“ The City of Philadelphia represents only a part of the citizens of Pennsylvania who reside in the watershed area of the Delaware River and its tributaries and depend upon those waters. If we undertook to evaluate all the separate interests within Pennsylvania, we could, in effect, be drawn into an intramural dispute over the distribution of water within the Commonwealth. ”

“ An intervenor whose state is already a party should have the burden of showing some compelling interest in his own right, apart from his interest in a class with all other citizens and creatures of the state, which interest is not properly represented by the state. See Com. of Kentucky v. State of Indiana, supra. Philadelphia has not met that burden and, therefore, even if her intervention would not amount to a suit against a state within the proscription of the Eleventh Amendment (and we do not intend to give any basis for implying that it does) , leave to intervene must be denied. ”

“ Otherwise, a state might be judicially impeached on matters of policy by its own subjects, and there would be no practical limitation on the number of citizens, as such, who would be entitled to be made parties. ”

“ The 1931 decree enjoined the State of New York and the City of New York from diverting from the Delaware River or its tributaries more than 440 million gallons daily, subject to a prescribed formula for the release of storage water during periods of low flow. ”

“ We do not write today upon a clean slate. New York City, as well as New York State, is a party to this action. It is true that the city was made a defendant in the original case. But that case was long ago decided. ”