Lake Washington was expanded and put into service as a reservoir in 1907, although it had been providing water to its predecessor, Monell's Pond, since 1852.
In 2016 the city briefly declared a water emergency and started using Browns Pond, its backup supply, when levels of perfluorooctane sulfonate (PFOS) near the federal Environmental Protection Agency guidelines of 200 parts per trillion (ppt) were found in the lake.