“Would you like some Hetch Hetchy water?” your waiter might say. San Francisco has long associated its water supply with Hetch Hetchy, but the city’s water comes from the mighty Tuolumne River system – Hetch Hetchy is just where some of that water has been stored over the last century. So if we take the water out of Hetch Hetchy, do we lose the water that San Francisco needs to survive and prosper?

No. San Francisco and other Bay Area communities will continue to receive a reliable supply of high-quality water from the Tuolumne River after Hetch Hetchy Reservoir is drained and the valley returned to the trusteeship of Yosemite National Park. Here’s how that can easily be achieved.

Hetch Hetchy is only one of nine reservoirs that comprise the San Francisco Public Utility Commission’s water system. Although Hetch Hetchy Reservoir is the most well-known, it stores less than 25% of the system’s water. San Francisco’s water-bank in Don Pedro Reservoir, downstream on the Tuolumne River, holds twice as much water as Hetch Hetchy.

Without a reservoir in Hetch Hetchy Valley, a minor new conveyance will be required to divert the Tuolumne River flows and stored water in Cherry and/or Don Pedro Reservoirs into the existing pipeline. On average, diversions of Tuolumne River supplies into the pipeline would be reduced by only 4%.

For most of the year it will be possible to divert the flow of the Tuolumne River into the pipeline just downstream of the National Park boundary. During late summer and early fall, it will be necessary to move stored supplies from Cherry or Don Pedro Reservoirs into the pipeline. In dry years, rivers flow will be sufficient for diversion for a shorter period, so it will be necessary to increase diversions from storage.

Hydrologic analysis shows that it will be possible to fully meet system demands in 4 out of 5 years. In the driest years, 20% of system demands will need to be met from additional water storage or supply resources.

This analysis is fully described in the Environmental Defense Fund’s studies Paradise Regained and Cherry Intertie Alternative. Similar findings are described in the UC Davis study Re-Assembling Hetch Hetchy and US Bureau of Reclamation study.

Facilities to make up the missing 4% of average supply can come from a variety of sources. These should be developed in co-operation with the SFPUC and its customers. A combination of the following options can easily make up this deficit:

• Development and replenishment of groundwater basins in the Tuolumne or Bay Area watershed, including “banking” groundwater in remote locations as many California cities do. San Francisco utilizes just 2.5 million gallons of groundwater today, compared with 14.5 million in 1930 – and does little to harness the 5.5 million acre-feet of rainwater it receives each year.
• Building new or enlarging existing surface storage – for example, a 20 foot raise of Don Pedro would enlarge that reservoir by the 360,000 acre-feet that Hetch Hetchy Reservoir currently holds.
• Purchasing supplies on the open market in dry years (similar to the agreement between Metropolitan Water District and the Palo Verde Irrigation District).
• Expanding wastewater recycling and water conservation efforts in Bay Area cities. Current water recycling by San Francisco lags way behind leading cities in California. Los Angeles recycles 189 million gallons per day compared with just 1 million per day in San Francisco.

By California standards, Hetch Hetchy is not a large reservoir. The storage capacity that will be lost by removing the reservoir – 360,000 acre-feet – is less than 1% of the capacity of the reservoirs in California. It’s just a fraction of the 6 million acre-feet of storage built in California during the last 20 years. Don Pedro reservoir, downstream on the Tuolumne River, already stores over 2 million acre-feet of water for San Francisco and other water consumers.

The annual water supply replacement needed when Hetch Hetchy is restored will be less than was required for other great restoration efforts in California - at Mono Lake, the Trinity River, or the Bay Delta.

In the words of three former superintendents of Yosemite, Dave Milhalic, BJ Griffin, and Bob Binnewies: “by investing in state-of-the-art recycling, conservation and groundwater systems, San Francisco can eliminate the use of Yosemite National Park as a water storage facility.”