The Earth as Modified by Human Action

by George P. Marsh

All arrangements which tend to obstruct this process of raising the flats adjacent to the channel, whether consisting in dikes which confine the waters, and, at the same time, augment the velocity of the current, or in other means of producing the last-mentioned effect, interfere with the restorative economy of nature, and at last occasion the formation of marshes where, if left to herself, she might have accumulated inexhaustible stores of the richest soil, and spread them out in plains above the reach of ordinary floods. [Footnote: The sediment of the Po has filled up some lagoons and swamps in its delta, and converted them into comparatively dry land; but, on the other hand, the retardation of the current from the lengthening of its course, and the diminution of its velocity by the deposits at its mouth, have forced its waters at some higher points to spread in spite of embankments, and thus fertile fields have been turned into unhealthy and unproductive marshes.—See Botter, Sulla condizione dei Terreni Maremmani nel Ferraress. Annali di Agricoltura, etc., Fasc. v., 1863.]

Dikes, which, as we have seen, are the means most frequently employed to prevent damage by inundation, are generally parallel to each other and separated by a distance not very much greater than the natural width of the bed. [Footnote: In the case of rivers flowing through wide alluvial plains and much inclined to shift their beds, like the Po, the embankments often leave a very wide space between them. The dikes of the Po are sometimes three or four miles apart.] If such walls are high enough to confine the water and strong enough to resist its pressure, they secure the lands behind them from all the evils of inundation except those resulting from filtration; but such ramparts are enormously costly in original construction and in maintenance, and, as has been already shown, the filling up of the bed of the river in its lower course, by sand and gravel, often involves the necessity of incurring new expenditures in increasing the height of the banks. [Footnote: It appears from the investigations of Lombardini that the rate of elevation of the bed of the Po has been much exaggerated by earlier writers, and in some parts of its course the change is so slow that its level may be regarded as nearly constant. Observation has established a similar constancy in the bed of the Rhone and of many other important rivers, while, on the other hand, the beds of the Adige and the Brenta, streams of a more torrential character, are raised considerably above the level of the adjacent fields.

The length of the lower course of the Po having been considerably increased by the filling up of the Adriatic with its deposits, the velocity of the current ought, prima facie, to have been diminished and its bed raised in proportion. There are abundant grounds for believing that this has happened in the case of the Nile, and one reason why the same effect has not been more sensibly perceptible in the Po is, that the confinement of the current by continuous embankements gives it a high-water velocity sufficient to sweep out deposits let fall at lower stages and slower movements of the water. Torrential streams tend to excavate or to raise their beds according to the inclination, and to the character of the material they transport. No general law on this point can be laid down in relation to the middle and lower courses of rivers. The conditions which determine the question of the depression or elevation of a river-bed are too multifarious, variable, and complex, to be subjected to formulae, and they can scarcely even be enumerated.

The following observation, however, though apparently too unconditionally stated, is too important to be omitted.

Rivers which transport sand, gravel, pebbles, heavy mineral matter in short, tend to raise their own beds; those charged only with fine, light earth, to cut them deeper. The prairie rivers of the western United States have deep channels, because the mineral matter they carry down is not heavy enough to resist the impulse of even a moderate current, and those tributaries of the Po which deposit their sediment in the lakes—the Ticino, the Adda, the Oglio, and the Mincio—flow in deep cuts, for the same reason.—Baumgarten, p. 132.

