Tons of dispersed camping along the river and down dirt roads going off the river. A magical place!!
It is a hit or miss as to whether or not it will be easy to find a dispersed site, but we have always found one every time we come out this way. Sometimes we find a spot immediately, other times, we drive around and "explore" the old logging roads to see what hidden gems we've yet to find.
From Oregon Highway 26, head south at the sign for "Lower Nehalem Rd." Coming from Portland, this will be on your left just past Baker's General Store and just before Elsie. The first five miles or so are paved before turning into a dirt and gravel road and lead to several nice campgrounds including the larger Henry Rierson Spruce Run Campground and smaller tent-only sites such as Morrison Eddy Campgrounds.
Continuing on, Lower Nehalem Road turns into Foss Rd. and you'll also go from the Clatsop Forest into the Tillamook Forest. Along the road, there are several dispersed camping site on your right, often right next to or near the river. These spots look like small dirt driveways or parking spots and the campsites are tamped down enough to be recognizable as a camp site, sometimes with stone fire rings. Beware that there is private land intermixed with the state forest, so look out for "no tresspassing" signs. We found several amazing sites just past mile marker 11. I took photos of the surrounding area to remember how to get there that I've included, but I'm not 100% sure that the GPS coordinates are correct.
Several well-marked sites (three or so) with a short walk down the the absolutely beautiful Nehalem River. The river was pleasant on a warm day with lots of rocks and salamanders to check out. According to the Tillamook Forest website, dispersed camping is free, though there are absolutely no facilities, so you'll have to bring your own water and toilet-type-items and pack out all trash. These sites are well-used and many (though not all) were full on the weekend. The only down-side was there were a few oddball young male campers roaming around making howling noises and "clearing" branches with machetes. In general, many of the sites we checked out along the way had a bit of a "what are you doing here" vibe from the folks who were already set up, so be ready to wander through a few spots before finding your perfect one.