The Mangapu River is a river of the Waikato region of New Zealand's North Island. It has its sources in numerous streams which flow generally northwards from the King Country south of Te Kūiti, the longest of which is the Mangaokewa Stream. These streams join to form the Mangapu close to Te Kuiti, and from here the river flows north, passing close to the east of Waitomo Caves, where the Mangapu caves have the largest entrance in the North Island (about 100 m long and 90 m deep), before flowing into the Waipā River at Ōtorohanga. The New Zealand Ministry for Culture and Heritage gives a translation of "double stream" for Mangapū. SH3 crosses the river on a 111 m bridge near Ōtorohanga built in 1966 and near Hangatiki on a 41 m concrete bridge built in 1977. The railway follows the east bank for about 6 km, crossing only small tributaries, though it did suffer from flooding, for example in 1905. The river at Ōtorohanga is classed as being in the worst 25% of similar sites as regards all measured pollutants, though some attempts are being made at restoration.

Pehitawa Kahikatea Forest Reserve

The reserve 18.5 ha of kahikatea forest on the east bank of the river, was purchased by the Native Forests Restoration Trust in 2001. Kahikatea forest once covered 41000 ha in the Waipa Ecological District, now reduced to 158 ha, of which 52 ha are in the Mangapu valley, though over 70 ha has been felled since 1975. A major drainage scheme was carried out just upstream from the reserve in the 1930s, including emptying of a lake. The forest floods in winter and flood protection would be uneconomic. Some kahikatea in the reserve are about 120 years old. The reserve also has swamp maire, mataī, tītoki and pukatea. Te Araroa long-distance walkway passes through the reserve and crosses the river on a suspension bridge.