are served by surface water systems, and 34% (101 million) are served by groundwater-supplied systems (as of 2009). Most groundwater systems are in small communities, and comprise 90% of the overall population of public water systems. For a surface water system to operate without filtration it has to fulfill certain criteria set by the EPA under its 2006 Surface Water Treatment Rule, including the implementation of a watershed control program. The water system of New York City has repeatedly fulfilled these criteria for most of the water processed through its facilities. Cities supplied primarily by surface water without filtration Boston, New York City, San Francisco, Denver, and Portland, Oregon are among the large cities in the U.S. that do not need to treat most of their surface water sources beyond disinfection, because their water sources are located in the upper reaches of protected watersheds and thus are naturally very pure. Boston receives most of its water from the Quabbin and Wachusett Reservoirs and the Ware River in central and western Massachusetts. New York City's water supply is fed by three watershed systems. The two larger systems, Catskill and Delaware, do not have filtration. The Catskill watershed is in one of the largest protected wilderness areas in the United States. Water from the two systems has been treated with ultraviolet germicidal irradiation since 2013. The Croton system, which supplies 10% of the city's water, has been filtered since 2015. San Francisco obtains 85% of its drinking water from high Sierra snowmelt through the Hetch Hetchy Reservoir in Yosemite National Park. However, to supplement the imported water supply, and to help maintain delivery of drinking water in the event of a major earthquake, drought or decline in the snowpack, San Francisco considers the use of alternative locally produced, sustainable water sources such as reclaimed water for irrigation, local groundwater and desalination during drought periods, all as part of its Water Supply Diversification Program. The largest source of water supply for Portland, Oregon, is the Bull Run Watershed. Denver receives its water almost entirely from mountain snowmelt in a number of highly protected watersheds in more than 9 counties. Its water is stored in 14 reservoirs, the largest of which is the Dillon Reservoir on the Blue River in the Colorado River. Water is diverted from there through the Harold D. Roberts Tunnel under the Continental Divide into the South Platte River Basin.