The sturgeon move around a fair bit. They are spread all over. Look for deeper holes (there are a million of them) but an easy one to find is downstream from Cedaroak around Lake Oswego. There is a big concrete structure. that boats used to dock up to. The water around there is 100 to 140' deep. You will want about 3x that in length to have a proper scope.
Look around on your FF for sturgeon near the bottom. If you don't get bit in 20 minutes then pull up and move a bit. If you move around enough you should find fish.
Also, watch the tides. The Willamette is tidal up to the falls. Best bite is on the outgoing.