In an annual reversal of roles, the little snail darter was pronounced a species on the rebound by federal officials yesterday while its nemesis, the giant Tellico Dam, may be facing extinction.

Until recently the rare three-inch perch lived only in the shadow of the nearly completed $120 million federal dam on the Little Tennessee River. Conservationists warned that when the Tellico was finished and the river was closed off the snail darter would be no more. In June the U.S. Supreme Court ruled the Tennessee Valley Authority had to stop work on the dam to save the snail darter and its singular habitat.

But yesterday the TVA and the Interior Department jointly announced that the snail darter - with a little nudge from the government - seems to have found a new home. In 1976, when the Tellico Dam controversy was reaching a shrill stage, federal experts moved 710 snail darters to the Hiwassee River, some 18 miles from the dam site.

TVA Chairman S. David Freeman said yesterday that samplings of the transplanted snail darters indicate they are flourishing in the Hiwassee with up to 3,700 fish now in the river.

That was the good news, Freeman told reporters at a new conference. The bad news is that silting and other construction problems have sharply cut the remaining snail darters at the dam site to only about 500. In fact, Freeman conceded, by the time a decision is made by TVA on what to do with Tellico there may not be any snail darters left there to worry about.

In a 113-page on the future of the project by the two federal agencies there were indications that the TVA, which has lobbied strongly for permission to complete the dam, may be softening its position.

The report, released yesterday, listed three major choices for the future of the dam, which is more than 90 percent complete. They were:

Completing the project as originally planned with protection for the snail darter accomplished by moving the rest of the fish to a new breeding area. The fish are covered by the 1973 federal Endangered Species Act, which was the basis for the Supreme Court's decision to halt work on Tellico.

Leaving the existing dam unfinished and building a separate new dam and reservoir on a tributary of the Little Tennessee River. This option was rejected in the study as too costly and without benefit.

Development of the river and surrounding land at the Tellico site without closing the dam except in a flood emergency, with a second alternative of removing the earthen portion of the dam and letting the river return to its natural course.

Freeman declined yesterday to say which option he would favor. The TVA chairman, who was appointed last year, said the TVA board would probably make a recommendation to Congress and the president on the dam's future after the remaining two board members are appointed.

Freeman and assistant Interior Department secretary Robert L. Herbst said yesterday that rising land values at the dam wite wuld insure that any decision would probably end up as profitable. The cost of completing the dam to recently enacted "superflood" standards would be slightly less than leaving it incomplete and taking the riverside development option, the officials said.

But Freeman noted that the no-dam option would mean more local jobs and only a relatively small increase of one tenth of a percent in TVA's fuel bill. The giant power authority would lose no generating capacity if the dam is not built, he said, and several valuable archeological and historical sites would be preserved.

The final decision on the dam, said Freeman, "is as much a question of values as cold hard fact."