I fish the rivers that feed Tillamook Bay in NW Oregon and encounter the same litter problem also. Several years ago I contacted Oregon Adopt-A-River and volunteered to clean up a 2 mile stretch of river twice a year. Since then it's a habit to carry a trash bag with me everytime I fish. The private property owners know me and allow me to fish on their property whenever I want and for return, I clean up their place. The Guide Shop on the Wilson River always accepts my extra trash to dispose of. I don't know if Wash State has a similar program, but if they do, it would be a win win deal.
Mark