Through the middle of Hilo Town runs the Wailuku River—'Destructive Waters' in Hawaiian—which carries the runoff from the saddle of the world's two tallest mountains Mauna Loa and Mauna Kea (when measured from the sea floor, these peaks are 40,000-plus feet). When tropical rain thunder down, no river is more aptly named, since Wailuku carries the force of a tsunami.
Wailuku River State Park consists of two separate parks, covering about 40 acres a few miles above Hilo. The upper portion is Boiling Pots (Pe'epe'e Falls) a run of rapids and swirling pools. People have died here, and most often, Boiling Pots are dangerous. But when water is low, the pots are primo swimming holes.
Rainbow Falls is the lower section of the state park—morning is the best time to catch the spectrum of color in the waters' mist. A trail sguiggles to the top of the falls, where swimming pools await. Though a hazard during high water, these pools are usually safer for swimmers than those at Boiling Pots. Both falls are on the tour-bus circuit, but adventure travelers can find room to roam.
Hawaii the Big Island Trailblazer has details on the park, as well as lots of other out-of-the-way treasures in Hilo and the Hamakua Coast. It's available on Amazon and BarnesandNoble.com