New state mineral prospecting rules, which go into effect April 3, are now available from the Washington Department of Fish and Wildlife (WDFW). The 2009 "Gold and Fish" pamphlet, which serves as a permit for prospectors is available in pamphlet form on WDFW's website or on this site (see above). Metal detectors are considered "Hand-held mineral prospecting tools".
Metal Detecting is permitted at more than 67 state parks throughout Washington. Users of metal detectors must register first with Washington State Parks and comply with posted regulations. Rules are summarized in pamphlet available from park rangers or by calling (360) 902-8500; Telephone Device for the Deaf (360) 664-3133. I have attached in in a pdf on this page.