A river that burned, Ohio's Cuyahoga sparkles once again.

One of the brightest stories in Cleveland is being told by a concrete box on the banks of the Cuyahoga River. In the red-brick, smokestack heart of this industrial city, the unassuming gray structure is swallowing Cuyahoga water - after it has passed through everything from industrial facilities and sewage treatment plants to farms and storm drains. Its purpose: to measure the water quality of a river once so polluted that it caught fire.

Now, to the relief of long-embarrassed Clevelanders, and in testament to the country's Clean Water Act, reports show marked increases in the amount of dissolved oxygen that fish and other living water-quality testers find in the hard-working Cuyahoga.

The Cuyahoga River is not the only star in this national cleanup effort. Compared with 1972, when Congress approved the first strict water-quality standards, the nation's rivers, streams, and lakes today are measurably cleaner and more life-supporting. Striped bass have returned to the Hudson River; salmon again battle the waters of the Connecticut River. And Lake Erie, for years so clogged with waste-fed algae that many considered it beyond salvation, once more sports some of the world's premier fishing waters.

But despite nearly $50 billion spent on water pollution control by federal and other public agencies, and billions more spent by private industry, many waterways still do not meet the ''fishable, swimmable'' standard Congress said rivers and lakes should meet by 1983. And even as Congress faces pressure to weaken water-quality standards, environmentalists warn that virtually uncontrolled toxic pollutants pose a major threat.

But for many, the Cuyahoga River remains the litmus test of progress in the battle against water pollution.

''I think the notoriety of the Cuyahoga was a large part of the incentive behind the environmental movement,'' says Bob Wysenski, an environmental scientist with the Ohio Environmental Protection Agency in Twinsburg, Ohio. Mr. Wysenski began working for the state on pollution control in 1972, the same year the federal EPA was created.

Three years earlier, the Cuyahoga caught fire in downtown Cleveland. Slick with oil from upstream refineries, oozing and churning with the refuse of dozens of steel and chemical plants, and choked with largely untreated sewage from more than 1 million households, the river shot forth flames and heavy black smoke as if to declare diabolically that it was not yet a dead river. Overnight, it became a national symbol, and helped galvanize a movement that, until then, had groped to make ''environment'' a household word.

Today most Clevelanders cringe when visitors recall the burning Cuyahoga. They note that it has been years since oil floated on the water's surface. The huge ore barges navigating up from Lake Erie must now share the river with increasing numbers of pleasure boats, and rarely do foul odors drive summertime patrons from the riverfront decks of restaurants and night clubs.

''After many years of hard times, there's a new interest in this part of the city, and the river is the center of that attention,'' says Raymond Haserodt, whose family tool-and-die business has been in ''the Flats'' - Cleveland's old industrial center - for more than 30 years. ''That interest probably wouldn't have been there if the river were still filthy,'' he says.

In the Flats, the concrete-boxed water-quality monitor is reporting a steady increase in the number of days that the river meets Ohio's dissolved-oxygen standard (no less than four milligrams per liter) - from 128 days in 1968 to 221 in 1982. Wysenski says he expects tests this summer to show a great leap in oxygen levels as a result of major improvements at Cleveland's sprawling Southerly Waste Water Treatment Plant.

Dissolved oxygen, which is depleted when a waterway is overloaded with decomposing wastes, is an important indicator of water quality because fish and other aquatic creatures require it. When Moses Cleaveland founded present-day Cleveland in 1796 on the banks of what the Indians called Cuyahoga (''winding river''), there were perhaps 50 varieties of fish swimming the waters. By the late 1960s there were virtually no fish in the river down in the Flats. But today there are catfish, shad, and some perch, and fishermen at the river's mouth occasionally report catching coho or walleye.

Of course, the Cuyahoga does not begin and end at the blackened steel mills, the chemical companies, and other plants of Cleveland's industrial center. The small, U-shaped Cuyahoga runs for 100 miles, beginning as a pristine stream, then first tasting civilization outside of Kent before confronting full-fledged urbanization at Akron. From the Tire City it runs north for 18 miles through the Cuyahoga Valley National Recreation Area before flowing through Cleveland to Lake Erie.

In terms of antipollution costs, it is easily a billion-dollar river. The Cleveland Southerly treatment plant alone has had $600 million in improvements since 1974. Republic Steel, the largest steel company now operating in the Flats , has spent about $85 million since 1972 on water-pollution control facilities that cost an additional $20 million a year to operate.

Still, some riverside residents say they are not convinced that the state's enforcement of the Clean Water Act has been strict enough.

But Robert Martin, assistant superintendent of the Cuyahoga Valley National Recreation Area, notes that closing a town's sewage treatment plant is not feasible if there is no alternative. Mr. Martin laments the fact that pollution still forces him to discourage recreational use of the Cuyahoga, which forms the backbone of the 18-mile-long park.

''But the EPA's job is a tough one,'' Martin says. ''They've ordered some small, inefficient treatment plants out along here to either improve or close. But what do you do if they don't have the money (to improve)?''

Bob Wysenski says a series of new pipelines are planned to carry the sewage of outlying areas and heavy-storm overflows to expanded metropolitan treatment plants. This will allow a number of small, low-quality plants to close.

According to Wysenski, the next big battle in the national war against water pollution will be in the control of toxic substances. Along the Cuyahoga, industries and sewage treatment facilities dump effluent containing high levels of ammonia and other chemicals into the river, while runoff from farms and gardens add dangerous pesticides.

The Clean Water Act is up for review in Congress during the coming months. Regulations for toxic substances - just now being implemented by the EPA - are coming under attack from the Reagan administration and chemical industries. EPA guidelines call for industry to reclaim and reuse the chemicals, but critics say the process will be too expensive for the benefits gained.

Supporters of a strong Clean Water Act say the administration, after cutting federal support for sewage treatment facilities and the EPA's budget in general, is now looking to weaken antipollution laws by allowing states to set their own, less stringent standards.

Yet in the eyes of Erwin Odeal, director of the Northeast Ohio Regional Sewer District, the time may have come when environmental benefits in enforcement of ever higher standards on a river such as the Cuyahoga are outstripped by economic concerns.

''When our water quality was much worse and with the height of the environmental movement, there was wide public support for very high standards and money was not seen as a limitation,'' says Mr. Odeal. ''But now many people are asking if maybe we can't afford all this. I think we're finding that these high standards for certain sections of the Cuyahoga are just not feasible.''

Bob Wysenski says he believes clean water and a strong economy need not be mutually unattainable. To help him remember the many roles of the nation's waterways, he keeps tacked to his door a picket sign from outside a talk he once gave on water quality to members of the United Steel Workers Union in economically depressed Youngstown, Ohio. The sign reads, ''Fish or Jobs.''

''I don't think we have to accept that kind of either-or,'' says the environmental scientist. ''But it helps me keep a perspective.''