We began developing the Second Supply Project in 1968 to meet our future water supply requirements and increase regional supply. In 1970, a plan was developed for another transmission main, Pipeline No. 5 (now  known as the Second Supply Project Pipeline) to convey additional water supply to the City of Tacoma and to blend water from the North Fork Well field with Green River water as necessary. Phase I, development of the North Fork well field, was complete in 1977.

Phase II, transmission main and treatment facilities, was made more complex by political issues and evolving environmental laws in the early 1980s, including the establishment of minimum instream flow levels in the river to protect fish and other instream resources. The Second Diversion water right on the Green River was granted in 1986. By 1995, an agreement with the Muckleshoot Indian Tribe (MIT) regarding the Green River established resolution of past differences over water resource issues and set a framework for cooperative work managing the river system's resources. All permits and permissions for construction of the Second Supply Project Pipeline were granted by 1997.

By 2001, a Partnership Agreement was enacted between Tacoma Water, the City of Kent, the Lakehaven Utility District and the Covington Water District covering each participants’ rights and obligations to the Second Supply Project. Although the Second Supply Project was a joint venture known as the Regional Water Supply System (RWSS), and operation and maintenance of the RWSS is jointly funded by the partners, the operation and maintenance of the RWSS is entirely carried out by Tacoma Water.

Major construction of the Second Supply Project began in 1999. In 2000, the Tacoma to Federal Way portion of the project pipeline was complete and Lakehaven Utility District began receiving water from another portion of Tacoma Water's system. In 2002, the upper 10 miles of the pipeline was in service, delivering water to the City of Kent and to the Covington Water District. Changes to the Headworks diversion facilities, including the addition of a new fish trap-and-haul facility, were complete in 2006.

The new Phase I water treatment facilities (chlorination, fluoridation and pH adjustment) were finished and in service in August 2005. Ozone treatment and an integrated operations center were done by 2007. By the end of October 2005, Second Supply Project water began flowing through the entire 34-mile-long project pipeline. In May  2006 we made use of the Second Diversion water right to meet demands for the first time. Water for municipal supply was stored behind Howard Hanson Dam for the first time in spring 2007.

The next major development for the RWSS was filtration of the Green River, which is now complete.