Many thousand acres of land have been recently permanently improved in Italy by the draining of swamps, and extensive operations have been projected and commenced on the lower Rhone, and elsewhere in France, with the same object. [Footnote: Very interesting and important experiments, on the practicability of washing out the salt from seacoast lands too highly impregnated with that mineral to be fit for cultivation, are now in progress near the mouth of the Rhone, where millions of acres of marshy soil can easily be recovered, if these experiments are successful.
See Duponchel, Traite d'Hydraulique et de Geologie agricoles. Paris, 1868, chap. xi. and xii.
In the neighborhood of Ferrara are pools and marshes covering nearly two hundred square miles, or a surface more than equal to eight American townships. Centrifugal steam-pumps, of 2,000 horse-power, capable of discharging more than six hundred and fifty millions of gallons of water per day have lately been constructed in England for draining these marshes. This discharge is equal to an area of 640 acres, or a mile square, with nearly three feet of water.] But there is probably no country where greater improvements of this sort have either been lately effected, or are now in course of accomplishment, than in our own. Not to speak of well-known works on the New Jersey seacoast and the shores of Lake Michigan, the people of the new State of California are engaging in this mode of subduing nature with as much enterprise and energy as they have shown in the search for gold. The Report of the Agricultural Department of the United States for January, 1872, notices, with more or less detail, several highly successful experiments in California in the way of swamp-drainage and securing land from overflow, and it appears that not far from 200,000 acres have either very recently undergone or will soon be subjected to this method of improvement.
Agricultural Draining.
I have commenced this chapter with a description of the dikes and other hydraulic works of the Netherland engineers, because both the immediate and the remote results of such operations are more obvious and more easily measured, though certainly not more important, than those of much older and more widely diffused modes of resisting or directing the flow of waters, which have been practised from remote antiquity in the interior of all civilized countries. Draining and irrigation are habitually regarded as purely agricultural processes, having little or no relation to technical geography; but we shall find that they exert a powerful influence on soil, climate, and animal and vegetable life, and may, therefore, justly claim to be regarded as geographical elements.
Superficial draining is a necessity in all lands newly reclaimed from the forest. The face of the ground in the woods is never so regularly inclined as to permit water to flow freely over it. There are, even on the hillsides, small ridges depressions, partly belonging to the original distribution of the soil, and partly occasioned by irregularities in the growth and deposit of vegetable matter. These, in the husbandry of nature, serve as dams and reservoirs to collect a larger supply of moisture than the spongy earth can at once imbibe. Besides this, the vegetable mould is, even under the most favorable circumstances, slow in parting with the humidity it has accumulated under the protection of the woods, and the infiltration from neighboring forests contributes to keep the soil of small clearings too wet for the advantageous cultivation of artificial crops. For these reasons, surface draining must have commenced with agriculture itself, and there is probably no cultivated district, one may almost say no single field, which is not provided with artificial arrangements for facilitating the escape of superficial water, and thus carrying off moisture which, in the natural condition of the earth, would have been imbibed by the soil.
All these processes belong to the incipient civilization of the ante-historical periods, but the construction of subterranean channels for the removal of infiltrated water marks ages and countries distinguished by a great advance in agricultural theory and practice, a great accumulation of pecuniary capital and a density of population which creates a ready demand and a high price for all products of rural industry. Under draining, too, would be most advantageous in damp and cool climates, where evaporation is slow, and upon soils where the natural inclination of surface does not promote a very rapid flow of the surface-waters. All the conditions required to make this mode of rural improvement, if not absolutely necessary, at least profitable, exist in Great Britain, and it is, therefore, very natural that the wealthy and intelligent farmers of England should have carried this practice farther, and reaped a more abundant pecuniary return from it, than those of any other country.
Besides superficial and subsoil drains, there is another method of disposing of superfluous surface-water, which, however, can rarely be practised, because the necessary conditions for its employment are not of frequent occurrence. Whenever a tenacious water-holding stratum rests on a loose, gravelly bed so situated as to admit of a free discharge of water from or through it by means of the outcropping of the bed at a lower level, or of deep-lying conduits leading to distant points of discharge, superficial waters may be carried off by opening a passage for them through the impervious into the permeable stratum. Thus, according to Bischof, as early as the time of King Rene, in the first half of the fifteenth century, when subsoil drainage was scarcely known, the plain of Paluns, near Marseilles, was laid dry by boring, and Wittwer informs us that drainage is effected at Munich by conducting the superfluous water into large excavations, from which it filters through into a lower stratum of pebble and gravel lying a little above the level of the river Isar. [Foonote: Physikalische Geographie, p. 288. This method is now frequently employed in France. Details as to the processes will be found in Mangon Pratique du Drainage, pp. 78 et seqq. Draining by driving down stakes mentioned in a note in the chapter on the Woods, ante, is a process of the same nature.
In the United States, large tracts of marshy ground, and even shallow lakes of considerable extent, have been sufficiently drained not only for pasturage but for cultivation, without resort to any special measures for effecting that end. The ordinary processes of rural improvement in the vicinity, such as felling woods upon and around such grounds, and the construction of roads, the side ditches of which act as drains, over or near them, aided now and then by the removal of a fallen tree or other accidental obstruction in the beds of small streams which flow from them, often suffice to reclaim miles square of unproductive swamp and water. See notes on p. 20, and on cedar swamps, p. 208, ante.] So at Washington, in the western part of the city, which lies high above the rivers Potomac and Rock Creek, many houses are provided with dry wells for draining their cellars and foundations. These extend through hard, tenacious earth to the depth of thirty or forty feet, when they strike a stratum of gravel, through which the water readily passes off. This practice has been extensively employed at Paris, not merely for carrying off ordinary surface-water, but for the discharge of offensive and deleterious fluids from chemical and manufacturing establishments. A well of this sort received, in the winter of 1832-'33, twenty thousand gallons per day of the foul water from a starch factory, and the same process was largely used in other factories. The apprehension of injury to common and artesian wells and springs led to an investigation on this subject by Girard and Parent Duchatelet, in the latter year. The report of these gentlemen, published in the Annales des Ponts et Chaussees for 1833, second half-year, is full of curious and instructive facts respecting the position and distribution of the subterranean waters under and near Paris; but it must suffice to say that the report came to the conclusion that, in consequence of the absolute immobility of these waters, and the relatively small quantity of noxious fluid to be conveyed to them, there was no danger of the diffusion of such fluid if discharged into them. This result will not surprise those who know that, in another work, Duchatelet maintains analogous opinions as to the effect of the discharge of the city sewers into the Seine or the waters of that river. The quantity of matter delivered by them he holds to be so nearly infinitesimal, as compared with the volume of water of the river, that it cannot possibly affect it to a sensible degree, and therefore cannot render the Seine water unfit for drinking. [Foonote: Coste found, in his experiments on pisciculture, that the fermentation, which takes place in the water of the Seine in consequence of the discharge of the drains into the river, destroyed a large proportion of the eggs of fish in his breeding basins. Analysis of Seine water by Boussingault in 1855 detected a considerable quantity of ammonia.]
Meteorological Effects of Draining.