It flows generally southwest from its origins in hill country to the east of Lake Rotorangi, reaching the Tasman Sea in the South Taranaki Bight 5 km (3.1 mi) southeast of Hāwera.

[1] The Ngāti Ruanui Claims Settlement Act 2003 said the river was a major source of resources, providing shellfish and fish, including piharau, kōkopu, tuna and pātiki,[2] birds such as kererū, pūkeko, tiwaiwaka, kāhu, kākāpō, kiwi, korimako, miromiro and pīpīwharauroa and plants, including koromiko, kōhia, hinau, piripiri, mamaku, and rewarewa.

[1] The steep valley sides of the lower river provided good defensive sites, as marked on the 1:50,000 map.

Otapawa pā was held by Tangahoe, Pakakohi and Ngāti Ruanui when it was attacked on 16 January 1866, during the Second Taranaki War.

The account of the attack in 'The adventures of Kimble Bent' put Māori losses at 7,[3] but others estimated 30 killed and 30 wounded.

[4] The river rises on a sandy mid-Pliocene Tangahoe Mudstone, formed in a shallow sea,[6] then its valley is cut down to early-Pliocene Whenuakura Group rocks (bioclastic limestone, pebbly and micaceous sandstones and massive siltstone), whilst the surrounding land is covered by mid-Pleistocene beach deposits of conglomerate, sand, peat and clay.

Quality varies over the length of the river, as shown in the table - In the summer of 2016 the MCI level at the viaduct was the lowest ever, probably due to earthworks by Fonterra, causing the rating to drop to poor.

[14] Fonterra's Whareroa dairy has a consent to draw 30,000,000 L (6,600,000 imp gal) of water a day from the Tangahoe's tributary, the Tawhiti Stream,[15] where flows, temperature, etc.

A deep and unstable cutting alongside, which proved to be often unpassable, served for traffic until, in 1996, South Taranaki District Council decided to reopen the tunnel,[22] which re-opened in 1997,[21] at a cost of $245,000.

The smaller spans are on piles driven into the rock, but the centre piers are concrete 5.5 m (18 ft) deep.