Environmental groups yesterday reacted angrily to new policies on use of western lands and water announced by Secretary of the Interior James G. Watt, including the decision to reverse a practice of allowing the federal government to preempt state water rights.

In statements Friday to the Western Governor's Conference in the Grand Teton mountains, Watt also said he wanted to open federal lands, including areas being studied for possible reserve as wilderness areas, for multiple uses.

A memorandum from an assistant solicitor at the Interior Department to Watt before the conference holds, a source said, that the executive branch of government can release lands for mining and other activities before Congress has had a chance to decide whether the lands will be set aside as wilderness areas.

Harry Crandell, staff director of the House subcommittee on public lands and national parks, said such an opinion "would not go unchallenged" and that Congress and previous Interior Department solicitors have opposed such a policy.

A Wilderness Society spokesman said, "If they go ahead with this it would mean many areas with significant wilderness values will be open to destruction by the mineral industry, without Congress having an opportunity to decide which areas deserve protection as national wilderness treasures.

"We have no doubt that such a legal opinion is without substantial merit."

Watt, in reversing the Carter administration opinion that the federal government has some right to water that passes through federal lands regardless of what state law says, apparently was heeding complaints from the states that the Carter policy went too far.

Watt said he wanted to reassert "the historic primacy of state water management." He said the decision "means federal land managers must follow state water laws and procedures except where Congress has specifically established a water right."

An example of the effect of such a policy, Watt said, was that the MX missile project could not just take water for construction. The government would have to apply to the states for water or, environmentalists say, adopt special laws to condemn the water for federal use.

A Pentagon spokesman said he was unable to comment without further study.

Brent Calkin of the Sierra Club said the Supreme Court ruled three years ago that the federal government does have some water rights, at least to take care of uses for which the land was set aside, such as in wildlife refuges.

But he said he feared the new policy might mean that even these rights would not be exercised under Watt, allowing mining and synfuel plants and other new development to draw huge amounts of water from wilderness areas, causing streams to dry up and destroying the wildlife habitat around them.

"If the secretary does not attempt to exercise the water rights the federal government has that is a breach of trust, and I think he should find employment elsewhere," Calkin said.

Other new initiatives mentioned by Watt include consideration of a huge exchange of federal lands for state lands that would give states a chance to consolidate their holdings for development.

Also he said he agreed with state demands that they be given the right to be consulted on federal energy development decisions within their borders.

Yesterday, Watt toured Yellowstone National Park and ignored the boos of hecklers sporting anti-Watt T-shirts. He joined park employes in an outdoor lunch on the banks of Firehole River and watched the geyser Old Faithful erupt on schedule.

Accompanied on his one-hour walking tour by Yellowstone Park Superintendent John Townsley, a crush of reporters and the hecklers, Watt stressed he did not favor and never has favored drilling or mining inside Yellowstone or any national park.

But he refused to answer when reporters questioned him on plans to mine or drill in national forests just outside the parks.