EVERY NOW AND THEN, the fates give Congress and a president a delectably inviting chance to serve the public and, as a bonus, actually live up to some of the billingsgate that passes for purposeful policy statement.

One of those chances is looming nigh: An opportunity to once and for all slay the billion-dollar Garrison Diversion Project in North Dakota, one of the most relentless public-works dinosaurs in a long line of such creatures.

Garrison is a classic.

In one swell foop, as they say, Garrison unites virtually every unhappy element one could conjure for a public project.

As originally conceived, the project would carry Missouri River water across the state for irrigation and municipal use. Its pollution potential has enraged Canada, which is worried that flora and fauna in the Hudson Bay watershed cold be endangered by Garrison waters carried by rivers flowing north across the border.

The project's irrigation subsidies would enrich a few of the state's 36,000 farmers. It would destroy thousands of acres of valuable wetlands and wildlife refuges. It would wipe out working farms to enhance others.

It gets even better.

At a time of massive wheat surpluses, Garrison would irrigate land to grow more wheat. As the government pays North Dakotans not to grow wheat, it will spend money to make more wheat-growing possible.

Garrison would also send a new flood of North Dakota potatoes to glutted market and depress farmers' prices nationally nearly $500 million, by some reliable estimates.

At a time when social programs are being slashed with abandon, Garrison would give each of about 400 farmers a $1.6 million federal water subsidy to grow food that the government will subsidize further with its crop-support programs.

At a time when President Reagan is railing against profligate spending on boondoggle projects, his fiscal 1986 budget proposes spending about $41 million on Garrison so that the federal Bureau of Reclamation can move ahead with its dream of feeding the dinosaur.

Given those curiosities, the

wonder is that Garrison hasn't died of a case of lard around the heart. Congress has argued and held its nose for 20 years over this huge irrigation scheme, but kept it alive solely at the insistence of a string of North Dakota legislators with power on the key appropriations committees.

A year ago, Garrison seemed doomed or at least stalemated. Environmental groups, led by the National Audubon Society, had blocked funding for a third straight year. The project's supporters pulled an end run. They got Congress to create a blue-ribbon commission to study alternatives and make recommendations for a "compromise."

Beautiful. Reason and logic would prevail. Foes would join hands. The system of political give and take would work. North Dakota would get its federal project. Bird, beast and ecosystem would be winners. Canadian apoplexy would vanish.

But the commission's "compromise" proposed more irrigation and a bigger subsidy for fewer farmers. It proposed tripling the amount of water carrying farm chemicals and salts into the James River, which has infuriated farmers downstream in South Dakota. It would expand the amount of subsidized municipal water for North Dakota communities. It would save some of the wetlands and refuges, sacrifice others.

Along the way, the compromise has left the Audubon Society with substantial amounts of egg on its face. Committed to working "within the process," the old line conservation group has alienated members in the Upper Midwest and got its paws stuck on a tar baby by supporting the compromise.

"The process worked because of the heroic efforts of the commission," said Glenn Paulson, an Audubon senior vice president, in the February issue of Audubon Action.

But a month later, another Audubon vice president, William A. Butler, was telling a House appropriations subcommittee that the Garrison compromise was an abomination. "If the compromise proposal came to us as a brand new project we would outright oppose it," Butler said, noting a number of unpalatable features in the commission plan.

A deal may be a deal, but it is not necessarily a given that the dinosaur will live. The House Interior water- resources subcommittee is scheduled to take up legislation this week that may decide the fate of Garrison.

Chairman George Miller (D- Calif.), who built a career chipping away at the federal water subsidies for big farmers in his home state, sent shock waves through the compromisers at a hearing in February when he warned that he found the immensity of Garrison hard to justify "at this particular time."

A week later, Miller came upon more Garison curiosities. An economic analysis provided by Marc Messing, who served on the commission staff, told in excruciating detail how the commission had ignored vital cost data that might have forced a more modest compromise.

Although the Bureau of Reclamation supports the irrigation proposals in the plan, the administration last week still was pondering whether to line up with the compromise.

There are a couple of other sides to this, however. A truly caring government, instead of rolling ahead with Garrison, probably could do North Dakota agriculture more good and spend less simply by canceling about $895 million that farmers there owe the Farmers Home Administration.

Or the government could leave well enough alone. A University of North Dakota poll in February reported that seven out of 10 respondents said they didn't follow the commission's work closely and five out of 10 said they had no opinion on its proposals.

Half or more, in other words, didn't know or didn't care. The fates may be trying to tell Congress something.