Canyons are inherently risky. Flash floods occur without notice on sunny days. Technical skills & surrounding topography knowledge required yet does not eliminate risk.
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The term "Upper Bridge" is referring to the popular starting point at Fish Creek Bridge on SR88. You'd need four topographical maps to fully appreciate the entire canyon. Horse Dam Mesa, Pinyon Mountain, Weavers Needle & Iron Mountain to be exact. A full trek of Fish Creek from the lower Salt River end to it's boxed-in origin near Windy Saddle is a hearty 16 miles!
Here we take on an easily accessible and scenic section of the canyon. Please note, that's easily accessible not easily traveled. Trail data given here is for a scramble going south to Lost Dutchman Canyon and back. This is to the point where Fish Creek Canyon abruptly veers due east.
Parking near the bridge on beautiful weekends may prove impossible. A weekday venture should thin out the crowds. Getting into the canyon can be a task your first time out. The key is to follow the west side to an alcove before heading down. It should go without saying but stay out of this canyon if rain is remotely possible. There's ten miles of canyon before this point. A severe flash flood is very probable in the area.
From the bridge you'll pass over broken beer bottles, graffiti and quite possibly a crowd of partygoers. The good news being it's over after the first bend in the canyon. The first quarter mile stretch up canyon is rather easy. No trail exist but you'll figure it out (don't climb the sheer canyon walls...lol) I've never explored the canyon bottom when the creek is flowing. I suppose it's possible to slosh through. Personally I wouldn't put forth that much effort for this canyon. It's interesting, just not quite up to spectacular par.
The real fun or hell depending on how you interpret it comes into play soon. Huge boulders encourage you to turn around. Your goal is to figure out the best route through each series of obstacles. I believe I made the wrong choice each and every time. =) This meant countless backtracking. There are simply too many trouble sections to offer much advice. I suppose you could conquer a few in various ways. You will need to take off your pack a couple times to make a squeeze here and there.
Like I said the canyon is interesting but there isn't a spectacular destination point in my opinion. The turn around point is really arbitrary. Just go in a couple hours and turn around. It's approximately twice as fast going out because you know the obstacles. I was surprised to see sections of trail further in with recent use.
While in the canyon notice some of the interesting rocks. You'd swear some came out of a cement mixer! Some of the boulders or slabs looked like they were loaded with geodes. I doubt that's the case but they have pockets with an edge that appears to be a cracked open geode. Also, one of the west sheer walls has an interesting, yet very inaccessible, alcove up high.
The book "Hiker's Guide to the Superstition Wilderness" has a ten-mile loop variation outlined on page 222. Read up on it. I would recommend making it a shuttle hike skipping the walk down SR88. That might be unpleasant and dusty for most. I do encourage walking the section of FS213 for non 4x4 users. It's a very scenic road. You won't be disappointed.
Check out the Official Route and Triplogs.