The Manawatu River (catchment area, 2,296 square miles) drains the central east coast of the North Island, rising in the eastern Ruahine Range to the north of Norsewood. It flows eastwards to the axis of the east coast inland lowlands where it turns a right angle to flow south-south-west towards Woodville for nearly 30 miles. It then turns abruptly westwards to flow through the Manawatu Gorge and thence south-westwards to the sea in the south Taranaki Bight. Between the town of Woodville and the Manawatu Gorge there is a confluence of the major tributaries Mangatainoka (171 square miles), Tiraumea (364 square miles), and Mangahao (125 square miles) Rivers which drain the southern part of the east coast inland lowlands, the west flank of the east coast highlands, and the east flank of the northern Tararua Range. The Manawatu River proper and its tributaries to the north-east of the gorge are deeply entrenched in terraces formed during the Pleistocene glaciations. At Ashhurst on the western side of the gorge the Manawatu is joined by the Pohangina River (211 square miles) and, near Rangiotu, by the Oroua (320 square miles). From approximately half way between Palmerston North and Rangiotu the Manawatu meanders over a coastal plain some 17 miles wide.
The Pohangina and Oroua rise in the eastern Ruahine Range in an area with a rainfall of up to 200 in. per annum. The upper part of the Manawatu, rising to the east of the axial range, is subject to periodic torrential rain. The largest recorded floods occurred in 1880, 1897, 1902, and 1953, with peak discharges of about 160,000 cu. ft. per second. Flooding to depths of between 4 and 24 ft resulted and there were river rises of up to 60 ft at the upper gorge bridge. Floods of over 100,000 cusecs occurred in 1906 and 1941, while flows of 50,000 cusecs have been common. The lowest measured flow was 430 cusecs in 1939. Extensive flood-control works have been established between Rangiotu and Foxton. These consist of a diversion channel and a massive flood-gate system. The river is tidal for a distance of about 4 miles to beyond the State highway bridge at Foxton.
Important towns on or near the river are Dannevirke and Woodville east of the gorge and Palmerston North and Foxton to the west. The port of Foxton, once important, is now but little used.
The gorge through the axial range, which affords an important road and rail link between east and west, is a feature of prime interest. Young marine sedimentary strata above the gorge show the former existence of a strait connecting the sea that lay both east and west of the axial range, which then existed as a series of islands. During the first Pleistocene, glaciation lowered the sea level with the emergence of the ridge and the development of a river which drained the area previously occupied by the sea. During the subsequent rise of the land, the river has cut its gorge in the hard rocks of the axial ridge.
The origin of the name Manawatu is obscure.
by Thomas Ludovic Grant-Taylor, M.SC., New Zealand Geological Survey, Lower Hutt.