The Hetch Hetchy Reservoir in Yosemite National Park has been a vital source of clean water for San Francisco Bay communities for almost 100 years.

Through its energy-efficient gravity flow system, it pipes some of the cleanest water in California to 2.6 million people and thousands of high-tech companies that rely on its purity.

Nevertheless, every so often an effort emerges to remove the O'Shaughnessy Dam and drain the reservoir.

Each time, the same conclusion is reached: There is simply no feasible way to replace the Hetch Hetchy Reservoir, return the valley to its original condition and still provide water to the Bay Area.

Advocates for restoring Hetch Hetchy persist in calling for removal of the dam. Most recently, they claim that San Francisco has violated the Raker Act, a law passed by Congress in 1913 that permits the city to use water from the Tuolumne River during the runoff season for municipal use.

According to this theory, San Francisco is violating the law by not fully developing all available water resources before it exports a portion of Hetch Hetchy water to neighboring Bay Area communities.

This is not only an incorrect interpretation of the law, it also fails to recognize the extensive efforts by the San Francisco Public Utilities Commission to maximize efficient use of its water supplies.

The requirements of the Raker Act are quite clear.

During congressional consideration, San Francisco proposed building the Hetch Hetchy Project to its maximum capacity, offsetting the cost by providing reservoir water to Bay Area farmers until the demand for domestic and municipal use increased.

San Joaquin Valley farmers were not keen on new competition and balked at this plan, and Congressman John Raker responded to these concerns by inserting language limiting San Francisco's use of Hetch Hetchy water to domestic and other municipal purposes only.

At no time was there any suggestion that San Francisco be required to exhaust local water resources before using Hetch Hetchy water for municipal needs.

Although not required to do so, San Francisco is aggressively pursuing numerous water supply projects at a cost of hundreds of millions of dollars, including recycled water projects, groundwater storage, desalination and surface storage expansion.

On average, the city diverts only 250,000 acre-feet of water from the Tuolumne River, from an annual average flow of 1.8 million acre-feet. These diversions represent only 0.7 percent of all Delta watershed diversions. These actions allow the city to serve 2.6 million customers, or 7 percent of the entire state's population.

I'm proud that San Francisco's water use is only 88 gallons per person, per day, one of the lowest in the state.

These facts illustrate San Francisco's aggressive pursuit of every reasonable water supply alternative. The actions taken by the city, along with the obvious legislative intent of the Raker Act, make it clear that an investigation is unnecessary.

It is important to note one glaring omission from dam removal proposals: the identification of an alternate water source if the reservoir ceases to exist.

If not from Hetch Hetchy, where would millions of Bay Area residents and thousands of businesses get water? The inevitable answer is the Bay Delta – the most stressed portion of the entire California water system.

Not only is it impractical to suggest that the Delta can serve millions of new customers, but any serious effort to consider such a proposal would trigger a statewide water war.

The cost of removing the dam is another obstacle. A 2006 study estimates removing Hetch Hetchy and replacing the water supply could cost as much as $10 billion. Even a thorough study to remove the dam could cost as much as $65 million.

To remove or in any way interfere with Hetch Hetchy's continued operation simply makes no sense.

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Dianne Feinstein is the senior U.S. senator from California.

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