A man slipped on a bridge at snowmelt-swollen Wapama Falls last week and got swept into Hetch Hetchy Reservoir in Yosemite National Park, authorities said.

Wapama Falls is the major cascade visible from O’Shaughnessy Dam that impounds Hetch Hetchy, which was 98 percent full when the accident was reported at 1:45 p.m. Thursday.

It’s less than a three-mile walk to Wapama Falls from the parking area next to the dam. Wooden bridges traverse a lower section of the falls before the falls tumble into the reservoir. The falls and their proximity to the bridges are billed as incentives for hikers to make the walk.

“If you've never taken a shower with your clothes on, cross the bridges below the falls in the springtime,” a YosemiteHikes.com page for Wapama Falls states. A June 1 article on sfgate.com is headlined “Yosemite's newest thrill: Running through a 'dangerous' waterfall.”

On Thursday last week, a man was on one of the bridges when he slipped, lost his footing, and got flipped into the falls, witnesses told rangers.

“We do not know what caused him to slip and fall,” Jamie Richards with Yosemite National Park public affairs said Monday. Asked if there was water on the bridge when he fell, Richards said, “We don't have an answer to that at this time.”

The man has been identified but the National Park Service, which is investigating his death, would not release his name and hometown Monday. Richards said the man was 66 years old. Autopsy results were not available Monday.

“At this point the family is still being contacted,” Richards said.

In late March this year, sections of wood beam railing on one of the bridges were broken or missing due to moving snow, avalanche, rockfall or intense runoff. Richards said bridge railings were fully repaired before the fatal accident on Thursday.

No other injuries have been reported at Wapama Falls this season, Richards said.

Two Los Angeles County men were swept to their deaths from the same bridge trail below Wapama Falls in late June 2011. Gregory W. Meyer, 53, of Whittier, and Richard Fox, 53, of Hacienda Heights, were swept off the bridge by high late-season runoff, authorities said. Fox’s body was recovered from Hetch Hetchy Reservoir.

According to the 2007 book “Off the Wall: Death in Yosemite,” in July 1978, a solo backpacker, 26-year-old Richard E. Dix, of San Mateo, went off-trail to sit above cascades at the base of the falls. Search-and-rescue personnel found his body sprawled in rocks below the base of the falls and above the reservoir.

“Wapama Falls descends about 1,400 feet,” the website SierraNevadaGeotourism.org says. “Like Yosemite Falls, it has three distinct parts. The uppermost section is a free drop of perhaps 300 feet, followed by a steep cascading stream which descends 600 feet in a steep sided gorge . . . Finally, the bottom drop, seen from the dam and intimately from the trail, is about 200 feet down an escarpment that is not vertical. When the water's high, it shoots outward to clear this descent. The remaining 300 feet is buried under the notorious reservoir.”

There have been no water quality impacts noticed by Hetch Hetchy Water and Power staff since the fatal accident, Charles Sheehan with San Francisco Public Utilities Commission communications said Monday. The Hetch Hetchy Regional Water System serves 2.6 million residential, commercial, and industrial customers in the Bay Area.

“With regards to this very unfortunate incident, we’re vigilant every day when it comes to monitoring water quality,” Sheehan said in a phone interview. “When there is an incident like this, we look at the monitors and determine any impact to water quality. Because it is raw and untreated water, it all gets treated downstream.”

National Park Service staff at Hetch Hetchy have been advising visitors to be careful at Wapama Falls and on the bridges.

“A lot of people have been using the hiking trails at Hetch Hetchy,” Richards said.

Between Jan. 1 and May 31, more than 2,300 people entered Hetch Hetchy from Evergreen Road, 81,000 entered Yosemite at Big Oak Flat Road and total visitors to the national park exceeded 1.2 million.

Contact Guy McCarthy at gmccarthy@uniondemocrat.com or (209) 588-4585. Follow him on Twitter @GuyMcCarthy.

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