Walking is way more risky than running the rapids in a boat. Walking takes a lot of time and definitely increases the likelihood of getting poison ivy, finding a bee nest, crawling over rocks with rattle snakes down in the holes, and twisting an ankle.
I have only ran the main salmon once in June 2011 at high to extreme levels. 15,500 CFS at corn creek at the start, 4 days later the river rose a couple of feet flooding some tents, and we finished the trip on 35,000 to 50,000 CFS. (60,000+ at white bird). We found most of the Rapids easy and straight forward read and run. Black creek at 15,000+ was a beautiful glassy wave train. We portaged two 14ft paddle boats around Chittam, but ran Chittam successfully with two 156R aire gear boats that were way overloaded. The portage was a dumb idea, prickly bushes and poison ivy, and one first aid due to a cut to one persons head from a branch. We had two flips during the trip goofing around in unnamed Rapids and rouge waves that appear at high water. Both rafts were righted in the middle of the river rolling over 3foot waves.
I recommend scouting, but not walking. And bring more beer than you think. We ran low but not out. Thank god for a jet boat run to riggins the day before the river rose.