The Earth as Modified by Human Action eBook

But we are ignorant of the proportions in which the marine deposits that form the soil of the polders have been derived from materials brought down by these rivers, or from other more remote sources.  Much of the river slime has, no doubt, been transported by marine currents quite beyond the reach of returning streams, and it is uncertain how far this loss has been balanced by earth washed by the sea from distant shores and let fall on the coasts of the Netherlands and other neighboring countries.

We know little or nothing of the quantity of solid matter brought down by the rivers of Western Europe in early ages, but, as the banks of those rivers are now generally better secured against wash and abrasion than in former centuries, the sediment transported by them must be less than at periods nearer the removal of the primitive forests of their valleys, though certainly greater than it was before those forests were felled.  Kladen informs us that the sedimentary matter transported to the sea by the Rhine would amount to a cubic geographical mile in five thousand years. [Footnote:  Erdhunde, vol. i, p. 384.  The Mississippi—­a river “undercharged with sediment”—­with a mean discharge of about ten times that of the Rhine, deposits a cubic geographical mile in thirty-three years.] The proportion of this suspended matter which, with our present means, could be arrested and precipitated upon the ground, is almost infinitesimal, for only the surface-water, which carries much less sediment than that at the bottom of the channel, would flow over the banks, and as the movement of this water, if not checked altogether, would be greatly retarded by the proposed cross-dikes, the quantity of solid matter which would be conveyed to a given portion of land during a single inundation would be extremely small.  Inundations of the Rhine occur but once or twice a year, and high water continues but a few days, or even hours; the flood-tide of the sea happens seven hundred times in a year, and at the turn of the tide the water is brought to almost absolute rest.  Hence, small as is the proportion of suspended matter in the tide-water, the deposit probably amounts to far more in a year than would be let fall upon the same area by the Rhine.

This argument, except as to the comparison between river and tide water, applies to the Mississippi, the Po, and most other great rivers.  Hence, until that distant day when man shall devise means of extracting from rivers at flood, the whole volume of their suspended material and of depositing it at the same time on their banks, the system of cross-dikes and COLMATAGE must be limited to torrential streams transporting large proportions of sediment, and to the rivers of hot countries, like the Nile, where the saturation of the soil with water, and the securing of a supply for irrigation afterwards, are the main objects, while raising the level of the banks is a secondary consideration.

CHAPTER V.