In regard to the level of the bed of the Po, there is another weighty consideration which does not seem to have received the attention it deserves. refer to the secular depression of the western coast of the Adriatic, which is computed at the rate of fifteen or twenty centimetres in a century, and which of course increases the inclination of the bed, and the velocity and transporting power of the current of the Po, UNLESS we assume that the whole course of the river, from the sea to its sources, shares in the depression. Of this assumption there is no proof, and the probability is to the contrary. For the evidence, though not conclusive, perhaps, tends to show an elevation of the Tuscan coast, and even of the Ligurian shore at points lying farther west than the sources of the Po. The level of certain parts of the bed of the river referred to by Lombardini as constant, is not their elevation as compared with points nearer the sea, but relatively to the adjacent plains, and there is every reason to believe that the depression of the Adriatic coast, whether, as is conceivable, occasioned by the mere weight of the fluviatile deposits or by more general geological causes, has increased the slope of the bed of the river between the points in question and the sea. In this instance, then, the relative permanency of the river level at certain points may be, not the ordinary case of a natural equilibrium, but the negative effect of an increased velocity of current which prevents deposits where they would otherwise have happened.] They are attended, too, with some collateral disadvantages. They deprive the earth of the fertilizing deposits of the waters, which are powerful natural restoratives of soils exhausted by cultivation; they accelerate the rapidity and transporting power of the current at high water by confining it to a narrower channel, and it consequently conveys to the sea the earthy matter it holds in suspension, and chokes up harbors with a deposit which it would otherwise have spread over a wider surface; they interfere with roads and the convenience of river navigation, and no amount of cost or care can secure them from occasional rupture, in case of which the rush of the waters through the breach is more destructive than the natural flow of the highest inundation. [Footnote: To secure the city of Sacramento, in California, from the inundations to which it is subject, a dike or levee was built upon the bank of the river and raised to an elevation above that of the highest known floods, and it was connected, below the town, with grounds lying considerably above the river. On one occasion a breach in the dike occurred above the town at a very high stage of the flood. The water poured in behind it, and overflowed the lower part of the city, which remained submerged for some time after the river had retired to its ordinary level, because the dike, which had been built to keep the water OUT, now kept it IN.

According to Arthur Young, on the lower Po, where the surface of the river at high water has been elevated considerably above the level of the adjacent fields by diking, the peasants in his time frequently endeavored to secure their grounds against threatened devastation through the bursting of the dikes, by crossing the river when the danger became imminent and opening a cut in the opposite bank, thus saving their own property by flooding their neighbors'. He adds, that at high water the navigation of the river was absolutely interdicted, except to mail and passenger boats, and that the guards fired upon all others; the object of the prohibition being to prevent the peasants from resorting to this measure of self-defence.—Travels in Italy and Spain, Nov. 7, 1789.

In a flood of the Po in 1839, a breach of the embankment took place at Bonizzo. The water poured through and inundated 116,000 acres, or 181 square miles, of the plain to the depth of from twenty to twenty-three feet, in the lower parts. The inundation of May, 1872, a giant breach occurred in the dike near Ferrara, and 170,000 acres of cultivated land were overflowed, and a population of 30,000 souls driven from their homes. In the flood of October in the same year, in consequence of a breach of the dike at Revere, 250,000 acres of cultivated soil were overflowed, and 60,000 persons were made homeless. The dikes were seriously injured at more than forty points. See page 279, ante. In the flood of 1856, the Loire made seventy-three breaches in its dikes, and thus, instead of a comparatively gradual rise and gentle expansion of its waters, it created seventy-three impetuous torrents, which inflicted infinitely greater mischief than a simply natural overflow would have done. The dikes or levees of the Mississippi, being of more recent construction than those of the Po, are not yet well consolidated and fortified, and for this reason crevasses which occasion destructive inundations are of very frequent occurrence.]

For these reasons, many experienced engineers are of opinion that the system of longitudinal dikes is fundamentally wrong, and it has been argued that if the Po, the Adige, and the Brenta had been left unconfined, as the Nile formerly was, and allowed to spread their muddy waters at will, according to the laws of nature, the sediment they have carried to the coast would have been chiefly distributed over the plains of Lombardy. Their banks, it is supposed, would have risen as fast as their beds, the coast-line would not have been extended so far into the Adriatic, and, the current of the streams being consequently shorter, the inclination of their channel and the rapidity of their flow would not have been so greatly diminished. Had man, too, spared a reasonable proportion of the forests of the Alps, and not attempted to control the natural drainage of the surface, the Po, it has been said, would resemble the Nile in all its essential characteristics, and, in spite of the difference of climate, perhaps be regarded as the friend and ally, not the enemy and the invader, of the population which dwells upon its banks.

But it has been shown by Humphreys and Abbot that the system of longitudinal dikes is the only one susceptible of advantageous application to the Mississippi, and if we knew the primitive geography and hydrography of the basin of the Po as well as wo do those of the valley of the great American river, we should very probably find that the condemnation of the plan pursued by the ancient inhabitants of Lombardy is a too hasty generalization, and that the case of the Nile is an exception, not an example of the normal regime and condition of a great river. [Footnote: Embankments have been employed on the lower course of the Po for at least two thousand years, and for some centuries they have been connected in a continuous chain from the sea to the vicinity of Cremona. From early ages the Italian hydrographers have stood in the front rank of their profession, and the Italian literature of this branch of material improvement is exceedingly voluminous, exhaustive, and complete.