Michigan, according to author Jim Harrison in the novel "Warlock," is really two places.

There is the inviting, unspoiled northland, "with its crystalline lakes and streams, small villages pervaded by the scent of pine forests . . ."

And there is the hideous south, a flat nightmare of vast, single-crop agribusiness and factory cities "as dim and blighted in their own way as Calcutta."

The money is in the south, but to live there, writes Harrison, "in terms of quality of life is something like owning the best (skin) diving equipment and only having a cesspool or a mud puddle to swim in."

Living in Washington used to be a little like that. The river that coursed through the Nation's Capital was a cesspool. Twelve years ago, a newcomer to the city, I stood on the banks of the Potomac at Mount Vernon and gazed out on a sea of green slime that stretched two miles to the Maryland shore.

President Lyndon B. Johnson looked at the same slimy sea and did something about it. In 1965, he called for a Potomac cleanup. Long after his death, LBJ's idea has borne fruit.

It took 10 years and cost more than $1 billion but it worked. The Potomac, benefiting from the 1972 Clean Water Act, is unquestionably cleaner today than it was a decade ago.

Now comes a new breed of federal government to report that the Potomac cleanup was too costly, a waste of money. In a 159-page report, the General Accounting Office has concluded that more than $120 million of the $1 billion was wasted and that the cleanup has not done everything it was supposed to do.

The GAO report calls for Congress, when it reviews and revises the Clean Water Act this year, to use the Potomac study as a model. One GAO recommendation is that Congress require all future river cleanup projects to demonstrate a positive cost-benefit ratio before funds are approved. If a cleanup can't yield more dollars than it costs, the GAO report implies, it ought to be scrapped.

This is the type of thinking that produces places like southern Michigan.

The GAO report concedes the improvement in Potomac River quality. Phosphorus and other pollutants declined dramatically. Dissolved oxygen, the lifeblood of a river, is up 10 percent. There are no more green-slime summers.

But the GAO balance-sheet boys nowhere acknowledge that this improvement, which has turned the river into a recreational and esthetic magnet for hundreds of thousands of people, has any value.

The $1 billion spent was almost all for upgrading and expanding sewage treatment facilities. Ten years ago, the river was a fetid stinkhole.

Those days are over. The cleanup worked. If it did not do all it was supposed to, if money was wasted along the way, that's an administrative problem. But it is not a reason to repudiate the entire program.

Says Paul Eastman, head of the Interstate Commission on the Potomac River Basin, "The GAO says about $20 million was wasted because some plans weren't implemented and some designs weren't constructed. That's like saying the Ten Commandments were a waste because we didn't follow them."

The GAO report ignores the single most important benefit of the Potomac River cleanup: the improvement in quality of life for all who are its neighbors and friends.

The report does not mention that five years ago there was a sign, where Rock Creek meets the river, warning against fishing there because of pollution. The sign is gone today. In April, my daughter and I happily pulled one fish after another from a place 10 yards from where the sign stood. We took them home and ate them.

The GAO report nowhere mentions that Washington is the home of the largest canoe club in the nation--the Canoe Cruisers Association--which counts the Potomac within a few miles of downtown as its major recreational outlet.

Perhaps the accounting wizards are too tired from staring at numbers to glimpse the broad view: the grin of a child catching a shiny herring that swam here from the sea; laughter from a family picnic at Hains Point; the hilarity of the annual raft race, where everyone takes a dunking; water-skiers and sailors below Wilson Bridge; kayakers and canoeists above Cabin John Bridge and the throngs at Fletcher's Boathouse when the white perch and striped bass are running.

The Potomac is the people's river. The captains of government and industry have other places to spend their leisure time--places such as northern Michigan, which are unspoiled, so far, by the offal of civilization and big business.

The people just have their backyard, and the stream that runs through it is the Potomac. They've seen what the river can be and are learning to love it. It's inconceivable that anyone will convince them to let the river be destroyed again to save a few pennies in their tax bills.

The government seems bent on repudiatating its own success with the Potomac, no doubt so it can divert future cleanup funds to some more "productive" programs.

To the growing legions who use the river and to anyone else who appreciates the simple satisfaction that comes from cleaning up a pigsty, there isn't a more productive program.