This is an old thread, and my experience with going down this canyon is even older - late '80's. A friend and I tried to mountain bike down the canyon from the Willard Basin road up above. Needless to say, we didn't make it and wound up hiking back out. The road starts out as a nice smooth dirt road, which then turns into two tracks, then a single track, then disappears completely! There is a route through there though, built by the CCC in the 1930's. Ever since, I've wanted to go back and rediscover the route. I'm sure it would need some clearing with the pruners and a saw. More recently, I've hiked part of the way up from the bottom; there are ledge-switchbacks cut into the rock on the south canyon wall. The trail crosses one of the waterfalls at one point.
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