Kansas v. Colorado

Background

Kansas and Colorado fought over water from the Arkansas River under a 1949 interstate compact. Kansas said Colorado’s post-1949 groundwater pumping unlawfully depleted water available to Kansas users. A Special Master found large depletions (1950–1994) and proposed remedies, including monetary damages and a method for calculating future compliance.

Reasoning

The Court reviewed the Special Master’s Fourth Report and decided whether to accept his fixes. It overruled Kansas’ exceptions and adopted the Master’s recommendations. The Court refused Kansas’ request for a permanent River Master, saying many disputes will require policy judgments, arbitration, or compact-administration processes instead. It affirmed using the complex H‑I computer model with a 10‑year rolling measurement period because the Model is inaccurate for short spans and more reliable over longer averages. The Court also interpreted earlier rulings to limit prejudgment interest to the damages the Master treated as “Late Damages” (post‑1984), so interest runs from 1985 on those amounts only.

Real world impact

The decision shapes how future water shortfalls and credits are measured and how much Kansas may recover. Colorado’s replacement program and a 10‑year averaging rule will affect when and how Colorado must replace water. State courts’ determinations about replacement credits may be used initially, but Kansas can still seek relief in this Court. The Court retained limited jurisdiction and sent the case back for a decree consistent with these rulings.

Dissents or concurrances

Justice Thomas joined except on the prejudgment‑interest issue. Justice Stevens would have allowed interest on damages accruing after 1968 (disagreeing with the majority’s narrower interest ruling).