A third to a half of the white sandy beach in this popular resort town has disappeared, carried out to sea and blown away by Tropical Storm Dennis.

Dennis has accelerated a natural process, the constant shifting of grains up and down the coastline and in and out of the shore.

Although jetties and seawalls are built, and sand is dredged and pumped, the beach is nearly as fluid as the ocean. Sand recognizes no state boundaries, moving back and forth between Delaware's southern beaches and Ocean City, Md., about 20 miles to the south.

"We have to think of beaches like a conveyer belt," said Tony Pratt, Delaware's shoreline manager. "The sand is increasing and decreasing all of the time."

Dennis's wind and waves are stealing so much sand from the shore that only a narrow band is left in Rehoboth Beach. The beach is expected to fill back in when the seas calm and the storm subsides, but over the busy Labor Day weekend visitors will be forced to spread out farther than usual from the commercial center at Rehoboth Avenue.

The storm's peculiar characteristics -- it sat and churned offshore for days before moving southwest today toward North Carolina and losing strength -- made it particularly hard on sandy strands on the mid-Atlantic. With gusts up to 35 mph whipping the shoreline, the sea was off limits to swimmers in Rehoboth Beach, Dewey Beach, Bethany Beach, South Bethany and Fenwick Island, Ocean City and the Virginia shore.

"When it is all said and done, this thing will be remembered for beach erosion more than anything else," said Hugh Cobb, a meteorologist with the National Weather Service in Wakefield, Va. "There's not going to be much beach left in Virginia Beach."

Much of the beach at Rehoboth has been under water from late morning to early afternoon all week, as high tides from Dennis pushed up to the wooden planks of the boardwalk. Yellow police tape has been stretched across beach entry points to prevent people from getting any where near the roiling waters. Officials could not say for certain whether swimming would be allowed by the weekend.

"The beach has been cut in half," said Chris Zimmer, a lifeguard with the Rehoboth Beach Patrol. "This weekend you will notice a significant amount of sand eroded."

Beach managers said the sand is not gone, just redistributed. Much of it has been deposited in sandbars under the water. Some also has blown down the coast, landing on other beaches.

"It hasn't been lost," Pratt said.

Sand naturally moves between the beaches every year, he said. Much of the sand from Delaware's southern beaches is eventually deposited in Cape Henlopen, just north of Rehoboth. Every year, about 200,000 cubic yards of sand land on that beach, the largest sand sinkhole in the state, according to Delaware's Division of Soil and Water Conservation.

Dewey Beach Mayor Bob Frederick said visitors this weekend will be able to see the effects of the sand movement.

"You'll see extremes when it comes to high and low tides," he said. "At low tide, the ocean will go pretty far out. At high tide, we will have a more intimate beach."

Unlike Ocean City, where sand has been pumped onto the beach every few years since 1988, Rehoboth received new sand last year for the first time since 1962. Delaware spent $1.2 million in 1998 to return sand to Rehoboth and other beaches south of here.

Rehoboth received 245,000 cubic yards of sand across a one-mile stretch between Philadelphia and Surf avenues.

In comparison, Ocean City has received 9.5 million cubic yards on eight miles of beachfront since 1988.

"It still looks good," said Lt. Warren Williams, a 37-year veteran of the Ocean City Beach Patrol. "When the tide recedes, there's not much of a change."

Williams attributed the good condition of the beach to a decade of replenishment programs. The town has spent $74 million in local, state and federal funds since 1988 to pump sand onto the beach and to build dunes, groins and seawalls.

"I was skeptical about whether it would work, but it's been a godsend," Williams said. "I'm really impressed with the way it has held up."

Nancy Howard, a spokeswoman for the Maryland Department of Natural Resources, said erosion programs do help save the beaches.

"The beach is going to erode no matter what we do," said Howard, who is also an Ocean City Council member. "This gives Mother Nature something to play with."

Staff writer Dan Eggen in Virginia Beach contributed to this report.