Well, there's Henry Hagg Lake/Res. not far from you. There are also smaller lakes in and around the islands on the Columbia too (Smith & Bybee, for example). I believe that the St. Louis Ponds would be a good bet as well. And there are probably many others too. Maybe some other OFFers can chime in.
St. Louis from Mid Spring - Early Fall. Anything small and moving, I'd personally rather use a beaded fly over a jig, slower presentation = staying in the strike zone longer, tiny worm with no weight or a stick float is great as well.
Dry flies are great at St. Louis as well.
I don't target Panfish anymore, but St. Louis used to be my favorite place, and I still get Bluegill once in awhile while fly fishing for the Trout.