River piracy at Kakariki, north-western Wairarapa, New Zealand

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Abstract
The part of Mangahao River that traverses the Tararua Range, upstream from Kakariki, was captured at some time in the Holocene Epoch (about 5,000-10,000 years B.P.) from the Hukanui Stream, which was formerly a major tributary ofthe Mangatainoka River. The captured segment now delivers high floods more than an order of magnitude greater than the mean annual flow of the Mangahao, and its diversion from the Hukanui Stream was a large drainage change. The capture site is on the line of the active Wellington Fault. The main factors that caused the capture were: (1) dextral displacement of the captured segment relative to the former Mangahao River; (2) in sequent erosion along the crush zone of the fault by a tributary of the former Mangahao; (3) aggradation of the floodplain of the captured segment during the Last Stadial of the Last Glaciation; and (4) easy lateral erosion of the fault crush zone by the captured segment during early Holocene time. The Mangahao provided a shorter route than the Mangatainoka to the local base-level (head of Manawatu Gorge) and the newly diverted water quickly excavated a deep ravine that now has a flood-plain 45 m lower than the early Holocene flood-plain of the Hukanui Stream at Kakariki.

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