The chronological arrangement of laws enacted by the Washington Legislature and signed into law by the Governor are called session laws. When you need read the text of a law as it was originally enacted, consult the session laws, also called the Laws of Washington.
Bluebook citation: <year> Wash. Sess. Laws <page number>
Washington Office of the Reporter of Decisions Style Sheet citation: Laws of <year>, ch. <number>
Indexes are useful when you don't have a citation to a particular chapter number and a search fails to retrieve the correct citation.
In the Laws of Washington, the last volume for each two-year legislative session includes a subject index.