Cite as: 514 U. S. 673 (1995)
Opinion of the Court
titative or specific entitlement against which depletions to usable flow could be judged. Nor were there any agreed upon criteria for establishing what flows were usable." Id., at 162-163.
As late as 1985, Colorado officials refused to permit an investigation by the Administration of well development in Colorado because they claimed that the evidence produced by Kansas did not " 'suggest that well development in Colorado has had an impact on usable stateline flows.' " Id., at 163 (quoting memorandum of J. William McDonald, chief of the Colorado delegation to the Administration). In light of the vague and conflicting evidence available to Kansas, we conclude that Colorado has failed to demonstrate lack of diligence, i. e., inexcusable delay, on the part of Kansas.
Accordingly, we overrule Colorado's exception to the Special Master's conclusion that the defense of laches should not bar Kansas' well-pumping claim.
B
The Compact prohibits "future beneficial development of the Arkansas River basin" that "materially deplete[s]" the usable flows of the Arkansas River. App. to Report 5 (Article IV-D) (emphasis added). Because some wells in Colorado were in existence prior to the Compact, both parties agree that a certain amount of post-Compact well pumping is allowable under the Compact. Report 182. Kansas and Colorado, however, dispute the extent of this allowance. The Special Master determined that the "highest annual amount shown to have been pumped during the negotiations, namely 15,000 acre-feet, should be allowed under the [C]om-pact." Id., at 200. Colorado makes both a legal and a factual challenge to this determination. Colorado's Exceptions 66-73, 73-84.
Colorado argues as a legal matter that the Compact does not limit the pumping by pre-Compact wells to the highest amount actually pumped in pre-Compact years; rather, Colo-
689
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