I did a little historical research at
http://www.cbr.washington.edu./dart/adult_rpt.html, and came to the conclusion that below the dam will be just fine. I'm gonna fish low in the river, just because it's closer to home. If the Willamette started throwing a lot of mud, I may go higher in the river.
My rationale is this: it looks like the run tends to stay strong at the dam for at least about 15 to 20 days after the peak bulge of fish. Assuming that the fish swim about 15 miles a day, and that the dam is at about river mile 120 from the mouth, fish should take about six days to reach the dam. If the peak was yesterday, the dam should be decent for another fifteen days, and the mouth should be decent for at least another nine days.
'Course, if I don't see squat near Kalama, I'll be beating cheeks upriver, just like everyone else.