The Beauties of Nature, and the Wonders of the World We Live In

by Sir John Lubbock

Fig. 48.—Diagrammatic section of a valley (exaggerated) R R, rocky basis of valley; A A, sedimentary strata; B, ordinary level of river; C, flood level.

The Reno, the most dangerous of all the Apennine rivers, is in some places as much as 30 feet above the adjoining country. Rivers under such conditions, when not interfered with by Man, sooner or later break through their banks, and leaving their former bed, take a new course along the lowest part of their valley, which again they gradually raise above the rest. Hence, unless they are kept in their own channels by human agency, such rivers are continually changing their course.

If we imagine a river running down a regularly inclined plane in a more or less straight line; any inequality or obstruction would produce an oscillation, which when once started would go on increasing until the force of gravity drawing the water in a straight line downwards equals that of the force tending to divert its course. Hence the radius of the curves will follow a regular law depending on the volume of water and the angle of inclination of the bed. If the fall is 10 feet per mile and the soil homogeneous, the curves would be so much extended that the course would appear almost straight. With a fall of 1 foot per mile the length of the curve is, according to Fergusson, about six times the width of the river, so that a river 1000 feet wide would oscillate once in 6000 feet. This is an important consideration, and much labour has been lost in trying to prevent rivers from following their natural law of oscillation. But rivers are very true to their own laws, and a change at any part is continued both upwards and downwards, so that a new oscillation in any place cuts its way through the whole plain of the river both above and below.

The curves of the Mississippi are, for instance, for a considerable part of its course so regular that they are said to have been used by the Indians as a measure of distance.

If the country is flat a river gradually raises the level on each side, the water which overflows during floods being retarded by reeds, bushes, trees, and a thousand other obstacles, gradually deposits the solid matter which it contains, and thus raising the surface, becomes at length suspended, as it were, above the general level. When this elevation has reached a certain point, the river during some flood bursts its banks, and deserting its old bed takes a new course along the lowest accessible level. This then it gradually fills up, and so on; coming back from time to time if permitted, after a long cycle of years, to its first course.

In evidence of the vast quantity of sediment which rivers deposit, I may mention that the river-deposits at Calcutta are more than 400 feet in thickness.

In addition to temporary "spates," due to heavy rain, most rivers are fuller at one time of year than another, our rivers, for instance, in winter, those of Switzerland, from the melting of the snow, in summer. The Nile commences to rise towards the beginning of July; from August to October it floods all the low lands, and early in November it sinks again. At its greatest height the volume of water sometimes reaches twenty times that when it is lowest, and yet perhaps not a drop of rain may have fallen. Though we now know that this annual variation is due to the melting of the snow and the fall of rain on the high lands of Central Africa, still when we consider that the phenomenon has been repeated annually for thousands of years it is impossible not to regard it with wonder. In fact Egypt itself may be said to be the bed of the Nile in flood time.

Some rivers, on the other hand, offer no such periodical differences. The lower Rhone, for instance, below the junction with the Saône, is nearly equal all through the year, and yet we know that the upper portion is greatly derived from the melting of the Swiss snows. In this case, however, while the Rhone itself is on this account highest in summer and lowest in winter, the Saône, on the contrary, is swollen by the winter's rain, and falls during the fine weather of summer. Hence the two tend to counterbalance one another.

Periodical differences are of course comparatively easy to deal with. It is very different with floods due to irregular rainfall. Here also, however, the mere quantity of rain is by no means the only matter to be considered. For instance a heavy rain in the watershed of the Seine, unless very prolonged, causes less difference in the flow of the river, say at Paris, than might at first have been expected, because the height of the flood in the nearer affluents has passed down the river before that from the more distant streams has arrived. The highest level is reached when the rain in the districts drained by the various affluents happens to be so timed that the different floods coincide in their arrival at Paris.