The Ruamahanga (catchment area, 1,340 sq. miles) is the most important river in the Wairarapa. Rising in the forested country of the northern central Tararua Range on the eastern slopes of Mount Dundas (4,935 ft), it flows into the open country of the Wairarapa lowlands west of Maurice-ville, passes close to the north-west of Masterton, the principal town of the Wairarapa, and thence southwards on the eastern side of the Wairarapa Plains, where it passes through Lake Wairarapa on its way to Lake Onoke, a beach bar-dammed lagoon on the shore of Palliser Bay.
The major tributaries draining from the forested areas of the eastern Tararua Ranges from north to south are the Waingawa, Waiohine, and Tauhere-nikau. Draining from the largely pasture-covered east coast highlands to the east of the Wairarapa Plains are the Tauweru, Ruakokoputuna, and Tauranganui Rivers.
The estimated minimum flow in 1948 was about 280 cu. ft. per second. Floods of approximately 100,000 cusecs occurred in 1880 and 1897.
The meaning of the Maori name is obscure.
by Thomas Ludovic Grant-Taylor, M.SC., New Zealand Geological Survey, Lower Hutt.