I love to eat crawdads also. Just go to BI-Mart and get a commercially made trap. I have made my own from rolling wire mesh and then making two cones, but they never turn out right. For $10 you can catch tons of crawdads and the commercial ones hold up longer than ones I have made.
Also page 18 in the fishing regs. I know that you "Fish for food" but becoming a mad hatter from too much mercury or getting a liver transplant from the other chemicals would not be fun. It looks like crawdads shouldn't be eaten out of the lower Columbia or lower Willamette. All resident species should not be eaten out of those zones. Seriously I would only eat migratory fish from those areas due to the PCBs, Dioxins, and Pesticides. Maybe the Clack or other rivers. I have had the most luck catching crawdads out of coastal streams than any other place I have tried in OR. I think the rotting salmon make them Huge.
Be Safe with your food,
Fisherwilly