He was chained to a boulder beside California's Stanislaus River, hiding from police boats and waiting for the water to rise.

"The Army Corps of Engineers had already dammed the river and were starting to flood a canyon that had been evolving for 9 million years," said Mark Dubois. "I figured if they were going to destroy that, they might as well take one other little critter with them."

Dubois' riverside melodrama three years ago had a finale that would make a scriptwriter for soap operas blush. With the water just two feet from his chains, a letter from the governor's office was rushed to his hiding place, promising that the canyon would be saved.

Last weekend, Dubois, 34, was in Washington to share his passion for untamed water with 170 people from about 35 states at the eighth annual National Conference on Rivers. For three days, representatives from about 65 private, state and federal organizations discussed ways to preserve wild rivers, oppose dams and generally plot strategy for the sometimes bitter war being fought between the Reagan administration and environmentalists.

The pros and cons of human sacrifice were not on the agenda.

"One of the benefits of this kind of conference is to show that others have been successful in taming these fearsome bureaucracies," said Brent Blackwelder, the Washington representative for the Environmental Policy Institute. "Environmentalists are no longer sitting on the sidelines. Politicians are seeking to be on our side. We are motherhood and apple pie."

For high-minded vitriol, there is no match for the unrelenting battle between this administration, as personified by Secretary of the Interior James Watt, and an eclectic army of environmentalists and outdoor organizations such as The National Wildlife Federation, The Sierra Club, the Izaak Walton League, Trout Unlimited and the National Audubon Society.

Last year 10 leading conservation organizations issued a statement at a joint press conference that read, "We are here to indict President Reagan and his administration for their extremism . . . in ignoring, circumventing or destroying the environmental laws and programs they are charged with administering." That sounds almost conciliatory compared to statements made since, particularly one by the director of the Wilderness Society that characterized Watt as "Mr. Rape, Ruin and Rot."

Reagan and Watt have led a vigorous counterattack, labeling conservationists as "environmental extremists" who will not be satisfied until the White House is a giant bird's nest.

The great irony in this escalating conflict is that the environmental movement has never been stronger, or more effective, than it is at present. Membership in conservation organizations has increased dramatically in the last two years. And fund raising is almost easy.

"People are always more liable to respond when there's a threat . . . when you can get them mad," said Pat Munoz, an official with the American Rivers Conservation Council, one of the sponsors of last weekend's conference.

During the last two years, conservation groups that have traditionally maintained a nonpolitical stance have been prodded by administration policies into open conflict with Reagan, Watt and the EPA.

An example of that polarization was the recent call by the National Wildlife Federation for Watt's ouster. "Our members are conservative folks. They voted 2 to 1 for Reagan over (Jimmy) Carter," said Ron Way, a spokesman for the Federation, which claims 4.2 million members and supporters.

Participants at last weekend's conference said Watt's contentious personality had done more to focus attention on environmental issues than any publicity campaign they could organize. Last week's debacle over rock and roll on the Mall was cited as a perfect example of the Watt rebound.

"If Watt had just announced that Wayne Newton was being invited to play on the Fourth of July this year and left it at that, there would have been little dispute," said one environmental lobbyist. "Instead he had to lash out against decadent youth . . . and the Beach Boys."

Since that episode, there have been renewed calls for Watt's resignation. No one at the rivers conference would admit to opposing such a move. But a few conceded that Watt has been more of a friend to his enemies than he would care to know.

"He has galvanized our support," said Chris Brown of the American Rivers Conservation Council. "People are asking, 'What happens to our fund raising in 1984 if Watt isn't around?' "