The Ngaruroro River (catchment area 970 sq. miles) rises on the north-east side of the Kaimanawa Mountains and flows along the south-west side of the Kaweka Range in forested country in a deeply incised gorge. It is joined by its only large tributary, the Taruarau, which drains the Ngamatea intramontane basin for some 5 miles before it, too, passes from the highlands of the main range on to the east coast lowlands. For some 13 miles it flows in a deeply incised course below broad Pleistocene terraces, before it passes on to the Heretaunga Plains, a deeply alluviated complex tectonic depression on the south-west shore of Hawke Bay. Alluvium carried down by the Ngaruroro and the neighbouring Tutaekuri and Tukituki Rivers has built up some 77 sq. miles of the plains to an average depth of 50 ft in the last 10,000 years.
The minimum flow is 300 cu. ft. per second. The largest flood on the Ngaruroro River occurred in 1867 when large areas of the plains as far north as Clive Square, in Napier, were flooded. A somewhat lesser flood in 1897 resulted in the river changing its course. In 1917 a flood almost as bad as that of 1867 occurred; the peak-measured flood (180,000 cusecs) was much smaller than any of these.
The Ngaruroro River charges a very important artesian basin in the Heretaunga Plains. Approximately half of its minimum flow of 300 cusecs passes into the underground aquifers, and eventually into the sea in submarine springs some 13½ miles off the coast. In the past, artesian pressures of nearly 40 ft were obtained at Clive, but recent very heavy use has reduced the pressures by nearly 20 ft.
The meaning of the name is obscure.
by Thomas Ludovic Grant-Taylor, M.SC., New Zealand Geological Survey, Lower Hutt.