Hey HaveRod,
Go to Clarkston, then take Hwy 129 towards Asotin. Once you get to Asotin, follow the sign that says 129 Anatone. This will take you up and over the hill, and drop you in on the Grande Ronde. Just before you cross the bridge, take a right on the dirt road. This will follow the river upstream all the way to Troy. The Washington hatchery at Cottonwood creek is about 3 miles from where you turn. Expect some company, it's no secret. But, they are all there for a reason. BTW-- I don't know how much fishing you've done, but that hole stretch from Boggan's(just across the bridge you don't cross, except to eat!!) to Cottonwood, and even beyond, if very fishy water. Lots of places to pull off and fish. As for the mouth, you are correct. Go to Asotin just like I said, only don't take the right turn to continue on 129. It's about 20 miles to the mouth, and then the road will follow the Ronde upstream for a couple of miles, before it crosses and goes up Joseph creek. That's all the access there is on the bottom end. This stretch normally fishes better in the fall, but with the low water all winter, who knows??? Sounds like maybe the fish aren't stacked at Cottonwood.......yet.

Good Luck