Unfortunately, the best Spokane Area duck hunting is around Moses Lake or the Tri-Cities, 2-3 hours away. The scabland ponds West of Spokane can occasionally produce, but the hunting is inconsistant and only until freeze up.
A boat helps a lot to be able to hunt water closer to Spokane in the form of the larger rivers and lakes.
Check out the WDFW website for more info.
Tom