WAILUA, Kaua'i — When rocks started crashing into the vegetation at the Fern Grotto during and after this year's heavy rains, State Parks officials at first felt they could get away with closing only a portion of the visitor attraction, but a geologist's review this week showed the risk was much greater.

The state shut down all access on Monday to the classic visitor destination, and although there is a tentative three-week timeline for reopening it, the reality is that it could take considerably longer, and visitors may never again enjoy the kind of unrestricted access they've had in the past.

"The rock specialist said there is a huge potential for more rocks falling down," said Kamika Smith, general manager of Smith's Motor Boat Tours. The company, started by Smith's grandfather, has been bringing tourists up the Wailua River to the Fern Grotto since the 1940s.

Peter Young, director of the state Department of Land and Natural Resources, said a state geologist was at the grotto late in the week, and it's beginning to look like it could take more than three weeks to make the area safe for visitors. It is possible, although the decision has not yet been made, that people will be permanently banned from the inner cave area of the grotto, he said.

The decision disappointed many of the hundreds of visitors who daily ride Smith's Motor Boat Service's large, flat-bottomed boats up the Wailua River to the grotto. And it has forced couples to move some of the many weddings that are scheduled at Fern Grotto.

"We have friends that got married there, and it was kind of a sentimental thing for us to see it," said Robert Hill, of Portland, Ore.

But Hill and his wife, Beth, said they were thrilled by the alternative offered by the Smith family—access to the botanical and cultural park, Smith's Tropical Paradise.

"They let us walk through the entire thing. Beautiful. It was amazing," Beth Hill said.

Smith said that visitors are still offered the river tour, which includes lectures on the history of the region as well as a musical group on each boat. To make up for the loss of the Fern Grotto portion of the boat tour, visitors are offered escorted tram rides through the park, he said.

"What they gave us in place of the grotto was spectacular," said Flo Gordon, of Minneapolis, who took the tour with her husband, Loren.

Smith said the boat tour company supports the decision to close the area until changes are made.

"Rocks were landing right on the viewing area," Smith said.

Young said the state has no plans to permanently close the area, and wants to reopen Fern Grotto as soon as it is safe.

"We know of the draw that the place has," he said.

Smith said he emphasized to state officials that his family company has 40 full-time employees.

"They depend on this," he said.

The scenario for reducing the threat has not been developed, Young said. It could involve a heavy fence to intercept rolling rocks, concrete to seal risky zones, or even bolts to keep sections of rock from moving.

"We are working with a consultant on various ways to make that place safe," Young said.

Meanwhile, Smith said that weddings scheduled for the Fern Grotto have been offered a venue at the riverside Smith's Tropical Paradise, and the strains of the Hawaiian wedding songs, "Ke Kali Nei Au" and "Lei Aloha, Lei Makamae," will sweep out across the waters of the lower Wailua River instead of echoing from the basalt walls of Fern Grotto.

The Fern Grotto is a large overhung cave that lies at the base of a cliff that forms the south wall of Wailua Valley. It gets its name from the long fronds of ferns that drape its walls. The solid rock forms a natural amphitheater that has reflected the voices of generations of Hawaiian singers.

Visitor officials and state parks authorities grew concerned when the ferns started dying in the early 1990s. Studies showed that the ferns had been getting most of their water from a century of sugar cane irrigation on the land above the grotto. When sugar —and irrigation — ended, so did the ferns' source of water. That issue was repaired in 2004 with another kind of artificial irrigation—water lines that drip water over the edge of the grotto, much as it did during the sugar era.

Then came the drenching of 2006. This year's heavy rains, while they may not have included the heaviest one-day downpour on record, did establish a record for month-long rain in March. The rains helped cause the fatal collapse of the Kaloko dam 10 miles north of Fern Grotto, and they weakened the structure of the geology above the grotto at Wailua.

When rocks first fell, Smith's and officials of the Division of State Parks simply closed off the trail near where the rocks were first found. Then a 4-foot-wide boulder crashed down and rolled directly onto a viewing platform that was under construction.

State officials called in a geotechnical consultant, who visited the site Monday and concluded the risk was substantial and immediate.

"It became very clear to us that there is a risk that we don't want to subject people to. There is more than a single place where there is a problem," Young said.

Reach Jan TenBruggencate at jant@honoluluadvertiser.com.