Cite as: 533 U. S. 1 (2001)
Opinion of O'Connor, J.
410-413; see also 3 Williston on Contracts § 1413, p. 2508 (1920) ("Interest is not generally allowed . . . where market rates or prices furnish no definite or exact test of the amount due" (footnote omitted)); 1 T. Sedgwick, Measure of Damages § 312, p. 614 (9th ed. 1912) ("Generally speaking, no interest can be recovered for breach of a contract, where the damages are in their nature unliquidated, until the amount is ascertained" (footnote omitted)). In fact, at the time, they were not allowed in either Colorado or Kansas. See, e. g., Clark v. Giacomini, 85 Colo. 530, 536-537, 277 P. 306, 308 (1929); Denver Horse Imp. Co. v. Schafer, 58 Colo. 376, 390, 147 P. 367, 372 (1915); Roe v. Snattinger, 91 Kan. 567, 568, 138 P. 581, 582 (1914); Evans v. Moseley, 84 Kan. 322, 332-333, 114 P. 374, 378 (1911).
Finally, and most important to this case, an award of prejudgment interest on unliquidated and unascertainable damages for breach of an interstate compact was unheard of at the time of the Compact's negotiation and approval. Unlike cases involving bonds or other instruments of credit, see, e. g., Virginia v. West Virginia, supra, at 232-236; South Dakota v. North Carolina, 192 U. S. 286, 317-321 (1904), monetary damages in cases of this sort, involving the apportionment of water between States, are notoriously difficult to ascertain. Indeed, despite 15 years of litigation over the Compact, and resort to a great deal of data, expert testimony, complicated methodologies, and sophisticated analyses on the subject, the final value of Kansas' damages still has yet to be determined. See ante, at 9, n. 2; see also Third Report §§ III to X (detailing and analyzing the numerous variables and data elements necessary to arrive at a determination of Kansas' damages). It thus is not surprising that, until 1987, we had never even suggested that monetary damages could be recovered from a State as a remedy for its violation of an interstate compact apportioning the flow of an interstate stream. And when we first allowed such damages in Texas v. New Mexico, 482 U. S. 124 (1987), we
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