I've been obsessed with Eaton Canyon ever since two college friends took me up in search of the route that bypasses the impressive lower falls. We failed to find the exit point and ended up at the power lines way above the canyon. (At which point my friend Tom decided he was more interested in zip-lining across the canyon along the power-lines instead... Fortunately he never realized his dream because an obscure technical detail called the "corona effect" would likely have killed him had he ever tried!)
The correct route is actually really easy to find nowadays, thanks to the
significant gang traffic and graffiti arrows pointing the way(!) Just go back
downstream from the falls until you can scramble up the ridge on the west side
of the canyon. It is a good solid third class route. It follows a scary
knife-edge ridge for a short distance before exiting to the right at one of two
points. The first option traverses on narrow toe-ledges past mountain
mahogany to a good use-trail. The second option continues up the ridge
another 20-30 feet and then drops steeply (but much more safely!) down to the
same use-trail. The use-trail contours for a hundred feet or so to a little
cliff. This is the hardest spot, but the route is obvious: climb around the
corner, then clamber down to loose slopes that run down to the canyon bottom at
last. Warning: I've terrorized several non-climber friends on this route
before accepting that most normal people simply find it too traumatizing to
enjoy.
Past the first waterfall it is possible to navigate all the way up to The
Gully -- several miles in all, a good long day trip there and back. The second
major waterfall has an easy 2nd class use-trail that scrambles around it on the
left (west/north) side of the canyon. There are several other minor falls and
pools, some of which require moderate climbing skill. One involves some tricky
moves up a small log right in the middle of a water fall -- great fun! A few
others require moderate climbing skills, but judging by the graffiti, gang
members manage it regularly, unfortunately. Eventually you reach a string of
several small waterfalls and pools. I'm pretty sure it's impossible to climb
them all, however my friend Sham and I managed to scramble up the brushy slope
to the right (south) and bypass them all, although neither of us would
recommend it highly unless you are equally obsessed. Past these falls it is
easy-going the last few miles to The Gully. There is a small fall and slide
there that you should be able to swim to and climb up using a hidden ledge
behind the falls, but it only buys you an extra 20-30 feet, because you run
into a 45 foot waterfall inside a dark grotto almost immediately afterword.
From the top there's nothing challenging until you reach the so-called
"point of no return" a mile or so below Idlehour. You can climb around this
first small water-slide on the right (north) side of the canyon, but don't
attempt it unless you are an experienced climber: it's got a good fall, but if
you can't make it back up you're screwed! From there you can make it almost
all the way to The Gully, stopping eventually at a slimy 40-some foot
waterfall.
The short section between this waterfall and The Gully waterfall in the grotto contains two 50 foot rappels and an non-reversible 20 foot water-slide (and one or two other minor pools and falls) -- probably the highpoint of the canyon, although there are cool gorges and pools and slides and waterfalls and verdant river-parks all along the lower half.
See Chris Brennen's Canyoneering in the San
Gabriels for a more conservative route description, including a shortcut
approach into the canyon a little way below Idlehour, and a questionable
description of a shortcut over a ridge that bypasses the bottom two falls.
Concerning the latter: Sham and I met two guys who showed us the route several years ago. It is very difficult to pick the "safe" route down, and even that is very loose -- hardly what I'd call "low-impact" hiking. I strongly advise against it. And to make matters worse, Brennen's directions are uncharacteristically vague. I stared at the cliff from the knife-edge on the other side (where you get an excellent view), and I couldn't make head nor tails of his description. If you get it wrong you'll miss the anchors for the rappel and end up down-climbing a heinously loose nasty cliff while pelting innocent passers-by in the canyon below with all manner of loose rocks and debris. I've done it, it's not pretty.