The draining of lakes, marshes, and other superficial accumulations of moisture, reduces the water surface of a country, and, of course, the evaporation from it. Lakes, too, in elevated positions, lose a part of their water by infiltration, and thereby supply other lakes, springs, and rivulets at lower levels. Hence, it is evident that the draining of such waters, if carried on upon a large scale, must affect both the humidity and the temperature of the atmosphere, and the permanent supply of water for extensive districts.[324]
_Geographical and Climatic Effects of Aqueducts, Reservoirs, and Canals._
Many processes of internal improvement, such as aqueducts for the supply of great cities, railroad cuts and embankments, and the like, divert water from its natural channels, and affect its distribution and ultimate discharge. The collecting of the waters of a considerable district into reservoirs, to be thence carried off by means of aqueducts, as, for example, in the forest of Belgrade, near Constantinople, deprives the grounds originally watered by the springs and rivulets of the necessary moisture, and reduces them to barrenness. Similar effects must have followed from the construction of the numerous aqueducts which supplied ancient Rome with such a profuse abundance of water. On the other hand, the filtration of water through the banks or walls of an aqueduct carried upon a high level across low ground, often injures the adjacent soil, and is prejudicial to the health of the neighboring population; and it has been observed in Switzerland, that fevers have been produced by the stagnation of the water in excavations from which earth had been taken to form embankments for railways.
If we consider only the influence of physical improvements on civilized life, we shall perhaps ascribe to navigable canals a higher importance, or at least a more diversified influence, than to any other works of man designed to control the waters of the earth, and to affect their distribution, They bind distant regions together by social ties, through the agency of the commerce they promote; they facilitate the transportation of military stores and engines, and of other heavy material connected with the discharge of the functions of government; they encourage industry by giving marketable value to raw material and to objects of artificial elaboration which would otherwise be worthless on account of the cost of conveyance; they supply from their surplus waters means of irrigation and of mechanical power; and, in many other ways, they contribute much to advance the prosperity and civilization of nations. Nor are they wholly without geographical importance. They sometimes drain lands by conveying off water which would otherwise stagnate on the surface, and, on the other hand, like aqueducts, they render the neighboring soil cold and moist by the percolation of water through their embankments;[325] they dam up, check, and divert the course of natural currents, and deliver them at points opposite to, or distant from, their original outlets; they often require extensive reservoirs to feed them, thus retaining through the year accumulations of water--which would otherwise run off, or evaporate in the dry season--and thereby enlarging the evaporable surface of the country; and we have already seen that they interchange the flora and the fauna of provinces widely separated by nature. All these modes of action certainly influence climate and the character of terrestrial surface, though our means of observation are not yet perfected enough to enable us to appreciate and measure their effects.