Canyons are inherently risky. Flash floods occur without notice on sunny days. Technical skills & surrounding topography knowledge required yet does not eliminate risk.
Moonshine canyon is one of my favorite non-technical canyons. I've visited it a half dozen times or so over the years, and never tire of it. The route is easy, and well within most hikers abilities, while the scenery, history, and old sheep bridge make the hike outstanding. For hikers looking for a narrow canyon experience, this is very highly recommended. This area of the San Rafael desert is not often visited, so expect much solitude and exquisite scenery. There is great primitive camping throughout the area, but please practice Leave No Trace Principles when camping in the area. Three Canyon, just across the road from Moonshine, and Chaffin Ranch Cold Water Geyser also provide interesting diversions while in the area. The area was popular with bootleggers during prohibition (hence it's name). The west fork contains a few remnants of the old stills built during this time. The sheep bridge, however, likely pre-dates the bootleggers. As of 2009, it is in poor shape, yet still standing proudly over the canyon. It's common sense, but do not try to use the sheep bridge. It is no longer stable, but is fascinating to view and try and image ranchers moving sheep over such a narrow bridge.
From the 4x4 trailhead, follow the ATV trail as it winds it's way down the wash to join Moonshine proper in 15 minutes or so. Once in Moonshine, head down canyon. The canyon quickly begins to narrow and deepen. There are a few boulders in this section to climb over and around. None are particularly difficult, especially with a little team work. Once in the depths of Moonshine, easy walking takes you to it's narrowest point, underneath the sheep bridge. Turning around at this point and returning makes a great short hike for those just wanting to see the narrows. (2-3 hours round trip). If your looking for a longer hike, continue down canyon as it begins opening and getting wider. A little over a mile from the sheep bridge (30-40 minutes), the West Fork comes in on the left (looking down canyon). Heading up the West Fork provides an interesting challenge. Just up from the junction is a 10-12m / 35-40 ft. dryfall. This can be climbed by most with little risk, it is fairly easy and very secure climbing. The pool beneath the dryfall, however, changes with every flood and can be shallow or fairly deep. Above the dryfall the slot has a few more relatively easy boulder sections before opening up. Again, however, this section seems to change a bit with each flood. Keep following the West Fork up canyon to the ruins of an old concrete vat used for moonshine production. Just past the vat the canyon forks. A short visit up the right fork will take you to Moonshine spring. To visit, got up the right fork until it forks again. Here, go west into the vegetated fork a short distance to the spring in an alcove. Continue up the west fork of Moonshine. When you reach another prominent junction, go right (up the west fork) climbing a couple of short dryfalls in the process. Once the canyon opens up, travel due east to reach the east fork of Moonshine at about the spot you dropped in at. Return up the ATV trail you came in on.
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WARNING! Hiking and outdoor related sports can be dangerous. Be responsible and prepare for the trip. Study the area you are entering and plan accordingly. Dress for the current and unexpected weather changes. Take plenty of water. Never go alone. Make an itinerary with your plan(s), route(s), destination(s) and expected return time. Give your itinerary to trusted family and/or friends.