America's dead sea rises and falls with a life all its own

Will she or won't she?

The Great Salt Lake, an eerie inland sea whose only escape is evaporation, has mystified settlers of this valley with its shifting shoreline.

If the water level rises just a few feet higher, the lake will flood businesses worth billions of dollars.

''Only Mother Nature knows right now,'' says Stan Elmer, chairman of a Great Salt Lake ''technical team.''

Salty and shallow, the 75 by 25 mile lake is just a remnant of its old self from glacial times, when it was as big as Lake Michigan. But it continues to fluctuate as much as 20 feet over several years. Winds alone can raise it over three feet. A one-foot rise can extend the shores by half a mile.

In the mid-1970s this ''dead sea'' of America rose quickly, causing the Utah Legislature to call for strategies and cost estimates for dealing with migrating shores. Solutions include the expensive building of dikes; diverting the lake's main water source, the Bear River; or pumping lake water over a hill and into an Air Force missile range in the desert.

But the rising has since stopped, easing the political problems of who would pay for any emergency steps. Now scientists are digging for clues as to ephemeral patterns in the lake, which is the lowest spot in a 22,000-square-mile , inward-draining watershed.

The only human records go back 135 years to Mormon settlements, and the cycles since then are as difficult to read as the Dead Sea scrolls. Longer-range theories are based on evidence ranging from sun spots to geologic mud cracks. Local bankers and developers quietly encourage talk of a decline in the lake's level.

The clearest clue comes from a sister ''terminal lake'' (with no outlet) in the deserts of Iran. Most of the time, the two lakes correlate in their ups and down, but the Iranian one conveniently precedes Salt Lake by three years.

But, says Mr. Elmer, ''Both lakes may be extremely random and we may be making something out of it that just isn't there.'' One man-made problem may be the diversion of Colorado River water to the Salt Lake valley in a few years. Its effect on the lake's level, however, may be minimal compared with that of natural causes.

Nonetheless, another few feet up and Salt Lake's damage to property could mount heavily. The Southern Pacific railroad, for instance, whose 12-mile solid trestle cuts the lake in half, would be sunk on its Western routes. Roadways and industrial development might get soggy foundations. And new shore resorts might be destroyed.

A dozen or so companies which ''mine'' the lake's minerals through evaporation, mainly salt and magnesium, would lose millions. During the 1976 high level, Great Salt Lake Minerals & Chemical Corporation spent nearly $1 million in protective dikes.

The train trestle, with only two openings, essentially blocks the free flow of the brine-filled water between the north and south. Because the south receives 75 percent of the fresh water, its salinity is half that of water in the north.

From satellite photos, the results are obvious: The north waters are pink, the south is blue. The two parts are developing different kinds of algae and bacteria.

Also, the mineral mining companies on the south arm can't compete with the companies in the brine-packed north. Some experts even suggest that the south could become a freshwater lake within two decades.

Receding waters can cause problems as well for the mining firms, who would be forced to relocate. In the past, drought periods brought out coyotes who raid high-and-dry islands where millions of migrating birds now seek safety.

The lake recently came under the drill of ''offshore'' oil explorers. An oil extremely high in sulfur was discovered after a $50 million four-year search by Amoco, but its uses are still hypothetical.