concurring in part and concurring in the judgment.
I join the Court’s opinion with the exception of Part II, which concerns whether prejudgment interest should begin accruing in 1985 only on damages thereafter arising (post-1985 damages) or also on damages then owing (pre-1985 damages). As Justice O’Connor explained in Kansas v. Colorado, 533 U. S. 1 (2001) (Kansas III), neither the Arkansas River Compact itself nor the common law at the time of the compact’s formation allows Kansas to recover any prejudgment interest. See id., at 21-25 (opinion, joined by Scalia and Thomas, JJ., dissenting in part). The Court did not adopt that view in Kansas III, but neither did it adopt the now-familiar rule that Kansas should be made whole with an award of prejudgment interest spanning the duration of Colorado’s breach, from 1950 to the present. See, e. g., Milwaukee v. Cement Div., National Gypsum Co., 515 U. S. 189, 195-196, and n. 7 (1995); West Virginia v. United States, 479 U. S. 305, 310-311, n. 2 (1987).
The Court instead crafted what it viewed as an equitable compromise, designed to apply sui generis to these States *107and their particular dispute, in which prejudgment interest would begin to accrue in 1985. See Kansas III, supra, at 14-16. Its compromise left open the door to the present litigation, for saying when prejudgment interest began to accrue did not answer on what the interest was accruing. The Court therefore must again decide what is too little or too much compensation for Colorado’s depletion of the Arkansas. That weighing is as unnecessary now as it was before. Kansas is not entitled to prejudgment interest, and its exception seeks only to compound the windfall it received in Kansas III. I therefore agree with the Court that Kansas’ second exception to the Special Master’s Report should be overruled.