Authorities continued their search yesterday for Mark Dubois, an environmentalist who has chained himself to a hidden rock in a California canyon in an effort to stop the Army Corps of Engineers from filling its New Melones reservoir.

After Dubois went into hiding Monday in the Stainslaus River canyon, threatening to allow himself to be drowned, the corps halted filling of the lake. Dubois heads a group that has worked for a decade to save the canyon and its archeological treasures from flooding.

Two reporters confirmed yesterday that Dubois had carried out his threat. They found him with his ankel chained to a large eyebolt which was anchored in solid rock. He told them the keys to a padlock on the chain are hidden 100 feet away from his perch near the water's edge.

The 6-foot-8 Dubois said searcher have come close to him without seeing him. "It's amazing how fast their boats go," he told the reporters.

California Gov. Jerry Brown, in a telegram to President Carter, asked that the impoundment be stopped, and a group of California congressmen has introduced legislation to preserve the unflooded nine-mile stretch of the river.

Dubois, 30, told the reporters that "As this lower river has gone (underwater) a part of me has gone. It's been painful, I've shed a lot of tears."

The Corps of Engineers, local sheriff's deputies and scores of volunteer searchers in helicopters, planes and boats have conducted a massive search of the shoreline since Monday, when his ultimatum - stop the impoundment of drown him - was received by the engineers.