O'Shaughnessy Dam and Hetch Hetchy Reservoir

O'Shaughnessy Dam holds Hetch Hetchy Reservoir in a granite valley in northwest Yosemite. A short paved walk from the parking area reaches the dam-top viewpoint, with the reservoir and Tueeulala and Wapama Falls in view. The dam crossing is the quick payoff; cross it and go through the tunnel to reach the shoreline trail toward both falls. The walk to Wapama Falls is about five miles round trip with 507 feet of elevation change, so treat that as a real hike, not an extension of the viewpoint stroll.

This is a separate outing from Yosemite Valley: about 38 miles and an hour and 15 minutes by road, with no public transit to Hetch Hetchy. The winding access road is open from sunrise to sunset and can close or require chains in winter weather. Vehicles or trailers longer than 25 feet, or wider than 8 feet, aren't allowed. Midday weekend delays can happen if the parking area fills, so leave time for the drive and carry water and sun protection. The dam viewpoint is paved and wheelchair accessible, with designated accessible parking. Pets aren't allowed on the dam or trails.

The reservoir fills a glacially carved, U-shaped valley at about 3,900 feet. Granite cliffs rise steeply from the water, with Kolana Rock to the south and Hetch Hetchy Dome to the north. Spring is the strongest waterfall season; Tueeulala often dries by early summer, and summer heat can make the exposed shoreline walk harder. For a photo, keep some water in the foreground and let the cliffs draw the eye toward the falls; the view from the dam is broad, while the shoreline gives you a closer angle on the rock and reflections.

Hetch Hetchy Reservoir is about eight miles long and holds 117 billion gallons. It is Yosemite's largest single body of water and supplies water to roughly 2.7 million people in the Bay Area. That drinking-water role shapes the visit: swimming and boating are prohibited, as is public access below the reservoir's high-water mark.

San Francisco considered Hetch Hetchy as a water source by 1882. After the 1906 earthquake and fire, the city pressed harder for a reliable supply; the 1913 Raker Act authorized dams at Hetch Hetchy and Lake Eleanor. O'Shaughnessy Dam's first phase was completed in 1923, then raised to its final height in 1938. The decision became a national debate over protecting a valley versus building water and power infrastructure, with John Muir among the preservation advocates. The dam is named for chief engineer Michael O'Shaughnessy.

People have lived in Hetch Hetchy for more than 6,000 years. The valley provided water, winter shelter, hunting and fishing, acorns, edible plants and a route for trade. Its name comes from the Miwok word hatchhatchie, meaning "edible grasses"; Nearby names include Tueeulala, Wapama, Kolana and Tuolumne.