The underlying dispute involves John and Stacy Ambler, who own private property inside Glacier National Park. The Flathead Conservation District and an intervenor, Friends of Montana Streams and Rivers, sought to enforce Montana's Streambed Act — a law regulating and providing a system for approving construction along streambanks — against the Amblers on that property. The Amblers sued for declaratory relief, and the district court granted their motion for summary judgment and denied the cross-motions for summary judgment filed by the Flathead Conservation District and Friends of Montana Streams and Rivers, concluding that the Flathead Conservation District lacks jurisdiction to enforce the Streambed Act against them.
The Ninth Circuit affirmed, working through four distinct legal questions. The court began with the jurisdictional baseline: in 1911, Montana ceded jurisdiction to the United States over private inholdings within Glacier National Park, and the United States accepted that cession in 1914. Montana reserved only the powers to serve process and to tax within the ceded territory. Under that framework, the court held, the United States has exclusive legislative jurisdiction over private inholdings in the park.
The court then addressed whether the Streambed Act was assimilated into federal law. State civil statutes enacted after a cession are assimilated only if they are part of the same basic scheme in effect at the time of cession. The Streambed Act, enacted in 1975, regulates streambank construction approvals. The pre-cession laws identified by the conservation district and the intervenor regulated dumping of sawmill debris near streams and permitted landowners to build docks and wharves along navigable waters without impeding navigation. The court held the Streambed Act is not part of that same basic scheme and therefore was not assimilated.
The court also rejected the argument that the Streambed Act should be treated as a criminal statute — and thus assimilated under the federal Assimilative Crimes Act regardless of when it was enacted. Although the Streambed Act carries a criminal component, the court applied the test from California v. Cabazon Band of Mission Indians: whether the conduct at issue violates the state's public policy. Because the Streambed Act provides a permitting and approval process for streambank construction rather than prohibiting such construction outright, the court held it is a regulatory law, not a criminal one, and was not assimilated when enacted in 1975.
Finally, the court rejected the argument, advanced by Friends of Montana Streams and Rivers, that public policy and federalism concerns support reading the cession statutes to give Montana authority to regulate private property within the park. Because Montana lacks concurrent legislative jurisdiction, the court held there is no basis to apply the Streambed Act to the Amblers' property.
The panel — Judges Owens, VanDyke, and Sung — issued the ruling as a non-precedential memorandum disposition under Ninth Circuit Rule 36-3. The case was submitted April 15, 2026, and decided April 17, 2026, on appeal from Magistrate Judge Kathleen Louise DeSoto of the District of Montana.