Regional Water Supply System

  • Seattle's Regional Water System Map. The Seattle area has so much water everywhere, but where exactly does the water you use come from?

Seattle's Regional Water System serves over 1.3 million people including customers in the City of Seattle and 22 other cities and utility districts that provide water. These water providers, also called purveyors, typically have their own storage and distribution systems.

Two watershed sources provide the majority of water for the system. About 70% is provided by the Cedar River Watershed. The remaining 30% comes from the Tolt River Watershed (29%) and wells at the Highline Wellfield (1%). From these sources, water travels by large pipelines to neighborhood storage areas and then on to home, business and institutional users where it is monitored and measured by water meters. Water is cleaned, treated and continually tested as it travels through the system to ensure that it is of the highest quality.

Water demand varies, depending on seasonal weather conditions. Summer and early fall months are typically times when the greatest amount of water is used, primarily for irrigation. The system provides an average of 160 million gallons per day (MGD) for the region.