Colorado River Water Conservation District v. United States, 424 U.S. 800 (1976), was a case in which the Supreme Court of the United States extensively refined the abstention doctrine to prevent duplicative litigation between state and federal courts. Background In the Southwestern United States, water scarcity was (and remains) a critical problem. The McCarran Amendment, , was a statute enacted by United States Congress in 1952 allowing the United States to be joined as a defendant in certain suits concerning the adjudication or administration of rights to use of waters. In 1969, the state of Colorado enacted a law to attempt to reorganize the procedure for legal determination of water claims within the state. The procedure that Colorado created divided the state into seven Water Divisions, each one encompassing one or more drainage basins for the largest rivers in the state. Each month, Water Referees in each division would rule on applications for water rights, or refer the case to a Water Judge, who would rule on referred or contested applications on a six-month schedule, applying the prior appropriation doctrine. A State Engineer, along with engineers for each division, were responsible for the administration and distribution of waters in each division. Facts and procedural history The federal government reserves the water rights on federal lands such as Indian reservations and national parks and forests. The rights to such federal lands in the state of Colorado affect the rights of users in Colorado Water Division No.7. On November 14, 1972, the government filed suit in the United States District Court for the District of Colorado, asserting jurisdiction under both (the federal question jurisdiction statute) and , which grants the United States district courts with original jurisdiction over all civil actions in which the United States is the plaintiff. The government sued for a declaratory judgment asserting its own reserved rights and those of several Native American tribes, against over 1,000 water users named as defendants. The government had previously filed similar suits in three other divisions. Shortly after the suit over Division No.7 was commenced, one of the defendants in one of the other suits filed an application in state court to join the United States as a party in a state court proceeding, pursuant to the McCarran Amendment. Several defendants in the federal court suit then moved to dismiss, challenging the federal court's jurisdiction to continue to hear matters pertaining