Wai manu literally means "bird water" or "river of birds" in the Hawaiian language. During the time of Ancient Hawaii it was an ahupuaʻa, or ancient land division with a small village. Most of the area is state forest land, with a few campsites available with reservations. It is located in the Hāmākua district of Hawaiʻi island. The Waimanu Stream watershed includes many smaller flows from Kohala Mountain to the Pacific Ocean at sea level. A system of dikes of hard lava rock force large amounts of ground water dropped from the tradewinds into this valley, making it very different from the smaller shallow valleys directly to the west. This ridge is administered as the Puʻu O ʻUmi Natural State Area Reserve.
Tributary Waihīlau Stream starts at about 3,500 feet (1,100 m) elevation at  and flows into Waimanu Stream in the valley. Waihīlau Falls is one of the tallest single drops of a waterfall in the United States. It descends from about 3,000 feet (910 m) at to about 400 feet (120 m) elevation. The name comes from wai hī lau which means "many trickling waters" since during heavy rains the stream splits into many smaller streams overflowing the cliff. Waimanu Stream empties into Waimanu Bay at sea level at Coordinates: . Other streams with waterfalls in the valley include Waiʻilikahi Falls and Lahomene Falls.
The valley is not accessible by automobile. A foot trail called Waimanu or Muliwai Trail leads down a steep path from the Waipiʻo Valley. At the south end of the valley, Waimanu Gap at 2,089 feet (637 m) elevation leads to the upper end of Waipiʻo Valley.
If you should choose to camp in Waimanu, a camping reservation from the state DLNR is required to camp onsite. The most common route to visit Waimanu Valley is hiking from Waipi'o Valley. The hike in totality is about 8 miles (one way) depending on when you begin your journey. If you start from the top of Waipi'o Valley, walk down 3/4 mile until the road levels. Follow the road toward the beach for 1/2 mil and arrive on the black sand beach. From there, find the beginning of the Muliwai Trail marked by an array of state signs. You will then start ascending what many refer to as the "Z Trail". This trail is almost 1000 feet of vertical switchbacks. It is very important to be aware of your footing and try to prevent knocking any loose stones to the trail below. After braving the "Z-Trail" hikers then walk through a Pacific Pine Rain Forest. From there, hikers cross about 12 different streams and the trail also gets less and less prominent as you make your way to Waimanu Valley. Be sure to take in every break in the rainforest as your descend into the magical valley. Please be advised to BRING MORE WATER AND FOOD than you think you will need. Also remember, what you pack in your MUST pack out.
- Lloyd J. Soehren (2004). "lookup of Waimanu". on Hawaiian place names. Ulukau, the Hawaiian Electronic Library. Retrieved 2010-04-13.
- "Kohala Mountain Watershed Management Plan" (PDF). Hawaii state Department of Land and Natural Resources. December 2007. p. 38. Retrieved 2010-04-13.
- U.S. Geological Survey Geographic Names Information System: Waimanu Stream
- "Origin of the Big Island's Great Valleys Revealed in Hawaiian chant". USGS weekly feature. USGS. February 16, 2006. Retrieved 2010-04-13.
- "Pu`u O `Umi". Natural Area Reserve System web site. Hawaii state Department of Land and Natural Resources. Retrieved 2010-04-13.
- U.S. Geological Survey Geographic Names Information System: Waihīlau Stream
- Lloyd J. Soehren (2004). "lookup of Waihilau Stream". on Hawaiian place names. Ulukau, the Hawaiian Electronic Library. Retrieved 2010-04-13.
- U.S. Geological Survey Geographic Names Information System: Wai‘ilikahi Falls
- U.S. Geological Survey Geographic Names Information System: Lahomene Falls
- Lloyd J. Soehren (2004). "lookup of Waimanu Gap". on Hawaiian place names. Ulukau, the Hawaiian Electronic Library. Retrieved 2010-04-13.
- "Waihilau Falls". World Waterfall Database. 2004. Retrieved 2010-04-13.
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