King County Water District No. 90 was incorporated on August 12, 1952, with a Water Service Area of approximately five square miles. The primary reason for the formation of the District was due to problems with wells located in the service area. During the summer months, many of the wells would become dry or stagnant. In the spring of 1956, after four years of planning, the District began to supply water to approximately 250 customers. Since that time the District has grown to its present size of just over 7,000 direct service connections in a service area of approximately 15 square miles (9,770 acres). As of December 31, 2015, the District is servicing over 7,800 accounts. The majority of the connections are single-family residential, but the District also serves commercial, irrigation and school service accounts.
The system is divided into 14 separate pressure zones in order to maintain adequate pressures throughout the service area. Under typical (non-emergency) operating conditions, the District relies on Seattle Public Utilities (SPU) for approximately 70% of its water supply. The District provides the other 30% of its water supply from a groundwater well and treatment facility that it operates and maintains. The District maintains and operates eight storage reservoirs, eight pump stations, one treatment facility, seventeen pressure reducing stations and over 110 miles of transmission and distribution mains to provide high quality service to all the areas of the District in accordance with District standards and applicable regulations.