The Rangitata.

The river, the hydrographic basin of which nest claims our attention, is the Rangitata, and although that portion of the Southern Alps proper which it drains is not so extensive as that at the head of the Waimakariri, next to be considered, it measuring scarcely ten miles along the summits of the Southern Alps, yet we find that the mountains there are not only of greater altitude; but, also, that the secondary ridges enclosing the system in question, are on both sides of such dimensions, that for a number of miles they are scarcely inferior in average altitude to the peaks in the central chain. The most southern branch of the Rangitata is the Havelock, which issues from a glacier of small proportions when compared with those at the head of the two rivers previously described. After a few miles, the valley enlarges to a breadth of more than a mile, and continues so wide for about fourteen miles to its confluence with the Clyde, having a nearly south-eastern course, which the river, for nearly seventy-eight miles as measured in a straight line, maintains to where it joins the ocean. The Havelock receives, in its course, numerous tributaries from both sides, of which the Forbes river, issuing from two glaciers that descend from Mount Forbes, is the most important.

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The second main branch, the Clyde, hag also its sources in glaciers of similar size to the former, its course to the junction haying a length of thirteen miles; it receives several tributaries, of which the McCoy, joining three miles below the terminal face of the Clyde glacier, is the largest. The bed of this branch, for the first six miles, is generally not so broad as the former, the water, often running in one channel and even when very low, being difficult to cross. After the junction of the Sinclair branch, however, the river-bed widens considerably, now flowing generally in several branches, and where the Lawrence joins it, being over two miles broad.

The Lawrence, the third main branch of importance, has its glacier sources on the south-western slopes of Mount Arrowsmith; in its upper portion, the valley has generally a gorge-like character, and even in its lower course, its bed is often considerably narrowed by gigantic moraines, shingle cones and fans, which the river could only partially remove. This branch is thirteen miles long from its principal glacier source to its junction with the Clyde; from here to the entrance of the Rangitata gorge, the river flows in numerous channels, its actual shingle-bed being generally a mile broad, whilst the valley itself—in which morainic accumulations, lacustrine deposits and alluvium abound, with roches moutonnées raising their ice-worn summits here and there above them—is often three to four miles broad. These Rangitata plains, about 22 miles long, doubtless represent the same area in which in the course of the Waitaki, the Mackenzie plains are situated, but owing to the fact that the glaciers were not of such gigantic proportions in the Great Glacier Period, all the phenomena are here on a smaller scale. However, it is evident that when the great glacier, which once filled these plains retreated, and before the lower gorge was excavated, a lake of considerable dimensions existed here, which after-being partly filled up in its upper portion, was drained in course of time by the lowering of its outlet. Numerous tributaries join the Rangitata in the middle course, of which the Butler, Potts, and Forest Creeks are the most important. From the upper plains, two broad openings lead into the valleys of the Ashburton and Rvakaia, of which I shall speak more fully when treating of the Ashburton system. At the termination of the lower of these openings, the so-called Trinity valley belonging to the Rangitata drainage, the gorge of the Rangitata begins. For about six miles, the river has cut such a deep and rocky channel through the front ranges, that it is impassable for man or horse, the bridle-track leads therefore along the ice-worn hills page 212bounding the gorge to the south, and where an ancient glacier channel can easily be traced. Where the river leaves the front ranges, it flows in a narrow gorge, about 600 feet deep with terraced banks on both sides, in which for the lower 300 feet, the alternating sandstones and slates, standing at a high angle, form perpendicular walls; but soon the rocks disappear and fluviatile and morainic accumulations take their place. These high banks, although gradually getting lower as we ascend the Canterbury plains, continue to accompany the northern banks of the river, for thirty-two miles, to the sea coast where they are still about twenty feet high. The southern banks however, disappear nine miles before reaching the coast, the channel of the river, on the so-called Island, being liable to shift with every freshet. The Rangitata, like all the principal rivers forming the Canterbury plains, does not receive any tributaries in its lower course their gigantic fan-like alluvial deposits being highest near their present river channels.