Page:The journal of the Royal Geographic Society of London. Volume 34, 1864. (IA s572id13663720).pdf/263

Rh by accumulating, over long periods, what would otherwise run off in streams as it fell, is the prime cause of the great, sudden, and, at first sight, apparently inexplicable floods that characterise all the rivers that have their sources in high mountains. Change of temperature is the secondary and immediate cause; but although this is the case, a flood may occur without any great or perceptible increase of temperature, for the wind, by transporting the snow to a lower altitude, occasions the same effect as a rise of temperature. It was noticed, during the survey, that the snow-line on the north-west side (the windy side) of the mountain ridges was higher than on the south-east side (the sheltered side), thus showing that the wind is a very decided cause in producing the effects now under consideration. The flood-marks on the mountain streams, more especially those running into the Te Anau and Manipori Lakes, show a rise and fall almost incredible. The power exercised by such torrents is forcibly impressed on the attention by an examination of their channels, where immense blocks, that have in the first place been disintegrated from the surrounding mountains by frost, are seen lying and being broken up, through the never-ceasing attrition of rushing waters, into shingle and sand, which are carried forward and deposited in the lakes. The mouths of the rivers all show that they are advancing into the lakes, however slowly that may be. The lakes are a very great feature in the natural history of the country, and perform a most important function in its economy. They act as regulating reservoirs to the mountain torrents already mentioned; for over their broad surface the floods find room to spread their volume, until there be time given for the accumulation to pass away in the steady flow of one river. The value of the lakes as a means of restraining such rivers as the Clutha and Waiau within safe limits, will more readily appear when it is considered that the Te Anau and Manipori Lakes (the two principal of the Waiau River system) alone cover 182 square miles, and that their surfaces have a rise and fall of 8 or 9 feet during the course of the year. The Clutha River, likewise, has the Wakatipu, 114 square miles; the Wanaka, 75 square miles; and the Hawea, 48 square miles: altogether, 237 square miles of lake to regulate its volume. These lakes have also a rise and fall of several feet. From the data now given, it will be evident that but for the tempering influences of the lakes, the Clutha and Waiau, in place of flowing along a well-defined channel in a perennial stream as now, would have been so fluctuating in volume that no channel could have contained them, and their valleys would have been long shingle-beds down to the sea—a continuation, in fact, on a grand scale, of such valleys as those of the Dart and Matukituki.

The greater extent of the lakes at a former period is evident