Mattz v. Arnett was a United States Supreme Court case in which the Court held that the land that had been the Klamath River Reservation and was incorporated into the Hoopa Valley Reservation in 1891 remained Indian country within the meaning of 18 U.S.C.

California game wardens seized 5 nylon gill nets from where they were strung across a point at Brooks Riffle on the Smith River, within one mile of its confluence with the Klamath River, less than 20 miles from the mouth of the Klamath, from Fee Land claimed by a lumber company within the Yurok Reservation.

[citation needed] On Oct. 21, 1971, the petition to intervene by Yurok Tribal member Raymond Mattz to resist the forfeiture of the gill nets, asserting that an enrolled Indian fishing in Indian country could not be statutory prohibited from using traditional gill nets, was denied by the trial court.

The state trial court found that the Klamath River Reservation in 1892 ‘for all practical purposes almost immediately lost its identity,’ and concluded that the area was not Indian country.

The state Court of Appeal affirmed, holding that since the area had been opened for unrestricted homestead entry in 1892, the earlier reservation status of the land had terminated.