The Supreme Court ruled yesterday that it was too late to correct a 20-year-old mistake by the federal government that cheated five Indian tribes out of nearly a quarter of their rights to western irrigation water.

The court said it had the power to reopen the case but that in the interest of "finality" did not want to upset the delicate balance among users of water from the Colorado River.

The 5-to-3 decision came over the strenuous objections of dissenters, who called the result a "manifest injustice" which raises questions about the quality of the representation the U.S. government has provided for the Indians. It is "proof (if any is needed) that those with direct interests--economic, historical, spiritual--in the outcome of a case are their own best representatives," Justice William J. Brennan Jr. said in dissent.

The mistake, to this day not fully explained, occurred while the government was representing the tribes in a court dispute among five western states over access to water from the Colorado. During the proceedings, government officials understated by about 22 percent the amount of irrigable land on the reservations. As a result, the tribes got roughly 22 percent less water from the river, according to yesterday's opinion.

Four years ago, the tribes asked that the proceedings be reopened to correct the error. Yesterday, even though the government admitted its mistake and joined in the tribes' plea, the justices refused.

Justice Byron R. White, writing for the majority, acknowledged that it would be permissible legally to reopen the matter. To do so, however, could open a "Pandora's Box" which would upset the water rights balance along the Colorado, White said. In the interests of finality, he said, the issue should remain closed.

He said the tribes were not using their current water allocations and that any enlargement of them "cannot help but exacerbate potential water shortage problems" for the surrounding states. "When viewed against the serious water shortages faced by all people, including other tribes" in the area, this is hardly the mark of manifest injustice, White said.

The Colorado runs from the Denver area in Colorado, southwest through Utah and Arizona and the Grand Canyon. It then cuts south, along the California-Arizona-New Mexico borders and into Mexico. The dispute, which began in 1952, was joined by all those states and the Quechan, Cocopah, Chemehuevi, Fort Mojave and Colorado River tribes.

Such controversies do not have to go through the U.S. District Courts but are handled directly by the Supreme Court, through the appointment of a special master who submits recommendations to the justices.

Under the standard established earlier in this case by the court, the amount of irrigable acreage determines the amount of water allocated. Thus, the understatement reduces the allocation gallon for gallon.

In its recent submissions to the court, the government has suggested that the mistake occurred because of the complexity of the dispute. Other officials have speculated that it was done for tactical reasons, to make the tribes' claims seem more reasonable and to get the court to accept the irrigable acreage standard for allocation, which was then a novel theory.

Some lawyers for the tribes say it was conflict of interest that led to the error. The United States has to represent not only the tribes but numerous federal water interests, such as reclamation projects, that seek access to the same river. To the tribes, the episode exemplified the poor representation they have received over the years.

The dissenters, Brennan and Justices Harry A. Blackmun and John Paul Stevens, said yesterday that they had failed to find "any justification" for what happened.

"There has often been reason to question the quality" of the government's representation of the Indians, Brennan wrote, "especially when rights to scarce water in the West" are at stake. "The United States," they said, "has sometimes been slow to press Indian claims when they conflicted with those of politically influential non-Indian interests.

" . . . As a matter of justice, the tribes desire this chance to defend rights which should have been theirs," Brennan wrote.

Justice Thurgood Marshall, who was U.S. solicitor general when the case was being adjudicated, did not participate.

White said the representation problem did not justify the reopening. "There was no demonstration that the United States . . . was involved in an actual conflict of interest," he said.

Reopening would create uncertainty, White stated. And "certainty of rights is particularly important with respect to water rights in the western United States. The development of that area of the United States would not have been possible without adequate water supplies in an otherwise water-scarce part of the country," said White, a Coloradan.

The court did rule in favor of a related but much smaller portion of the tribes' claim that raised separate issues. Another part of the ruling in Arizona vs. California et al., granting the tribes permission to intervene in the dispute, also disappointed tribal lawyers.

Under the ruling, tribes are permitted to participate independently in such cases but do not have an automatic right to participate.

Overall, said Arlinda Locklear, an attorney for the Native American Rights Fund, "this is a very bad decision."