A boat opens up a whole lot of opportunity. If you don't have sonar just go pick up one of the "Fish-n-River" maps at one of the local boat shops. Northwest Jet, Sundown, and I believe Wholesale Sports all have them. They will show you the depth (on average) and help to recognize structures and depth changes in the river. They also list the most prolific fishing holes on the river. They will have a little tab identifying the best holes for sturgeon. Now these are the holes that are most heavily fished so you might have some competition, but it will get you a place to start.
I always find the best fishing downstream of the merging waterways. The Yakima, Snake, Walla Walla and even the flume at Ringold. Find the area downstream of the merger that the water flow slows down or swirls from the two currents coming together and look for an underwater depression in that area.
I am by no means a pro sturgeon fisher, so take this with a grain of salt. Just my observations over the years.