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Geology of the Provinces of Canterbury and Westland, New Zealand : a report comprising the results of official explorations

The Opihi

page 218

The Opihi.

Of the rivers of which the hydrographic basins lie entirely in Canterbury, but which have no glacier sources, the Opihi is the most important. Its principal sources are all situated in the high mountain chain beginning at Fox Peak and running in a nearly north and south direction to Mount Nimrod, by which the Waitaki system is bounded on the eastern side. Of the confluents of which this river is formed, the Opuha is the largest, the main sources coming from the eastern declivities of Fox Peak and the Dobson range. After passing through the Opuha plains, it enters amongst low palæozoic ranges till it joins the Opihi amongst the limestone hills, 20 miles above its mouth. The Opihi proper has its sources in the ranges near Burke's Pass, and after leaving these mountains, it flows for about 12 miles, diagonally, through the middle portion of the Opihi plains, when, meeting the old morainic accumulations which here cross the open valley, the river enters the Opihi gorge, a deep fissure, cut through a hill consisting of palæozoic rocks. This is certainly one of the most remarkable phenomena in the physical geography of New Zealand, where a river, instead of excavating its bed through loose alluvial and moramic deposits, has actually cut a deep channel through hard rocks. I have already noticed the same fact when speak ing of the lower Waimakariri gorge. For a distance of five miles the Opihi flows through this picturesque gorge, its nearly vertical banks, about 600 feet high, being clothed with luxuriant forest; it then enters the limestone region where it is joined by the Opuha. After flowing for six miles through a broad valley with low limestone ridges on both sides, it is joined by the Tengawai, the main sources of which are situated near the Mackenzie Pass. This stream receives numerous affluents from the eastern slopes of the high ranges at the head of the Hakataramea. Its valley is generally broad, and bounded by low hills on both sides.