Whether the five Indian Tribes could intervene in this original action to participate in the adjudication of their water rights.
Holding
Yes. The Tribes could intervene in the later proceedings concerning their reserved water rights.
Reasoning
The States’ sovereign-immunity objection failed because the United States had already brought the Tribes’ water-right claims in this original action. The Tribes did not seek to assert new claims against the States; they sought to participate in the continuing adjudication of claims that the United States had asserted on their behalf. Their participation therefore did not enlarge the Court’s judicial power or compromise state immunity.
The Court’s original-action rules use the Federal Rules of Civil Procedure only as a guide, but the Tribes at least met the standard for permissive intervention. Their water rights would be determined in this litigation, and the United States’ representation would bind them to the judgment. Given the direct importance of the litigation to the Tribes’ welfare, their participation should not be discouraged.
Intervention did not authorize the Tribes to relitigate matters already decided. It allowed them to participate only in issues that remained properly open for decision, and the States had not shown that intervention would cause prejudice or undue delay.