 The dam has been raised many times since then to increase capacity.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.It is not known yet where the PFOS came from; a pond on the Stewart Air Guard Base from which Silver Stream, one of Lake Washington's tributaries, rises, had levels of 5,900 ppt.