By JAMES DAMSCHRODER

The Union Democrat

For a day, the Tuolumne River below the O'Shaughnessy Dam at Hetch Hetchy Reservoir was restored to its pre-dam spring-runoff condition.

A 36-hour pulse water release starting on Tuesday and peaking with 6,500 cubic feet per second of water being released into the river on Wednesday at 1 p.m., was intended to simulate spring-runoff pre-dam conditions to analyze the affect the increased water-flow would have on the ecology of the river.

The flood was conducted by the San Francisco Public Utilities Commission and the National Park Service.

The massive amount of water being released at the discharge's peak would be similar to the water released from snow-melt during the recent heatwave, said Margaret Hannaford, general manager of Hetch Hetchy Water and Power.

"It was pretty impressive," Hannaford said of the amount of snow-melt the heat wave caused.

Experts are interested in how the high water-flow conditions will affect fish and amphibians in the river, said Tim Ramirez, division manager for natural resources at Hetch Hetchy.

"We're trying to get better information," Ramirez said about the ecology below the dam. "We historically have studied the trout, but we're trying to broaden that into including amphibians."

Studies of the massive water release will last about a week, Ramirez said. Biologists with the SFPUC and and NPS will examine if the flood will push fish downstream. If so, the biologists will examine if the fish fight back upstream as conditions return to normal.

Water would normally be released because spring runoff will spill over the dam. But, typically, the water is released in small increments over a long period of time, Ramirez said.

Onlookers on Wednesday watched as water violently shot from three different valves on the dam's concrete wall and from a fourth valve about a hundred yards downstream of the dam.

"It changes the river's course," said Hetch Hetchy watershed keeper Mike Patterson. "It will scour trees and brush along the riverbank."

The dam has been steeped in controversy since Hetch Hetchy was first considered a potential site for a reservoir in 1882.

Preservationists, led by John Muir, fought to keep the dam out of the valley, but San Francisco's chronic water shortage led to the passing of the Raker Act in 1913, which authorized O'Shaughnessy construction.

The dam was again surrounded in debate only a few years ago, when there was momentum to drain Hetch Hetchy Reservoir and restore the glacier-cut valley to pre-dam conditions. Some say the valley would rival Yosemite Valley in its beauty and grandeur.

But the effort to remove the dam and release one of San Francisco's main water sources into the Tuolumne River was dealt a blow when initial studies indicated that the cost and manpower to remove the 312-foot high dam would be expensive: $3 billion to $10 billion.

Some onlookers on Wednesday got to see the Tuolumne River swelled up like it would have looked before the dam was constructed.

"It's really cool looking," said Carl Saltzberg, 18, of Seattle.

"I just feel sorry for the animals at the bottom of the dam."

Contact James Damschroder at jdamschroder @uniondemocrat.com or 588-4526.