The Rakaia River, with its tributaries the Mathias and Wilberforce, drains a 40-mile stretch of the main divide between the Waimakariri and Rangitata Rivers, together with much of the Arrowsmith (to 9,171 ft), Rolleston, and Craigieburn Ranges. Its main source, over 90 miles from the sea, is fed by meltwater from the Lyell and Ramsay Glaciers, the latter flowing from a név below Mount Whitcombe (8,656 ft), the highest mountain on the main divide north of the Classen peaks. Included in the Rakaia catchment of about 1,000 square miles are Lake Coleridge; Lake Heron, a moraine-dammed lake once draining into the South Ashburton; and Lake Lyndon, at the foot of Porters Pass.
The Whitcombe, Mathias, and Browning Passes (4–5,000 ft) are all relatively low main divide crossings from the Rakaia, the last-named an important route to the West Coast in the days of gold mining in the Wilberforce.
The whole catchment upstream of the Rakaia Gorge was intensively glaciated during the Pleistocene Ice Ages, and the plain through which the Rakaia now flows in a braided channel before reaching the sea, over 30 miles from the mountains, was built with materials eroded from the hills by ice during these glaciations. After the retreat of the glaciers, the hills became covered with thick forest, at least to 4,500 ft, but only relatively small areas of bush remain, and the thin plant cover allows fast run-off of water following storms. A flood exceeding 200,000 cusecs was recorded in 1950, while flows as low as 3,000 cusecs occur for long periods in the late winter.
With regard to the name itself, it is possible that Rangaia is the proper form, as “ng” becomes “k” in the South Island dialect. Rangaia may be translated “to arrange in ranks” and perhaps refers to the methods adopted by the Maoris to ford the river. But the origin of the word is most obscure.
by George Leslie Wickenden, Sheep and Wool Instructor, Farm Advisory Division, Department of Agriculture, Wellington.