Cite as: 514 U. S. 673 (1995)
Opinion of the Court
the Spronk method in favor of the Durbin approach, as modified by the Larson coefficients.
III
A
The Special Master concluded that Kansas was not guilty of inexcusable delay in making its well-pumping claim, and that Colorado had not been prejudiced by Kansas' failure to press its claim earlier. Id., at 170. Colorado has excepted to this determination. Colorado argues that the equitable doctrine of laches should bar Kansas' claim for relief. See Colorado's Exceptions to Special Master's Report (Colorado's Exceptions) 24-64. We overrule Colorado's exception.
The defense of laches "requires proof of (1) lack of diligence by the party against whom the defense is asserted, and (2) prejudice to the party asserting the defense." Costello v. United States, 365 U. S. 265, 282 (1961); see also Black's Law Dictionary 875 (6th ed. 1990) (" 'Doctrine of laches,' is based upon maxim that equity aids the vigilant and not those who slumber on their rights. It is defined as neglect to assert a right or claim which, taken together with lapse of time and other circumstances causing prejudice to the adverse party, operates as bar in court of equity"). This Court has yet to decide whether the doctrine of laches applies in a case involving the enforcement of an interstate compact. Cf. Illinois v. Kentucky, 500 U. S. 380, 388 (1991) (in the context of an interstate boundary dispute, "the laches defense is generally inapplicable against a State"); Block v. North Dakota ex rel. Board of Univ. and School Lands, 461 U. S. 273, 294 (1983) (O'Connor, J., dissenting) ("The common law has long accepted the principle 'nullum tempus occurrit regi'—neither laches nor statutes of limitations will bar the sovereign"); Colorado v. Kansas, supra, at 394 (In the context of a suit seeking an equitable apportionment of river flows, facts demonstrating a delay in filing a complaint "might well preclude the award of the relief [requested]. But, in any event, they
687
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