It arises at about the 10,800 feet (3,300 m) elevation along the eastern slope of Mauna Kea (19°48′26″N 155°25′13″W / 19.80722°N 155.42028°W / 19.80722; -155.42028 (Wailuku River source)).

It flows generally eastward, descending steeply from the mountain and entering the Pacific Ocean at Hilo.

There is a plan for restoration of the Hilo Bay watershed that includes characterization of the Wailuku River.

The high flood marks can be seen dated in concrete, on the stairs going down to the river behind the Hilo Public Library.

According to legend, the river was created in a battle between the god Maui and the lizard monster Kuna.