I have hiked down into Kalauao Valley on several occasions and just recently found an alternate access/exit route. The traditional route down into Kalauao begins at a point about 10 minutes into the Aiea Loop hike (the upper starting point). Stuart Ball, in his well-known *Hikers Guide to Oahu*, describes this trek quite well, so for a detailed narrative, pick up his book.
From the Aiea Loop, the trail to the valley first descends gradually through thick stands of strawberry guava. Every time I hike through this section, my mind conjures images of the Jack Nicholson movie "The Shining" (remember the scene where one of the victims was being pursued through rows of hedge-like shrubs?). After a couple of minutes, the trail opens up at the base of a powerline tower. Then the descent through shrubs recommences.
Eventually, the path opens up and widens in a section of eroded red dirt lined with eucalyptus trees. Off to the right, the ridge drops off steeply into kukui-lined Kalauao Valley. Look for a ribbon-marked mango tree on the right, for the trail leaves the ridge at that point and drops steeply to the valley floor. However, don't count on the ribbon being there. For whatever reason, people sometimes remove these trail markers.
Though steep, the quarter-mile descent into Kalauao is manageable because of a host of readily available tree branches, roots, and rocks to place hands and feet. A aptly-situated cable/rope is even at your disposal to help you on your downward trek. If you are uncertain at any point, remember to lower your center of gravity and slide on your okole if need be. A dirt-covered rump is always preferable to a broken limb or gashed body part.
Once you reach the river, take note of the area and ribbons marking the spot. This is particularly important if you are returning the same way, for it is possible to walk right by this junction. Usually, heaps of ribbons on trees mark this spot. However, on my most recent hike into Kalauao, someone--perhaps a local very protective of the valley--had removed all but one of these markers. If need be, leave your own marker there (a piece of plastic or material will work).
The trail continues upstream. You'll cross Kalauao river nine times before arriving at a decent-sized pool and waterfall. At each river crossing, ribbons are affixed on both sides of the bank. Look for these. Also take some time to admire the plant life along the banks and the animal life in the river. I've seen prawns (although much less on recent hikes) and o'opu. Please do not pillage the stream for these!
The pool/falls is a nice spot to grab a bite to eat and kick back and celebrate your accomplishment. If the water is flowing, take a jump into the pool (it's about 8-9 feet deep at its deepest point), sit under the waterfall, or both. Be careful if you have open cuts, for feral pigs populate the valley and leptospirosis, a nasty water borne virus from the feces and urine of these animals, is a possibility.
Most folks who hike into Kalauao exit the same way they came in. However, further upstream, another trail out of the valley exists. I found this route by chance in early June. Accompanied by a couple friends, I had trekked into Kalauao via the traditional route. After a swim in the pool, we decided to do some exploring upstream before retracing our steps. During this exploratory phase, we spotted what appeared to be a rough trail that ascended straight up the ridge (this spot is less than 50 yards above the pool). Since my hiking companions were willing, I led them up this "trail" that soon dissipated into a non-trail. Having committed ourselves, we hauled and hacked our way upward, following the route of least resistance.
After about 30-40 minutes of this, we ascended to a distinct trail that contoured mauka to makai along a line about two-thirds of the way up the ridge. At first, I thought that this was a pig trail because I spotted nary a ribbon or marker or some other sign of human passage along the path. As it turns out, this contour trail ends at a spur ridge that climbs up to a point just past the second powerline tower on the Aiea Loop Trail.
On a subsequent hike, I followed the contour trail mauka until it reached the river. The trail down is much more gradual than the traditional descent route described earlier. Along the way, you'll work your way in and out of a couple of small ravines, pass under a sizable banyan tree, and cross over a barely- bubbling intermittent stream. Eventually, in 40-50 minutes, you'll reach Kalauao Stream. I marked this junction with several orange ribbons. To get to the pool/falls from this upper junction, one must cross the river five times. Among the sights you'll see on this section is a grove of thick-trunked palm trees.
If you do attempt this loop hike version, I'd recommend that you descend via the steep traditional route and, after a rest/lunch/swim break at the pool, exit via the upper contour trail. Remember that there are five stream crossings to be made after the pool; these are marked by ribbons. Plan on about 20-25 minutes to reach the upper junction from the pool and another 40- 45 minutes to hike along the contour trail back to the Aiea Loop.
Kalauao Stream, by the way, continues on for quite a distance above the upper junction. I've met hunters on the trail who have told me they regularly travel two to three hours upstream beyond that point. If you attempt upstream exploration, keep an eye on the weather conditions on the summit. Hiking in a river valley when flash flood conditions exist falls into the category of living dangerously. In three words--don't do it.
Give Kalauao a try if you have some energy and time to burn. The hours and effort you commit to this venture will be returned to you ten-fold.