The Waipaoa River, with a catchment area of 830 square miles, rises in the heavily forested Raukumara Range south of Mt. Arowhana (4,724 ft). The major tributary, the Waikohu, rises near Matawai in the saddle in the Raukumara Range through which the Gisborne-Opotiki road passes. The Te Arai, also flowing from the west, rises in the eastern part of the coastal highlands south-west of Poverty Bay and drains an area that is largely grassed farm land. The Waingaromia drains the central area of coastal highlands to the north of the Poverty Bay Flats, joining the Waipaoa at Whatatutu. The Waihora whose catchment lies to the south of the Waingaromia has its confluence with the main river at Te Karaka.
The Awapuni Lagoon communicates with the river through a long channel, and the river itself, after running for more than a mile behind a sand bank, finally enters the sea a few miles north-west of Young Nicks Head. What appear to be two old channels of the river, the Taruheru River and Waikanae Creek, flow through Gisborne on the opposite side of the valley from the present river channel.
The western part of the catchment of the Waipaoa is rugged, rising to 4,000 ft. More than three-quarters of the total area is in soft, very clayey, lower Tertiary and upper Cretaceous marine sediments. In the Waingaromia and upper Waipaoa, and also at Waerengaokuri, a significant proportion of the sediments are the lower Tertiary (Dannevirke Series) bentonites which, when wet, flow under gravity. The rainfall pattern of the East Coast produces periodic high-intensity downpours, sometimes amounting to an inch an hour for several hours. Occasionally rainfalls as high as 10 in. in 24 hours occur, and a runoff as great as 5 in. an hour has been recorded. The sediment content of the Waipaoa River is very great and the river bed tends to aggrade, causing serious problems in river control. The river meanders widely over its lower plain, and river-straightening works have been carried out at Matawhero in the lower valley, where in some places the valley floor has been built up more than 20 ft since the Taupo pumice eruption some 1,850 years ago.
The minimum flow of the Waipaoa was 90 cu. ft. per second in March 1948. Since 1853 floods covering a large part of the lower valley are recorded. In 1853 the valley was covered with water from Waerengaohika to Gisborne. Severe floods occurred in 1867, 1879, 1906, 1910, 1914, 1916, 1932, 1944, and 1948. The largest recorded flow was in 1932, with 145,000 cusecs. This was almost equalled in 1948, with 140,000 cusecs.
The Maori name of the river is associated with Wai-a-Paoa, “the river of Paoa”. Paoa, a great chief, lived with his people in the forests of the mountainous interior of the East Coast district. According to legend, he built a canoe and then created the river in order to launch it.
by Thomas Ludovic Grant-Taylor, M.SC., New Zealand Geological Survey, Lower Hutt.