Only a small fraction of U.S. lakes and streams have been damaged by acid rain, and the damage is not likely to worsen significantly, a federal task force has concluded in a report that will probably cause a furor among conservation groups and congressional supporters of acid-rain controls.

The National Acid Precipitation Assessment Program, in a draft report that is scheduled to be released today, states that current research suggests "that there will not be an abrupt change in aquatic systems, crops or forests at present levels of air pollution."

The conclusion runs counter to arguments by some scientists that acid-rain damage is widening in the United States and poses an increasing threat to the nation's lakes and forests. The report strongly supports the Reagan administration's position that acid rain does not warrant expensive new pollution controls.

The acid-precipitation program, established by Congress in 1980, has become the focus of the administration's policy on acid rain. Its latest report, originally due in 1985, was to mark the halfway point in a comprehensive 10-year research effort.

The Natural Resources Defense Council, which obtained a draft of the report and released it to reporters yesterday, called the document "nothing more than political propaganda."

"This is the Pollyanna-in-blinders approach," said Richard Ayres, senior attorney for the group. He accused the task force of ignoring studies inconsistent with the administration's position, selectively quoting from others and accepting unrealistic projections about emission levels in an effort to demonstrate that "the problem will go away by itself."

As an example, he said, the report embraces a controversial scientific theory that acidified lakes and streams eventually reach a "steady state" in which additional acid rain causes no additional damage. The report concludes that most U.S. waters, except in the Southeast, have reached that stage.

The finding is a crucial one, suggesting that the environment will suffer little damage if acid-rain controls are not installed for several more years, if ever.

J. Laurence Kulp, research director for the acid-rain task force, was not immediately available for comment. Courtney Riordan, research director of the Environmental Protection Agency and a member of the task force's science committee, acknowledged that the "steady state" theory has not been proved, but he added: "The consensus of the community is that it is likely to be so."

According to Ayres, the theory has been disproved by scientists in Canada, who have documented increasing acidity in one long-studied lake despite a reduction in acid rain over the period of the study.

The environmental group also criticized as "misleading" the report's assertion that only a small fraction of U.S. waters have become acidified. The report summary gives such data in terms of pH, or acid content, rather than using what Ayres said is the more accurate measurement of a lake's ability to neutralize acid.

As a result, he said the report states that only 2 percent of lakes in the Upper Midwest had a pH of less than 5.0, although an EPA survey found that 10 percent of the lakes in the Upper Peninsula of Michigan had lost their ability to neutralize acid and were acidified. (A pH of 5.0 is 100 times more acid than ordinary water.)

The environmental group also criticized the report's inclusion of an Energy Department projection that emissions of sulfur dioxide, a key component of acid rain, will decline sharply after the turn of the century.

The projection assumes that older coal-fired power plants will be going out of service by then, although utility companies have increasingly elected to repair older plants rather than build new ones. The projection also assumes that nuclear power generation will triple over the next 40 years, although no new nuclear plants have been ordered in nearly a decade.