The Great Salt Lake, which has no outlet, is rising more dramatically than it has for 90 years after an unusually long spell of cool, wet weather over this flat, dry desert.

The lake has flooded mineral installations and wildlife refuges, undermined railroads and is lapping at the highways that keep this desert city alive.

For a century, Utahns enjoyed floating on their backs in this natural wonder, the largest lake west of the Mississippi and the second saltiest lake in the world. Residents used to regret that the lake seemed to be evaporating.

Officials have mustered only feeble responses so far to the rapid rise.

The state legislature passed a bill prohibiting the lake from rising higher than 4,202 feet above sea level. Without noticeable hesitation or remorse, the lake broke the law on Feb. 1.

The Great Salt Lake has an actual average depth of only about 15 feet, but scientists use the height of its surface above sea level as a measure. It now stands at 4,203.2 feet. Just 9 1/2 inches more, said Ted Arnow, district chief of water resources for the U.S. Geological Survey here, and "it will be the largest seasonal rise we have ever had since we began recording" in the 19th century.

Damage is estimated at $30 million and could go to nearly $400 million if the lake again reaches its record 1873 level of 4,211.5 feet.

Companies taking magnesium, salt and phosphates out of the lake have been forced to raise and shore up dikes, since the water is now 10 feet higher than when many of the companies located here.

Rising waters in lakes and rivers throughout the Salt Lake basin have flooded parks and wildlife refuges, poisoned agricultural land with difficult-to-eradicate salts and threatened to inundate Interstates 80 and 15, which link this city with the rest of the country.

Arnow has a recent snapshot of water completely filling a culvert under the Southern Pacific Railroad causeway that bisects the lake. A few years ago, he said, "there was enough room for a boat to go through there. That's the major transcontinental railroad. I'd think the Defense Department would be a little concerned if that went out too."

Shallow as the lake is--nowhere deeper than 34 feet--its banks on this high desert plateau are so flat that a rise of just a foot can inundate several square miles of land. The lake level dropped just 20 feet from 1873 to its recorded low point in 1963, but that was enough to uncover 1,400 square miles of land. It has since risen 12 feet, recovering 700 square miles of land and creating a total lake area of 1,700 square miles, enough to cover Rhode Island easily.

Three rivers feed the lake. It has no outlet; water can escape only by evaporation or breaching the banks.

Gerald Williams, hydrologist in charge at the National Weather Service office here, noted that only two or three extra inches of rain a year are enough to keep the lake rising.

Some scientists blame the slower evaporation of recent years on unusual amounts of volcanic ash in the atmosphere or on slightly lower solar radiation in a time of heavy sunspots. The unusual rains in recent months may have been stimulated by the odd equatorial weather system, dubbed El Nino, whose warm air has brought harsh storms to the West Coast.

Temple A. Reynolds, executive director of the state's Department of Natural Resources and Energy, said officials have considered long-range solutions to serious flooding in the past. "But by the time they got ready to move," he said, "the lake had gone back down and they did not want to spend the money."

The legislature gave Gov. Scott M. Matheson a $957,000 emergency appropriation to handle the crisis. Most of that money is going to the state transportation department to protect roads threatened by flooding. Rural counties on the west and south edges of the lake already have declared themselves disaster areas and asked for state aid.

"We wish the government acted a little faster and reversed the problem before the damage gets out of hand," said Peter Bahrens, president of the Great Salt Lake Minerals & Chemicals Corp.

He said the company spent $1 million in the last year raising and shoring up dikes to try and keep its installation, employing 350 people, in business. It also has given the state $200,000 for an engineering study of a plan to pump water out of the lake and into the desert.

The Salt Lake's cousin, the Dead Sea, has also experienced long periods of rise and fall, although extensive irrigation since 1948 has forced a drop in its level.

But the Great Salt Lake's water levels remain so difficult to predict that residents here have begun to punctuate many conversations with the latest reports from the stretch of I-80 skirting the southern end of the lake where water laps on both sides of the roadway.

Micky McKenzie, an airport bus driver, drove the road last weekend. Said McKenzie: "If I-80 goes under and then I-15 goes under, we will be left with a choice--either flying or swimming."