In an onslaught threatening some of North America's most prized vacation and commercial shorelines, the Great Lakes have risen to record levels this summer, continuing a 12-year trend that some researchers say marks the beginning of a dangerous, prolonged era of high water.

Throughout the vast lakes region, communities, businesses and homeowners are building seawalls, breakwaters and barricades to stave off the rising water. Millions of dollars in federal, state and local funds are being spent in the effort, and regional officials predict that costs will rise with the water.

Only the shores of Lake Ontario, gateway to the St. Lawrence River and the Atlantic Ocean, have been spared, because of efforts to drain water from the lowest of the lakes. On the other four -- Superior, Michigan, Huron, and Erie -- familiar shorelines are being inexorably altered.

Dozens of lakefront homes in Michigan, Ohio and New York have fallen into the water in the past two years, claimed by sudden erosion in what had been seen -- mistakenly -- as part of a benign phenomenon that would soon subside.

The International Joint Commission (IJC), a Canadian-American water-control body in the lakes, calls it an "emergency" and warned last month of possible "severe flooding and erosion" this year.

Lake levels are only a few feet higher than average heights observed since 1860, when systematic record-keeping began. But a rise of just one foot can put several yards of gently sloping beach under water or vastly intensify the wave action of a storm.

Scientists ascribe the floodtide to a decade of higher-than-average precipitation over the 1,000-mile wide, 750-mile long lakes area that is home to 37 million Americans. But some also outline a new theory that predicts radically higher levels -- perhaps up to 5 feet above present record highs -- as the norm.

The revisionist view of the dynamics of the world's largest resrvoir of fresh water is based on pioneering analysis of prehistoric geological evidence gleaned in recent years from shoreline sediments and carbon-dated residues. Curtis Larsen, now of the U.S. Geological Survey in Reston, Va., originated the theory after weeks spent roaming and investigating the Lake Michigan coast around the Illinois-Wisconsin border.

Author of several scientific papers on the subject, Larsen found evidence suggesting that about every 500 years over the past 2,000, Lake Michigan has risen to about 585 feet and stayed there for perhaps 100 years each time.

His theories challenge longstanding convention that the lakes rise and fall about one to two feet annually from seasonal precipitation, with slight high-low fluctuations in a cycle of about 12 years duration.

Based on 126 years of records, the total difference between record low and record high was thought to be about 4 1/2 feet. But Larsen's data, which many other researchers endorse, indicate much longer, more severe changes of about 10 feet between low and high water in Lakes Michigan and Huron -- and presumably, the other three lakes as well.

"We're talking a range of longer-term fluctuations as high as three to five feet above present levels," Larsen said in a telephone interview. Records kept since 1860, he said, "are actually from a low phase in the lakes."

If Larsen is correct, the rise in the lakes would increase flood and erosion damage from storms and require construction of dikes and pumping stations to hold out the water. At worst, it could cause abandonment of parts of some cities that dot the lakes' shores, such as Chicago, Toledo or Cleveland.

Engineers, politicians and planners are being forced to rethink assumptions that guided lakeside land use and water-control policies.

Examples of these new attitudes abound. In Grand Haven, a community near Muskegon, Mich., homeowners who once took pride in privacy are jointly paying for new breakwaters to protect their expensive houses perched on the dunes above Lake Michigan.

"Ten years ago, you never heard the word 'erosion,' " said Clifford Pfaff, who with his family has lived for 20 years in a one of Grand Haven's most historic beachfront houses. But after two monster storms heavily damaged 100 feet of soil and sand dunes between house and lake, Pfaff built two seawalls to retard waves beating onshore, and four splashwalls on the dune to buffer it from erosion.

Then came a storm last December packing 75 mph winds and huge waves. Pfaff lost part of the dune. Now, he and five neighbors are sharing the costs of a new breakwater and bank restoration.

Here in Chicago, north side lakefront condominium owners along fashionable Sheridan Road have lobbied for protective measures. Beaches that once were backyard amenities have disappeared, and now their buildings are often pummeled by storm-tossed waves and caked with winter ice.

Sheli Lulkin, a condo association leader, said, "We don't have any time to waste. The lake is coming up." She has sought antierosion aid from the Chicago City Council, Illinois legislature and federal authorities.

Rep. Sidney R. Yates (D-Ill.) has obtained a $1 million appropriation for a U.S. Army Corps of Engineers project to build breakwaters and revetments to keep north side lakefront streets from crumbling into the water. Other members of Congress are taking similar actions for their own hard-pressed shorefront constituents.

