Tenaya Canyon is one of the classic scrambles in the vicinity of Yosemite Valley. The official NPS map given out to visitors at the park entrance has a specific warning in the center of the map stating: Hiking in Tenaya Canyon is dangerous and not recommended.. They may as well just say "Free beer and pizza" if they are directing their warning to the curious scrambler.
Make no mistake the route is a serious undertaking, but it can be rather fun too. It should only be attempted in late summer or early autumn when water levels are low enough. At times of high water the route is considerably more dangerous. Route-finding is somewhat tricky. Most parties seem to either do the descent in a day, or take two days to do the ascent. The fastest parties can make it in both directions in a single day, but be sure you have sufficient stamina and confidence in your solo skills if attempting both directions.
The length of the route from the Mirror Lake TH to Tenaya Lake is about 10 miles, all but about three miles is cross-country and much of this fairly rugged. Most parties take 7-8 hours when descending the canyon using rappels which take time to set up and execute (there are four bolted rappel points in the canyon). The canyon was annually descended from 1909 to 1937 by one S.L. Foster. Today there are perhaps a dozen or two (mostly) descents each year during the 1-2 month low water period (Sept-Oct).
If descending from Tenaya Lake, the closest parking is at the Sunset TH just west of the lake. Overnight parking is allowed here. Follow the Forsyth trail southwest for about half a mile until shortly after it has headed in a southeast direction and begins a short climb. Leave the trail, cross to the northwest side of the creek, and continue downstream. At the start, you should be heading in a direction almost due south. Follow the easy streambed (often dry in Sept-Oct) for a mile until you reach the top of a large granite bowl leading down to Glacial Valley. This point has become known as Admonition Point due to the rustic NPS sign found here warning visitors to stay out of Tenaya Canyon. You have reached the start of the route.
If ascending from Yosemite Valley, the closest parking is at the stables, but be aware that overnight parking is not allowed here. Overnight parking can be found at the JMT backpackers lot located between Curry Village and Happy Isle. The free shuttle bus can drop you off even closer at the the Mirror Lake TH, but the bus doesn't usually run early enough for most parties. From the Mirror Lake TH, head east for several miles to the end of the maintained trails. There are trails on both the north and south side of Tenaya Creek (either can be followed), both ending where they join at a bridge crossing the creek on the far east end. This is where the Tenaya Canyon route starts.
Tenya Canyon can be broken up into roughly four sections, each described here. Route-finding is somewhat different depending on whether you are heading up or down the canyon. An attempt is made here to describe both directions, but be aware that no amount of text will substitute for route-finding skills. The descriptions here may be very helpful or completely useless depending on the party and their individual experience. It is a good idea to have a backup plan in case you run into trouble, namely returning the way you came. Don't put yourself into the position of forcing your party to continue ahead against your better judgement.
This is the section between the Mirror Lake trail and the Inner Gorge. In general it is an easy (class 2) section with tricky route-finding only at the beginning (west end). From the bridge at the end of the maintained trail, there are use trails leading off on both the northwest and southeast sides of Tenaya Creek. The trail on the southeast side is harder and will disappear in about 1/4 mile. From here you have two options. If you follow the creek directly (on the southeast side), you will soon come to a beautiful 30-foot waterfall. Following the creek to the right of this is class 3 with low water, slippery class 4 with just a little bit more water. There are huge boulders and just figuring out how to get around this is an interesting challenge.
Your second option is to head southeast where the use trail ended, following a dry side creek up about 150ft. Another use trail then heads east across some easy slabs to the top of the waterfall section described above. This second option is far more obvious during a descent than an ascent. This entire first challenge can be bypassed if you start on the northwest side of the creek from the beginning at the bridge. A use trail is found that traverses roughly 50-100 feet above the creek level across leaf-strewn slopes under shady oaks. Though less interesting, this is by far the fastest option and can easily save half an hour's time, possibly more. The use trail gives out about halfway up the Lower Gorge and leaves you off in the flatter upper section walking up the broad, boulder-strewn creekbed.
There is little elevation gain for the last mile in the Lower Gorge, and the hiking is easy, if tedious. There are excellent views of Half Dome, Clouds Rest, and Watkins Pinnacle to enthrall you. Eventually the canyon will narrow to little more than 50 feet across, and it will be obvious you have reached the Inner Gorge. If descending, keep an eye out for the use trail on the right (northwest) side about halfway down this section where the angle of descent starts to pick up again. If you don't find the use trail it is not critical. You should have less trouble than you found making your way through the Inner Gorge.
