I've been into Cottonwood Canyon, but it's been years (2007). At about 3.5 miles in, the bridge (7.7K') had been taken out by an avalanche and that's where we turned around. The gain to that point was 2,300' and the county highway was still 1,600' above. I went back in a few days later to cut back some brush - it's a beautiful trail, but doesn't see much use.