Anti-government activists seemed primed for a violent clash with federal authorities this summer in the Klamath Basin along the California-Oregon border.

The federal Bureau of Reclamation had shut off water for most of the region’s 1,400 farms, denying access to the same irrigation canal in Klamath Falls, Ore., where during a drought two decades earlier, activists tried to pry open its headgates and clashed with U.S. marshals.