This trek begins from the upper parking lot that one would use for the Aiea Loop. After about 15-20 minutes of hiking the initial section of the Loop trail, you'll see a path descending a slope on your left. Take it.
The initial half mile or so is a gradual descent through strawberry guava and Christmas Berry groves. Eventually the path will became a wide dirt avenue. At a large mango tree on the right, the trail veers off the wide avenue and descends quite steeply down the hillside. This mango-tree turnoff isn't well marked; in fact, I walked right by it the first time I did the hike. Eventually the dirt avenue will reach the end of a residential cul de sac, so if you see houses, you know you've gone too far and have missed the turnoff.
Once you begin the steep descent, be careful because it can be slippery even on days when it hasn't rained. Fortunately, there are a sufficient number of trees for handholds and rocks and roots for footholds to make the climb down to the gulch negotiable.
Eventually you'll reach the streambed. From there, you'll head upstream on a trail that skirts along side of it. Be careful while rock hopping the river. The stones are deceptively slippery. According to an informed source, one should also be wary of hunters and wayward sorts who are known to frequent that remote valley. Be forewarned.
After about 45 minutes to an hour of upstream hiking, you'll reach a circular pool that spans the width of the river. That is where the official hike turnaround point is. I have read that the pool is particularly nice a few days after a heavy storm. On the day I reached the pool, the water was murky and stagnant.
The trail, although rugged and not well marked, continues upstream. If you have time and are adventurous, continue on. Otherwise, retrace your steps back to the parking lot.
Kalauao offers hikers an alternative to ridge climbing. Only adventurous sorts should attempt this challenging trek.