Tank Hill History
Location: Clarendon & Twin Peaks
Bus: #33 or #37 @ 17th & Clayton
Contact: Greg Gaar, (415) 752-5983
Tank Hill is one of San Francisco's secret treasures. Its name comes from the Clarendon Heights Water Tank built in 1894 by the Spring Valley Water Company to store drinking water pumped from Laguna Honda. Tank Hill became city property in 1930 when Spring Valley was acquired to establish the San Francisco Water Department. The prominent water tank was removed in 1957, and all that remains is its round foundation. Residents remember seeing goldfish flowing down Belgrave Avenue when the old tank was drained. In 1960, Tank Hill was sold as surplus property for $230,000. In 1977, developers proposed building 20 houses, but the community convinced the city to buy Tank Hill back for $650,000 from the recently created Open Space Program.
At an elevation of 650 feet, Tank Hill's main draw is its panoramic view from Point Reyes to Bayview Hill. But what makes the 2.8 acre rocky promontory unique is that it is a remnant of San Francisco's indigenous landscape, containing 60 species of native plants. The easiest access is a stairway at Twin Peaks and Clarendon.
The hill's rich native plant community contains larval food plants for endangered butterflies (Mission Blue and Callippe Silverspot) as well as habitat for garter snakes, alligator lizards, western meadow larks, kestrals and hawks. The spring wildflower show is one of the city's most spectacular sights as the hill changes colors from yellow in March, to purple in April, to pink in May. The site also exposes some beautiful Franciscan radiolarian chert rock outcrops, the city's oldest natural features, formed on the ocean floor 130 million years ago.
Tank Hill is a fragmented natural habitat, meaning that it is an island of natives surrounded by an ocean of exotic plants. The greatest threats to this native plant heritage are the invasive exotic plants commonly called weeds. Since non-native plants have no natural predators here, they can destroy a 10,000 year-old native plant community in just a few years. Weeds invade when the ecological balance is altered. Soil disturbance, extreme chemical change or erosion, elimination of the sunlight, or increased moisture can create a niche for a weed to thrive and spread.
The hill's worst weed threat comes from non-native trees, mainly eucalyptus. Planted after the Pearl Harbor attack to conceal the water tank from Japanese planes, the trees now shade the natives, collect and drip moisture, drop tons of debris, and alter the soil composition. These impacts help to spread weedy English ivy, Cape ivy, sheep sorrel, oxalis, and erharta grass. Erosion allows weeds to attack native habitats rather like the way a wound on the body can become infected. Erosion is caused by people wandering off trails or dogs digging up the ground. The hill is not a dog run, and dog feces and urine are problems that support weeds.
Designated as one of the Recreation and Park Department's thirty Significant Natural Resource Areas, Tank Hill is part of the department's Natural Areas Program. The program focuses on native plant propagation and planting, weed and erosion control, and public education. Volunteers are needed and appreciated. To volunteer in any natural area, call Kristin Bowman at 415-753-7265. To work on Tank Hill, call Greg Gaar at 415-752-5983.
– Greg Gaar