This is the Wailua River, which originates at the center of the island on the highest peak, Mt. Waialeale. The river and its valley were considered very sacred by the ancient Hawaiians. Poliahu Heiau is one of seven heiaus of major importance that used to be in the valley. On the right bank of the river is Kamokila, a re-created Hawaiian village that is also worth a visit.
There are no road signs showing where Poliahu Heiau is, so most people don't even know it's there. You actually would have passed on entering the parking lot for the falls, but circling through the parking lot and heading back downhill for a hundred yards or so is really the easiest way to get to it. If you do that you will see a low rock wall on the right. Just past the wall is a place to turn in. There will be a guard rail overlooking the river with informative placards. To the right is the heiau itself with another informative placard in front of it.