Not really a valley, Tonquin Valley is a broad, open pass between the Astoria River valley and the Tonquin Creek valley. It's notorious for dense swarms of mosquitoes and other biting insects, as well as wet, muddy trails. I chose my dates for early September to avoid these obstacles. The only problem with that is that most of the vegetation is on the decline and the weather can be dicey.
Make sure to give yourself plenty of time to relax and explore all this area has to offer.