Kansas v. Colorado
The Supreme Court rejected every objection Kansas raised to a special master's latest recommendations in its long-running water-sharing fight with Colorado, refusing to appoint an independent 'River Master,' expand Kansas's interest award, or shorten a 10-year measuring period used to track Colorado's compliance.
The ruling keeps in place a detailed framework for enforcing the 1949 Arkansas River Compact, relying on a complex computer model and state-court determinations rather than creating new machinery, and sends the case back for a final decree.
“This Compact is not intended to impede or prevent future beneficial development of the Arkansas River basin”
Quoting the 1949 compact provision at the heart of the dispute over Colorado's water use.
How it got here: This is an original Supreme Court case between two states; a Court-appointed Special Master issued a Fourth Report resolving remaining remedy issues, and Kansas filed exceptions to several of his recommendations.
The Case in Depth
What happened
Kansas and Colorado have fought for decades over sharing the Arkansas River, which flows from Colorado into Kansas. A 1949 interstate compact promised to divide the water fairly, but Kansas said Colorado let farmers pump too much groundwater from new wells, drying up water Kansas was owed. Kansas sued in 1985, and after years of litigation and expert reports, the Court had already found Colorado liable and ordered damages; the parties then fought over how to calculate and enforce that remedy going forward.
The question before the Court
In this decades-long fight over the Arkansas River, could Kansas force a special river master, more interest payments, and shorter compliance checks on Colorado?
Why it matters
Farmers and water users in both states depend on how the Arkansas River's flow gets measured and divided. By approving a 10-year measurement window and a computer model instead of a stricter yearly check, the Court gave Colorado more flexibility, while preserving Kansas's right to challenge specific water-credit decisions later.
What changes now
The case goes back to the Special Master to prepare a formal decree consistent with the Court's opinion. The Court will retain ongoing jurisdiction so the Special Master can resolve any future technical disputes over the computer model, water credits, and compliance monitoring, with the parties encouraged to use arbitration, negotiation, or informal dispute resolution before returning to litigation.
What this does not decide
The ruling does not resolve every outstanding technical dispute over how the water-tracking computer model should be calibrated or how future compliance credits will be calculated; the Court explicitly left many of those questions for the Special Master to take up later as more information becomes available.
Concurrences and dissents
Concurrence — Justice Thomas
Justice Thomas joined the Court's opinion except for the interest-calculation part. He argued that neither the compact itself nor the common law existing when the compact was formed actually entitles Kansas to any prejudgment interest at all. He believed the Court's earlier compromise on interest was already too generous to Kansas, so he saw no need to weigh whether the current interest calculation was adequate — Kansas should get none.
Dissent in part — Justice Stevens
“Nothing in our Kansas III opinion compels such a result.”Stevens arguing the majority wrongly limited Kansas's interest award based on the Court's earlier ruling.
Justice Stevens joined the Court's opinion except on the interest question, where he would have ruled partly for Kansas. He argued the Court's prior decision had implicitly approved paying interest on all damages from 1969 onward (when Colorado knew or should have known it was violating the compact), not just damages arising after 1985. He would have awarded Kansas interest on damages from 1969 through 1985 as well.
How the Court got there
The legal reasoning, step by step
- The Court explained that appointing an independent 'River Master' to police compliance is a discretionary tool used only when disputes are likely to be resolved through mechanical, largely non-controversial calculations, as with a simple mathematical curve used in a past Texas-New Mexico river case.
- Because future disputes here would involve a highly complex computer program (the Hydrologic-Institutional Model, or H-I Model) requiring judgment calls closely tied to the parties' legal rights, and because the states could use the compact's own arbitration process or informal negotiation instead, the Court found a River Master unnecessary.
- On the interest question, the Court read its own prior ruling (Kansas III) as having implicitly approved a specific method for calculating additional interest — one that applied interest only to damages arising after 1985, not to older damages already fixed by the special master's earlier report. The Court found no basis in the prior opinion or the resulting dollar figures to expand that interest calculation as Kansas wanted.
- On the measurement-period question, the Court weighed the compact's text, which does not fix a time period for measuring water credits and debits, against real-world accuracy problems: the computer model was shown to be unreliable over periods shorter than ten years but highly accurate over longer stretches, so a ten-year window served the compact's actual purpose better than a one-year window.
- The Court also found that letting a state court (the Colorado Water Court) make an initial determination of water-replacement credits did not improperly let Colorado 'judge its own case,' because the Special Master's recommendation expressly preserved Kansas's right to challenge any such credit determination directly in the Supreme Court's original jurisdiction.
- Finally, the Court agreed that several remaining technical disputes over model calibration and future compliance did not need immediate resolution, since some were moot and others would be easier to resolve accurately as more real-world data and modeling experience accumulated.
Doctrinal impact
Cases affected by this decision
Reaffirms Kansas v. Colorado (Kansas III) (533 U.S. 1)
The Court relied on its earlier equitable compromise on when and how prejudgment interest should be calculated.
Distinguishes Texas v. New Mexico (482 U.S. 124)
The Court said this river's complex computer-model disputes differ from the simpler, more mechanical Pecos River disputes that justified appointing a River Master there.