid and, without irrigation, were practically valueless." The Court also noted that ambiguities in the agreement, such as the status of the water rights related to the land, are to be "resolved from the standpoint of the Indians," as a rule of interpretation. The Court held that: The power of the Government to reserve the waters and exempt them from appropriation under the state laws is not denied, and could not be. That the Government did reserve them we have decided, and for a use which would be necessarily continued through the years. The Court has continued to recognize the principle derived from Winters in both Indian and non-Indian contexts. In 1976, the Court noted that it "has long held that when the Federal Government withdraws its land from the public domain and reserves it for a federal purpose, the Government, by implication, reserves appurtenant water then unappropriated to the extent needed to accomplish the purpose of the reservation." Under the Winter s doctrine, the priority and extent of Indian reserved water rights is affected by the purposes of the Indian reservation, the date when the Indian reservation was created, the quantification of water sufficient to accomplish those purposes, and the sources of water that may be used to fulfill the particular water rights. Although the federal government may act under a number of constitutional authorities to regulate water, in most instances it has deferred to the states. States either adhere to a riparian or prior appropriation system of water allocation. In a riparian system, landowners adjacent to a waterway share a common right to use the water, with a limitation of reasonableness. In times of water shortages, all riparian rights holders must share the burden of the shortage proportionally. Other states, typically the drier western states, use a prior appropriation system of water rights. Under prior appropriation, water users who make beneficial use of a water supply, regardless of their location relative to it, obtain a right to that water under a seniority system that reflects the order in which the right was obtained. The date that the user put the water to beneficial use is known as the priority date. Some states incorporate elements of both the riparian and appropriation doctrines. Because riparian rights' holders must share the burden of any water shortage proportionally, Indian reserved water rights generally do not have a noticeable impact in riparian jurisdictions. In prior appropriation systems, recognition of a tribe's water rights is often times more contentious because, in times of shortage, junior users may receive none of their allocations after a tribe with senior takes its share under the Winters doctrine. Tribes often have seniority because the laws, treaties, executive orders, and other legal agreements that created the Indian reservations (and thus the priority date for purposes of seniority) predate other settlement of the area. Allocation of scarce water is further complicated by the fact that a tribe's reserved water rights under the Winters doctrine are not lost if the tribe does not maintain continuous use of the rights. As a result, junior rights holders may be unaware that a tribe has senior reserved rights, leaving the junior rights holder with little or no allocated water in some instances. Tribes may also acquire water rights under state law. That is, if water is available under the state water allocation system and a tribe requires water beyond what it receives under its federal reserved water rights, it may seek to acquire state water rights to supplement its federal reserved rights. Under Winters , the reserved water rights are tied to the purpose (and in some cases, purposes) of the reservation, as embodied in the particular law, treaty, agreement, or executive order that created the reservation. It is unclear, however, whether the primary purpose standard the Supreme Court adopted when applying Winters in non-Indian reserved water rights cases governs Indian reserved water rights. Under the primary purpose standard, reserved water rights may be applied only for the primary purposes of reservations, not for secondary purposes. In Cappaert , the Supreme Court held that water rights are limited to the "amount of water necessary to fulfill the purpose of the reservation, no more." In United States v. New Mexico , the Supreme Court further clarified that the test is whether "the purposes of the reservation would be entirely defeated" without that water. Lower courts generally define Indian reservation purposes broadly, which reflects the reasoning in Winters that Indian reservations were created in order to transform and sustain a new lifestyle for the tribe. In one case, the Arizona Supreme Court refused to limit Indian reserved rights to only the primary purpose of a reservation. The court noted the importance of providing the Indians with "a permanent home and abiding place," as well as the need for "broader interpretation [of the purposes of Indian reserved rights] in order to further the federal goal of Indian self-sufficiency." The court emphasized the need for a broad interpretation of the purposes of the reservation, recognizing that it is difficult to ascertain "the true reasons for which Indian reservations were created." Similarly, the U.S. Court of Appeals for the Ninth Circuit interpreted the primary purpose of a reservation to encompass a number of related purposes. It found that the primary purpose of a reservation was to create a homeland for the tribe, which included water rights that could be used not only for irrigation but also for fishing. Other courts have rejected such broad interpretations, noting the inherent risks in placing no limits on the water rights associated with a reservation and accordingly limiting water rights only to purposes that were specifically contemplated when the reservation was created. Although state law generally restricts changes in water use from the originally designated purpose of the water right, Indian reserved water rights are not subject to such restrictions. As discussed later in this report, courts quantify the amount of Indian reserved water rights based on the purposes of the reservation. Once quantified, some decisions have allowed Indian reserved water rights to be used for purposes other than those considered in the quantification of the right. Other courts, however, have refused to allow tribes to change their water use from the purpose contemplated when the reservation was created. For example, the Wyoming Supreme Court refused to permit tribes to apply their reserved rights to instream flows. Finding the water to fulfill a water right is often the most controversial element of water rights claims, particularly in times of shortage. The question of which water sources may be used to fulfill reserved rights is not clearly resolved. Although Indian reserved rights generally attach to whatever water sources may be within or adjacent to the reserved lands, it is generally understood that reserved rights do not necessarily require that the water source be encompassed within the reserved lands. Rather, courts have allowed tribes to draw water from various sources as necessary to fulfill the reservation purpose, limiting the potential sources only to the extent that the waters must be unappropriated at the time the reservation was created. In other words, an Indian reserved water right cannot trump senior rights that existed at the time of reservation. One question on which courts have disagreed is whether the reserved water right may draw from groundwater, or if it is limited to surface water. In In re Big Horn , the Wyoming Supreme Court rejected applying the Winters doctrine to groundwater. While acknowledging that groundwater and surface water sources are often connected, the court held that there was no precedent for applying the Winters doctrine to groundwater. On the other hand, the Arizona Supreme Court has held that reserved rights may include claims to groundwater, if the groundwater is necessary to fulfill the purpose of the reservation. Finding that the government's reservation must have contemplated water "from whatever particular sources each reservation had at hand," the court explained that the "significant question for the purpose of the reserved rights doctrine is not whether the water runs above or below the ground but whether it is necessary to accomplish the purpose of the reservation." Although the court recognized that many possible water sources could be used to fulfill reserved rights claims, it also noted that groundwater should only be claimed if other sources were insufficient to accomplish the purpose of the reservation. When degradation of water quality would undermine the water's use for reservation purposes, courts have recognized water quality as another element of Indian reserved water rights. Federal courts have ruled that reserved water rights holders can seek legal protection from water quality degradation by other water users.