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The islands are one of Toronto's most bucolic places - a beautiful waterfront park in Lake Ontario formed out of an archipelago of sandbars.

But here at their southwestern tip, the scene is anything but breathtaking. Erosion caused by the pounding action of waves from Lake Ontario is chewing away at the islands. It's not a pretty sight.

Mature cottonwood trees are tumbling into the lake, as are the islands' dunes, considered an ecological rarity because there aren't any others in this area of Canada for more than 100 kilometres. So much of the islands has been washed away that old water and gas mains buried decades ago, thought to be safely away from the then-existing shoreline, now lie exposed in open water. Even the city's popular nude beach at Hanlan's Point, once a wide expanse of buff-coloured sand, is beginning to be stripped away.

"It's sad. It's valuable real estate washing away," observed Warren Hoselton, Toronto Islands park supervisor, who has been watching with increasing alarm as the lake claims parts of his park. He is concerned that the erosion is set to worsen. "We're at the tipping point now."

The problem of disappearing real estate at one of Toronto's local landmarks has an unusual cause. In Toronto parlance, the islands are being eaten by the spit.

For decades, Toronto has been dumping construction waste from downtown building sites into Lake Ontario.

The rubble has been shaped into a five-kilometre-long peninsula jutting into the lake near the islands and known locally as the Leslie Street Spit.

The spit is blocking the lake currents that, in the past, built up the islands from sand washed off the Scarborough Bluffs.

Now, whenever storms blow across the lake from the south, their large, rolling waves cut away at the exposed southwesterly tip of the islands. With no fresh sediment from the bluffs to replenish what is washed away, the islands are slowly losing ground to the lake.

The rate of erosion has been estimated at about eight metres a year, a relatively sedate pace but one that is still worrisome, given that the islands are only a few hundred metres wide at many points and, in some areas, trees that previously resisted the waves have toppled over. Once tree cover is lost, exposed sand is no match for waves, and is quickly washed away.

According to an estimate by the Toronto and Region Conservation Authority, up to about a sixth of the park could be lost over the next century if no solutions are found. But the problem could easily worsen. A pickup in the pace of erosion would occur should the high water levels existing in the mid-1990s return and coincide with big winter storms.

"All we need is one high-water year and we're really in trouble," said Joanna Kidd, a spokeswoman for the Toronto Bay Initiative, an organization that tried to preserve the islands' rare dunes but was running a losing battle with the erosion.

The worst damage on the islands is around Gibraltar Point, where the entire near-shore area is suffering.

The point lies more than a kilometre to the west of the park's most popular picnic area at Centre Island, and about 3½ kilometres from the residential community at the eastern tip of the islands.

No buildings have been washed away, but the point's washroom nearly fell into the lake after a particularly bad winter storm a few years ago, according to Mr. Hoselton. The building was only saved by placing giant limestone boulders between it and the waves, although this is providing only a temporary respite.

Moranne McDonnell, an erosion specialist at the Toronto and Region Conservation Authority, says that sand on the lakebed offshore from the islands is also washing away, from the same forces causing erosion on land.

This makes it difficult to protect the shoreline with giant rocks, because they'll tumble into the lake. "Anything you place along the shoreline will eventually become undermined and will fail," Ms. McDonnell said.

Next in the path of the waves is an arts centre that overlooks the lake, followed by one of Toronto's four drinking-water filtration plants, which would eventually become vulnerable if nothing is done.

The islands are considered dear to many Torontonians, who view them as the city's version of New York's Central Park. More than 1.2 million people visit them every year, brought across Toronto inner harbour from the city's downtown by a fleet of municipal ferry boats.

The city has already protected much of the islands behind bulkheads, cement sandbags and breakwalls, but plans to save the vulnerable southwestern areas have been stalled since the late 1980s for budget reasons.

Proposals for halting the erosion are under study by the conservation authority. It has looked at ringing the shoreline with a massive, offshore breakwall nearly a kilometre long that would block storm waves.

The drawback is that it would recreate a stagnant pool of water in the lake, so an alternative approach being considered is to dot the offshore with a series of smaller breakwalls that would allow some water circulation and still dissipate much of the power contained in storm waves.

Some consideration is even being given to trying to mimic nature by replenishing the sand that once came from the bluffs, using material dredged from elsewhere in the harbour area.

Stopping the erosion won't be cheap and it won't happen any time soon. Ms. McDonnell says breakwalls could cost anywhere from $10-million to $13-million, and would take years to complete.