Along the South Shore, bulldozers and earthmovers have been at work for months, building and bunkering sand and fill to save an important sweep of the city's 23-mile-long beach.

Forty miles to the east, hapless vacation-home owners in another state watch as the sand dunes that keep their houses high and dry subside before an onslaught of Lake Michigan's rising waves.

In Minnesota, Wisconsin, Indiana and even New York, the story is the same: docks, boat ramps, barriers and boardwalks are being inundated by the waters they were meant to enhance or hold at bay.

While unusually dry spring and summer weather has brought drought to the South and East and massive forest fires to parched West Coast canyons, the opposite is true for the Midwest and the nation's inland seas, the five Great Lakes.

Never in their recorded history have the lakes been higher than they are this summer. According to the Army Corps of Engineers, which oversees navigational conditions in hundreds of harbors, canals and waterways in the Great Lakes-St. Lawrence Seaway system, lake levels are a few inches to more than four feet higher than average.

"Since shorelines tend to have a relatively gentle slope, just two inches of higher water can mean two feet less beach," said James Fish, executive director of the Great Lakes Commission, a regional coordinating organization. "With a wind, the water can get driven four to six feet up the beach . . . and that means trouble."

Just 20 years ago, the reverse was true. The lakes were unusually low, sometimes as much as two feet below normal. Total fluctuation "has been just under six feet," said Ross Kittleman, an area engineer for the Corps. "On an ocean coast, that's nothing. But when the rise is spread over 20 years, it's a tremendous change," he said.

Kittleman observed that the consequences of high water have been magnified because developers and vacation-home buyers forgot during the Midwest's economic boom in the late 1960s through the mid-1970s how high the lakes once were.

"Erosion rates vary from place to place," said John Wolf of the Corps' Detroit office, "but an average along the Great Lakes is a foot or two a year" of shore carried away. Strong waves and high water can knock down an entire bluff by undercutting it, he said. "Then you can lose 15, 20 feet in one night if you've got a clay bluff that falls in."

Snow and rainfall in the lakes' watershed is the sole source of the high water that has accelerated erosion. But because the Corps can regulate water levels to some extent, some communities blame the Corps for keeping water levels high to ease the passage of deep-draft vessels in the underused seaway.

Corps engineers discount the critics. Kittleman points out that Lake Superior, the largest of the five in area and volume, is being used as a reservoir to hold back some of the excess water by restricting water flowing through the St. Marys River. "But the total impact of that is about three inches overall," he said.

That estimate establishes a certain scale of reality about the continent's unique inland seas. The lakes were formed just a few thousand years ago by glaciers of the fourth Ice Age, and "they are still evolving very rapidly," said Charles Johnson of the Corps' Chicago office. "It's a very powerful, wonderful process, and people interfere with it at their own peril."

Yet interfere they do. Aside from the thousands of vacation homes, from cottages to swank mansions, that dot the shorelines of the five lakes, most of Chicago's lakefront is artificial, created from fill first dredged out of the Chicago and Calumet rivers more than a century ago and later taken from the massive Indiana sand dunes at the south end of the lake.

The beaches, lagoons and boat basins that so improve the city are constantly being repaired by the Chicago Park District, which spends millions annually grooming and restoring them. Northward, such posh communities as Evanston, Kenilworth, Winnetka, Glencoe and Highland Park have fashioned a variety of barricades, walls, breakwaters and barriers of concrete, stone and steel to stabilize their shores.

Similar engineering works can be found armoring lakeside land in communities throughout the region. But in the long run, said Johnson, who holds a master's degree in engineering hydraulics, "you're going to lose when you fight nature. If you go up and down the shoreline, you'll see lots of ruined barricades, because the shoreline is going to erode.

"It will continue to erode. The water is going to cut into the high ground, and send it tumbling into the depths of the lake. Some day, you are going to have a marshy river bottom where Lake Michigan now is. That's the way the Earth works."