Page:Transactions NZ Institute Volume 5.djvu/137

Rh only in connection with the Taieri, I shall simply refer to them at that stage when we come to view the storage room on the lower parts of the plain, and shall devote this portion principally to the nature of the larger river and its floods.

A comparison of the Taieri with the chief rivers of Europe or North America will show that in proportion to the country drained it is much in excess of the most of these as a flood producer. Thus, while it pours down its water at the rate of 1.666 cubic yards in the second for each square mile drained above Outram, the Mississippi in flood flows only at the rate of 0.044; the Ohio, at its mouth, 0.122; and the Yazoo 0.372 cubic yards per square mile drained. In Great Britain, the Tyne is the only one which approximates to the Taieri in this respect, its rate of flood discharge being 1.12; but small streams which have been gauged in meadow land have yielded as much as 1.2. The Yellow River, in Ireland, is as high as 4.12. On the continent of Europe the Loire appears to be pre-eminent for its floods—yielding at Pont de Fleurs as much as 4.18 cubic yards per second for each mile drained. Many of the Indian rivers far exceed any of the above—the Irvitz, especially, delivering at the rate of 16.5 cubic yards per second for each square mile of its gathering ground.

Now these figures of course represent very rough comparisons between the several streams named, no two of which are alike in physical conditions, either as to amount of rainfall or configuration of drainage area; but in a general way they enable a classification to be made which may yet be further improved as information is gathered. Apart from the relative quantities of rainfall, there are many other circumstances which tend either to aid or impede floods. Thus a stream draining a large tract of country is much less liable to heavy floods than one draining a small one; and also the general nature of the country as to inclination has a most noticeable effect upon the rate at which the water finds its way to the river, and consequently a like effect upon the amount of its volume. Another most important consideration is the nature of the strata, or the amount of vegetation in the district drained; for when the rocks are of a close compact nature, with comparatively few joints or crevices, and the ground bears but little vegetation, the water will run quickly oflf; but where the ground is porous, and the vegetation rank, a much longer time will elapse before the underground basins are filled, and the ground so saturated as to shed the water off as the rain falls. Surface lakes have a comparatively greater effect in moderating floods than either of the above causes, by receiving the water as it comes from the creeks, and allowing it to spread in thin films instead of rushing down a river channel in deep volumes.

Having got these general results, we may now endeavour to examine