Starting from Happy Isle, go south along the path on the west side of the Merced River to the Nature Center. This is near ground zero for the massive rockfall of July 10, 1996 that killed one hiker and destroyed much of the Nature Center. There is a display on the southeast side of the building depicting the event that makes for an interesting read. The area south of here is still heavily damaged from the accompanying air blast that felled ten acres of trees in its path. It's eerie how all the fallen trees point in the same direction.
Having your fill of nature education, find the start of the horse trail on the southwest side of the display area. A service road is west of the trail, and the start is marked by a sign indicating No Hikers. But ignore that part. This trail is used by the stable guides taking tourists on horseback up to Nevada Falls in order to avoid the more popular JMT on the east side of the river. If you are polite enough to move off the trail when stock approaches and explain you are heading to Illilouette Gorge if anyone asks, you will not be cited or otherwise harrassed. The horse trail is a quiet respite from the regular foot trail, and you'll likely encounter no one on the 1/2 mile you follow it.
Heading south, go past a water tank on the right side (thus the service road) and cross a bridge. The trail turns south and joins an east-west trail between two bridges, cross the bridge to the right, and nearly immediately turn left. Leave the trail heading upstream, following the creek on the northwest side.
This is a fairly easy and short scramble, with a great view of Illilouette Falls at the top of the gorge as a fine reward. It is less than a mile in length and requires no special climbing skills and no serious bushwhacking. The creek cascades down the left side of the gorge, closely skirting the west side of Panorama Cliffs. It is fairly impossible to follow the gorge up on the left side. The right side is another story. The forest follows the gorge up nearly the entire way, covering the jumble of boulders found between the creek and the southeast face of Glacier Point on the right. There are old climbing routes on this side of Glacier Point that have fallen into disuse due to rockfall hazards and perigrine falcon closures.
Travel through the gorge is helped by several factors. A great deal of organic material covers the boulders making them generally safe and easy to scramble on. Even better, one can find vestiges of a climbers' trail along the base of Glacier Point that skirts some of the heavier forest. Take your time to enjoy the hike and you can always find a class 2 way around the obstacles.
Two thirds of the way up, most of the forest gives out as you get your first view of the falls. They are stunning. The 370-foot drop has significant flow even in fall. In Spring it is a torrent and your progress may be stopped altogether if the flow is high enough. As the trees give out, continue boulder hopping to the base of the falls. Mist and chilly temperatures will greet you as you near. Most of the gorge is in shade for all but a few short hours of the day, so it can provide a cool respite when the Valley is sizzling in summer. A large shallow pool is found at the base of the falls, but the water is quite chilly - not so good for a swim! If you look carefully towards the top you may see hikers on the Panorama Trail taking in the views above you.
In the fall, water flow is low enough that you can scramble much of the way in the creekbed itself. Many of the boulders are huge, so it may be more difficult ascending. I found descending along the watercourse far more enjoyable (and challenging) than returning back via the forest.
For a more serious scramble, you may want to try scrambling out of the gorge. The west side of the gorge offers the only practical scramble routes. I tried two routes during my visit, but turned back on both due to exposure and difficulty of climbing. The first route I tried was an obvious ramp across from the falls to the northwest. This led up about 150 feet of class 3-4 climbing before I got spooked off. A second route is found further north along the same side of the gorge. Just north of a bulging buttress, a small side gorge heads up about 50 yards. Follow this to near its end, then scramble onto the buttress on your left. There is exposure here, but good foot placements. Climb up the north-facing side of the buttress on class 3 face climbing with exposure. This can be followed up for almost 400 feet, to a height above the top of the falls (but still not out of the gorge). Here again I turned back. It appeared one might traverse south across a seriously exposed grassy ledge. Might be wise to rope up. That's as far as I got before retracing my steps. If anyone has further info on this scramble, I'd love to hear of it.
Another interesting option would be to rappel into the gorge, climbing down from Glacier Point. If anyone is interested in joining me, send email - I'd love to try it. :)
None needed unless attempting to scramble out of the gorge to the Panorama Trail. This is a more serious class 5 undertaking, and a rope and gear would be recommended.
If you have information about this route that doesn't pertain to any of the other sections, please add it here.