Environmentalist Mark Dubois may or may not be chained to a rock in a remote canyon, promising to let himself be drowned, but his protest has achieved temporary success.

As search teams fanned through the rugged Stanislaus River canyon today, hunting for Dubois, the Army Corps of Engineers ordered a halt to the gradual filling of its New Melones Reservoir.

Dubois, director of Friends of the River, a group that has fought the corps project for a decade, had told the engineers by letter last week that he would enter the canyon Monday and chain himself to a rock in protest.

Dubois' friends said he in fact had gone into the Stanislaus canyon to await death by drowning, and the corps' district engineer, Col. Donald M. O'shei, took the protester at his word.

O'shei ordered the gates of the New Melones dam opened enough to maintain the lake level at an elevation of 803.5 feet-enough, presumably, to protect Dubois from inundation.

An aide to Michael Blumenfeld, deputy under secretary of the Army in Washington, said the department was satisfied that O'shei had acted properly in suspending the filling of New Melones.

Meanwhile, the aide said, the Army is "doing all that we can to remove Dubois" from the area so that impoundment of the lake can continue.

The problem today, however, was finding the hidden environmentalist. Calaveras and Tuolumne County sheriffs' deputies joined state and federal officials in the fruitless search for Dubois.

Three boats carrying park rangers traveled the shoreline, while 18 other searchers on shore pressed the hunt. A state police helicopter and a state fish and game service airplane assisted from the air.

But as of this afternoon, there was no sign of Dubois, who is said by friends to know the remote nooks and crannies of the canyon intimately, well enough, at least, to evade the searchers.

Dubois and Friends of the River have conducted a years-long effort to prevent the corps from filling the lake, in hope of saving archeological and recreational assets from inundation.

The environmentalists contend that the canyon's treasures would be lost or damaged if the corps raises the level of the lake beyond 808 feet elevation.

The 808-foot level is crucial. At that point the corps will be able to test the hydroelectric power turbines on the $337 million dam project and then turn it over to the Department of the Interior, which is to operate it.

Officials at the Bureau of Reclamation and its parent Interior Department in Washington today were keeping in close contact with corps officials, following progress of the search in the canyon 120 miles east of here.

Although O'Shei ordered a halt to filling the lake, an Army official in Washington indicated that the corps may soon find itself with a difficult operational problem if Dubois is not found.

The lake is being filled in part with runoff from the melting snowfields in the Sierra Nevadas. Other dams upstream and downstream are being regulated to keep the New Melones level at 803 feet.

But officials said water behind Tulloch Dam, the next dam downstream from New Melones, is building quickly, and, if released precipitously, could cause flooding and bank erosion problems in populated areas.

California Resources Agency Secretary Huey D. Johnson Monday urged President Carter by telegram to order a halt to filling New Melones.

He said there is no compelling reason to raise the reservoir because the water and power from New Melones are not expected to be used this year.