The Inner Gorge
This is the most interesting section of the entire route. The walls close in steeply on two sides, almost claustrophobically, and there is little sun that penetrates to the creek level. Consequently, one will find unusual fern gardens and delightful pools (some with decent-sized fish in them) to dazzle your senses. The scrambling is highly varied - large boulders, polished granite, a little bushwhacking at times, some optional rappels. Published descriptions of this section vary widely, which mainly shows that there are many options, so I will describe it in general terms only. Besides, route-finding for yourself is a good deal of the fun to be had here.
There are four fixed rappel anchors, all of them located on the northwest side of the stream. Since these are generally only used for descents, they are numbered starting with the one furthest upstream. The first is located just above the top of the Inner Gorge, on some ledges about 80-100ft above the creek. If coming from Lost Valley above, it is pretty obvious when easy progress is stopped by a cascading watercourse down into the gorge. Look for the ledges to your right and the anchor further down amongst the ledges. Anchors #2-4 are located further down, #3 & #4 are closer together and about a hundred yards from the bottom of the gorge. Some parties report only finding or using 2 of the rappels, so clearly they are not all necessary.
Most of the time, it is never necessary to climb more than about 50 feet above the creek on one side or the other to bypass obstacles. It should never be necessary to climb up the streambed directly except for one section lower down, just below anchor #4. At the top of the gorge, one encounters an impasse if following the creek directly. Steep walls surround you, and you can see the trees of Lost Valley tantalizingly out of reach above of you. A fractured, near-vertical mossy wall on your left is the descent route from anchor #1 above. The wall might go at 5.8 or 5.9, but it looks dangerously slick and unstable. Regular humans must backtrack about 100 yards down the canyon and choose a route high up to the left or right. Most parties report using the northwest side, a steep talus climb with some bushwhacking followed by some traversing ledges about 200 feet above the creek. It is also possible to climb about 300 feet up on the southeast side, followed by some bushwhacking and then class 3 slabs to reach Lost Valley.
If you have ideas of finding an enchanting forested valley in a remote locale, perhaps lured by the name, forget it. There's really very little that is enticing. It is often hot and exposed, the thin forest understory thick with reeds and brush. We didn't see anything that might make a decent camping spot. Mostly you just want to get through it. The lower part is easy, just follow the valley floor not far from the creek on whichever side seems less difficult. The upper part is a bit tricky. The key thing to remember is that you do not follow the granite slabs on either side of Pywiack Cascade (where the main flow of Tenaya Creek follows). The upper section is too slick and steep.
The correct route is about a quarter mile to the south, up (or down if descending) some incredibly steep slabs about 800 feet in elevation that look impossible to climb without a rope and almost certain death. Once on them, you find the angle is not so steep and even tennis shoes will grip fairly well - but the height will play tricks with your head if you lose confidence. From below we watched a party of 10 (with quite a few novices) descend without mishap, though not without apprehension. Finding this route from below is pretty easy as you have to go out of your way to continue upstream to Pywiack Cascade. The wide open bowl is pretty obvious rising over 800 feet up on your right. From above, it is not so easy. From the top of Pywiack Cascade, head southwest up slabs aiming for a rounded boulder-strewn ridge.
Perched atop is an unusually spherical rock some 15 feet in diameter that makes an excellent landmark (and a good bouldering challenge). Continue southwest, descending talus and a bit of bushwhacking until you find the top of the bowl marking the 800-foot slab descent. The south side of the slabs offers the easier route, particularly in the lower reaches where it will help you avoid the often slick rock where the water is trickling down from above.
This is the name given by S.L. Foster to the hanging valley above Pywiack Cascade. It is about two miles in length, has a gentle grade, and makes for easy class 1-2 walking. The upper end of the valley offers the only challenge, where the creek runs mostly north-south, cascading down a wide granite bowl about 300 feet high. Parties have reported climbing the granite slabs on either side of the creek, so choose what looks easiest to your party. These are far less steep than the slabs bypassing Pywiack Cacade at the head of Lost Valley. At the top of the bowl you should find yourself at Admonition Point - the rest is an easy mile walk out to find the trail and then another half mile to Tenaya Lake.
Most parties use a rope in descending the canyon. If you are comfortable soloing class 4 rock, you should have no trouble in Tenaya Canyon. If you are ascending and want to bring a rope, be sure to have at least one good climber in your group - the slabs are not easily protectable for an ascent, and someone will likely have to solo the four sections to set up a rope for the rest of the party.