Michigan, with extensive shoreline on four of the lakes, has taken the lead in helping homeowners defray the cost of moving their houses back from threatened shoreline and limiting how close houses may be built to eroding shorelines. Other states are earmarking funds and legislatures are pondering aid packages to expand protective efforts.

Meanwhile, according to Corps of Engineers hydrologists, the surface of Lake Michigan stood at 581.08 feet above sea level at the end of June, several inches above the previous high set in 1974, and about 30 inches higher that its long-term average monthly high.

Other Corps records show that Lake Superior was at 601.66 feet, several inches below its all-time high established in 1985. Lake Huron was at the same level as Michigan, while Lake Erie stood at 573.70 feet, several inches above its record level set in 1973.

Only Lake Ontario, at 246.65 feet, was near normal, about 18 inches below its historic high, set in 1952.

There is so much excess water in the system that if a drought set in this summer, researchers say, it would still take about four years for the lakes to subside to levels once thought "average." Normal precipitation would bring on such levels in about six to eight years.

"Maybe what we thought was the norm is not what Mother Nature says the real norm is," said James Fish, executive director of the Great Lake Commission. "We've believed there's an engineering solution to everything and that government ought to be able to solve it. But maybe this is one problem that doesn't solve that well."

Increasing the high water effect is the fact that the Earth's crust beneath the northern portions of the lakes is still slowly rebounding from the last Ice Age, when glaciers advanced across much of North America, their great weight pushing the land down.

This "isostatic rebound" is raising the crust several inches a century around Superior, northern Huron and Michigan. Although slight, it is enough of a change to add to rising waters in the southern and western portions of Michigan, Erie and Ontario, much as lifting one end of a dishpan sloshes more water into the other end.

The high water has increased the phenomenon called "set-up," in which storm winds pile up water as they cross a lake, increasing the storm's impact. Lakes Michigan and Erie, with long, narrow shapes, relatively shallow bottoms and heavily populated industrial shorelines, are especially vulnerable to to this.

Last December, a storm from the west howled across Erie, causing a "set-up" difference estimated at up to 16 feet in lake levels between towns at the two ends of the lake. In Hamburg, N.Y., 12 homes were destroyed, hundreds of residents were evacuated and prime shoreline was damaged.

"People had backyards of 150 feet and now they're 10 feet. I've seen it," said Hamburg town supervisor Jack Quinn.

Some shore dwellers, awed and angered at seeing dunes protecting their homes lose 20 feet in a single storm, blame hydroelectric and shipping interests, the Army Engineers and the IJC for manipulating canals, locks and dams around the lakes to keep levels unusually high. They say high water reduces shipping costs by allowing heavily laden vessels to work the lakes. They also contend that Canadian utilities generate cheap hydroelectricity by dumping extra water into the lakes.

Many politicians and lakes officials deny this is the case.

At their May conference, the Great Lakes governors asked the IJC to study the feasibilty of increasing the outflow from Erie to Ontario through the Niagara River, a natural chokepoint that restricts the movement of water from the upper lakes to Ontario. Proposals include deepening the river or cutting a new channel through an island in the river upstream of Niagara Falls.

Researchers say some of the flood distress may be traced back 150 years to inadvertent development of vulnerable coastal areas by white settlers ignorant of prehisoric indications that there had been much higher water levels.

Said William Wood, director of the Great Lakes Coastal Research Laboratory, "What we have is a situation of an intersection of nature with the social and economic sector. These two things compound the situation."

Throughout the Great Lakes, "beach nourishment" efforts are under way, with state and local park authorities trucking in sand to raise beaches higher than the water or to replace sand lost to storms.

Some communities and homeowners have built walls into the water at right angles to their shrinking beaches. These "groins" can slow currents so that sand settles out of water sweeping alongshore.

Charles Collinson of the Illinois State Geological Survey holds out little long-term hope for most of these efforts. "All the private beaches on Illinois' Lake Michigan shore are going or gone. We're going to be down to the big public beaches and not much more." Once erosion carries sand out beyond about 10 feet of depth, "it's lost for good, he said. "We'll be living with artificial beaches."

He foresees a time when many shoreline communities may be forced to erect high barriers of stone, called riprap, to protect themselves and their expensive, manmade beaches.

Wood, director of the research lab, warns, "Once a community commits itself to shoreline protection, it takes on a constant expense. People who think next summer will be better are mistaken. It's not going to get that much better . . . a drought will be only a temporary lull. If a homeowner on the coast can move his house, he should do so."