The challenge with multi day trips in WA and other geographic locations where the headwaters are relatively close to the river’s base level (ocean, Puget Sound, Columbia, etc.) is that our rivers are generally fairly short, and the section of “good boating” can be limited. The rivers start out STEEP (often too steep), mellow out into a zone where paddling is nice, and then quickly transition into the flat, meandering lowland section that precedes base level. Often times these lower flat sections of river are also developed, or at least agricultural, so they don’t always offer good camping or a wilderness feel. Plus, with the ice age glacial topography in WA which often times results in “U”-shaped river valleys (vs. “V” shaped), roads are often built utilizing the easier terrain in the river valley. So many of our longer, mellower river valleys have roads rather than a wilderness feel.
That said, WA has many fantastic shorter (2-3 day) hike-in wilderness runs IF you’re comfortable paddling harder, steeper, committed sections of whitewater. A few classics include the Elwha and Tshletshy Creek in the Olympics, and the Lost River and Bridge Creek / Stehekin in the North Cascades. There are many more, but those tend to be the ones talked about, probably due to (fairly) straightforward trail access.
Stepping down a notch in difficulty, to runs that are maybe up to class III in difficulty, limits the options of multi day runs that require hiking to access. A few of the rivers on the western side of the OP could fit that category – you can hike up the Queets, Hoh, and Bogachiel to access some moderate stretches of whitewater. And on those you can extend your float down past the “road free” sections and still have some feeling of wilderness with decent camping.
If you don’t require a wilderness feel to your hike, there are also some nice sections of river that are accessed via gated timber roads. The WF Satsop and Canyon Creek on the southern OP offer some fairly long class II / II+ / III sections that go through amazingly scenic canyons. But since Green Diamond Resource Co (formerly Simpson Timber) has started gating their roads to keep out vehicles, the only way the public can access these gems is on foot. You could easily do a 2, or even 3 day float on these. Similarly, there’s gated access to the Chehalis and the Kalama, both of which offer really nice LONG sections of class II / III (with a couple of portages).
If you’re willing to consider some really nice overnight river trips that don’t include hiking access (except for maybe a portage or two), I’d add the Wynoochee (southern OP) and the Cispus to your list. Both of these have fairly long sections of moderate whitewater that make for nice overnight trips.
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