Outside of the normal legislative process, Washington State laws can be amended through the an initiative or referendum. There are two types of initiatives (Initiatives to the People and Initiatives to the Legislature) and two types of referenda (Referendum Measures and Referendum Bills). A brief explanation of each of these is available at the Washington Secretary of State's website here. A more in-depth overview of each process is available in Jeffrey T. Evan's article Direct Democracy in Washington: A Discourse on the Peoples' Power ot Initiative and Referendum, 32 Gonz. L. Rev. 247 (1996).
The following video from TVW, featuring UW Law Professor Hugh Spitzer, provides information about the history of the initiative process as well as its potential benefits and pitfalls.
As initiatives and referenda are outside of the "normal" legislative process, the information gathered to uncover the history and intent will be different. In addition to the sources outlined below, it can be fruitful to review relevant publications from the sponsoring organization and news stories to determine intent.