SEPA was enacted in 1971 during the First Extraordinary Session of the Washington State Legislature: 1971 Laws of Washington, 1st Ex. Sess., ch. 109, p. 623 (PDF). The original bill was introduced in the Senate as S. 545 and passed the Senate and House of Representatives in May, 1971. The governor signed the bill into law on May 19, 1971.
SEPA has been amended several times, including:
The text of these laws can be found on the Office of the Code Reviser's website.