To Build a Dam

Beneath the lake lies evidence of the dam’s construction, completed in 1892 through intensive labor by workers and horses. To prepare the area to be flooded, workers removed brush and trees from 333 acres so rotting vegetation would not affect the reservoir’s water quality. In 1874, laborers dug Tunnel No. 1 through a rocky hillside to divert San Leandro Creek during construction; the tunnel originally stood 30 feet above the canyon floor and later delivered water to homes. To prevent leakage and the creek’s return, laborers dug three nested trenches, including a largest ditch measuring 900 feet by 150 feet, a second ditch or puddle-pit 90 feet from bank to bank and about 140 feet long creek-wise, and a third ditch where they built three concrete walls before sealing the trench with concrete grout. Laborers then filled the two larger trenches with clay and selected material from nearby areas, kept the clay wet and workable with water from a stream pump, and drove gangs of horses across the puddle-pit to compact the layers into a water-resistant surface.

Castro Valley, California · United States