The tan riffle shell pearly mussel isn't a creature many people worry about, but it and its aquatic cousins across the country are starting to cause nightmares for the folks who build big dams on the nation's rivers.

The pearly mussel is about to be placed on the government's list of endangered species. That would mean no government agency could build any project that might disturb its habitat. And it would mean that the Tennesse Valley Authority's proposed dam on the Duck River, where the small mussel lives, would be in jeopardy.

The pearly mussel is only one example. According to the U.S. Fish and Wildlife Service, its endangered species list includes, or will soon include, about 10 crayfish, 30 fish, 40 mussels, 40 freshwater snails and one shrimp - all living in habitants imperiled by government dams.

For the TVA, other dam-building agencies, and members of Congress who like dams, the list of endangered species is itself a growing danger. The TVA is lobbying for help to keep its dams safe, and some congressional aides are working on legislation to keep them and other projects free of such environmental challenges.

The endangered projects are widely spread. There's the TVA dam in Columbia, Tenn., on the Duck River where the pearly mussel lives and breeds. There's Darby Creek in Ohio, where the Corps of Engineers is planning a dam that threatens the soon-to-be-listed northern riffle shell, another mussel. There are three dams planned for the Flint River in Georgia where two species of fish, two of snails and one of mussels would be imperiled.

The confrontations between dams and wildlife are expected to multiply soon because the Fish and Wildlife Service plans to classify some 1,700 species of plant life as endangered. They will be the first of their kind. Now, the list includes 609 species or subspecies of fich and animals.

For example, there's a rare species of snapdragon in Maine that the Fish and Wildlife Service says would be obliterated by the giant Dickey-Lincoln public power project. And, it says, the La darge Dam on Wisconsin's Kickapoo River would wipe out the northern wild monk's head, a flower.

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The endangered list was created to preserve rare species of fish and wildlife that might disappear from the planet. Such large projects as dams are threats because reservoirs flood the fish's breeding grounds and slit over feeding sites.

No one paid much attention to the preservation law until the celebrated case of Tennesse's snail darter, a three-inch fish that flourished only in a 17-mile stretch of the Little Tennessee River. That stretch would be flooded if TVA's Tellico Dam were completed.

In a decision that rocked the dam-building world, the Sixth U.S. Circuit Court of Appeals last week ruled that construction had to stop on the Tellico, even though it is 90 per cent complete. The $116 million dam has been building since 1966.

The snail darter decision and the threat of similar rulings in other cases has provoked a flurry of lobbying in Congress and some anxious attention by the Senate Public works Committee.

Larry Calvert, TVA's Washington representative, says, "If the courts say that a project started years ago can be stopped. It isn't just a case of the snail darter. That is the problem."

TVA's directors are using the energy shortage as an argument for completing the dam quickly, saying that Tellico would provide power enough to heat 20,000 homes in the TVA region. Its critics contend that Tellico is primarily an industrial development project, not a hydroelectric plant, and that it would add less than one-tenth of 1 per cent of TVA's total generating capacity.

As soon as the appeals court ruling was announced, the TVA began talking of seeking special legislation exempting Tellico from restrictions of the 1973 endangered species act.

That hopes faded when Sen. Howard H. baker Jr. (R-Tenn.), ranking Republican on the Public works Committee, rejected exemption for a single case. Instead, he called for a review of the endangered species act to see if a balance could be reached between environmental interests and public works projects.

Congressional aides working on the issue are aiming to trim the scope of the endangered species act, but have not yet decided how to do it. Some feel the law should be tailored to preserve very important species of wildlife - the bald eagle or the American crocodile, for example - but not the many species that differ only slightly from other species.

Critics of the act point out that the snail darter, for example, differs very little from approximately 50 other species of darters found in Southern waterways from Texas to Tennessee.

Preservationists contend, however, that a strong law is needed to stem the loss of small species as a result of pollution and large construction projects.

"Ten per cent of all known species - of fish, plants and wildlife - have been wiped out in the last 100 years," said Mark Imlay of the Fish and Wildlife Service. "We would guess that another 15 per cent would disappear inthe next 20 years."