Cathlamet sees the most fishermen from March to May for the spring salmon season. The B10 season boats are pretty much all downstream in July for ocean and August when the river opens. In September the boats are scattered out more with a lot of them up by Cathlamet and further upstream chasing the salmon in the fall. In August Cathlamet should be pretty quiet as far as fisherman although the launch ramp may be busy at times with go-fast boats if the weather is real nice.
Some folks, though, may be more inclined to be down in the estuary for fishing than moored at Cathlamet, although if the brewery and taco truck are still there and in business what's a couple of days off from fishing?
The Cathlamet gatherings in the past have been in the early spring and it seems we were lucky with the springer seasons being off for whatever weekend was picked. The regs can be goofy for the spring salmon and usually shut down for a while in the middle of the run when quotas are reached.
August might be a more pleasant time, for sure. I recall at the first Cathlamet gathering about 20 years ago we woke up the morning of April 4 with a dusting of snow in the cockpit. Cathlamet can be a neat walking around town so a warm summer weekend might be best.