RIVERS
Many rivers are subject to a considerable elevation of the level of their waters. This is periodical or irregular in its occurrence, according to the nature of the producing cause. Casual temporary floodings, as the effect of extraordinary rains, are common to the streams of most countries, and sometimes occasion great changes of the surface, and destruction of life and property. One of the most remarkable instances of this kind in modern times, occurred on the 4th of August, 1829, in Scotland, when the Nairn, Spey, and Findhorn rose above their natural boundaries, and spread a devastating deluge over the surrounding country. The rain which produced this flood fell chiefly on the Monadhleadh Mountains, where the rivers in question have their feeders, situated between the south of Loch Ness and the group of the Cairngorums. Sir Thomas Dick Lauder, in his interesting account of this inundation, considers the westerly winds, which prevailed for some time previously, after a season of unusual heat, to have produced a gradual accumulation of vapor, somewhere north of our island; and the column being suddenly impelled by a strong north-easterly blast, it was driven toward the south-west, till arrested in its course by the lofty mountains upon which it discharged itself in torrents perfectly unexampled. The rain fell occasionally in heavy drops, but was for the most part broken by the blast into extremely minute particles, so thick that the very air itself seemed to be descending in one mass of water upon the earth. It deluged every house whose windows were exposed to the south-east. The lesser animals, the birds, and especially game of all kinds, were destroyed in great numbers, by the rain alone; and the mother partridge, with her progeny and mate, were found chilled to death amidst the drenching wet. At Huntly Lodge, according to an accurate observation, between five o’clock of the morning of the 3d of August and the same hour of the succeeding day there fell 3¾ inches of rain, or about one-sixth of our annual allowance of rain descended there in twenty-four hours. This was at a considerable distance from the mountains – the central scene of the rain – where its quantity must have been prodigiously greater, sufficient to account for the tremendous flood that followed, far exceeding in its rise, duration, and havoc, any other that ever affected the same locality. The Findhorn and Spey assumed the appearance of inland seas; and, when the former began to ebb, a fine salmon was driven ashore and captured at an elevation of fifty feet above its ordinary level. Most of the rivers of the temperate zones are subject to these irregular floodings from the same cause, especially those which take their rise in high mountain regions, the St. Lawrence being the most remarkable exception, the level of which is not affected by either rains or drought. The vast lakes from which this river issues furnish its channel with an inexhaustible supply of water, and present a surface too extensive to be sensibly elevated by any extraordinary rains. A strong westerly wind, however, will affect the level of the St. Lawrence, and occasion a rise of six feet in the waters to the eastern extremity of Lake Erie. An easterly wind also upon the Orinoco will check its current, elevate the upper part of the stream, and force its waters into the channels of its tributaries, giving them a backward flow, and causing them to be flooded; and a northerly wind will drive the Baltic up the mouths of the Oder, and raise its level for a considerable distance. In a similar manner, the Neva rises when a strong wind blows from the Gulf of Finland; and that occurrence – taking place coincidently with high water and the breaking up of the ice, would create an inundation sufficient to drown the whole population of St. Petersburg, and convert that brilliant capital, with all its sumptuous palaces, into a chaotic mass of ruins. We have the materials of this statement from M. Kohl. The Gulf of Finland runs to a point as it approaches the mouth of the Neva, where the most violent gales are always those from the west; so that the mass of waters on such occasions is always forcibly impelled toward the city. The islands forming the delta of the Neva, on which St. Petersburg stands, are extremely low and flat; and the highest point in the city is probably not more than twelve or fourteen feet above the average level of the sea. A rise of fifteen feet is therefore enough to place all St. Petersburg under water, and a rise of thirty feet is enough to drown almost every human being in the place. Hence the inhabitants of the capital are in constant danger of destruction at the period referred to, and can never be certain that the 500,000 of them may not, within the next twenty-four hours, be driven out of their houses to find, in multitudes of instances, a watery grave. This is not a chimerical danger; for, during its short continuance, the city of the Czar has experienced some formidable inundations. The only hope of this apparently doomed city is that the three circumstances may never be coincident, namely, high water, the breaking up of the ice, and a gale of wind from the west. It is nevertheless true, that the wind is very often westerly during spring, and the ice floating in the Neva and the Gulf of Finland is of a bulk amply sufficient to oppose a formidable obstacle to the egress of the water; so that it will not be surprising if St. Petersburg, after suddenly rising like a meteor from the swamps of Finland, should still more suddenly be extinguished in them.
The periodical rise of rivers is either diurnal, semi-annual, or annual, and proceeds from a variety of causes. Where streams descend immediately from mountains covered with snow, the heat of the sun melting the snow produces high water every day, the increase being the greatest in the hottest days. In Peru and Chili there are small rivers which flow only during the day, because they are fed entirely by the melting of the snow upon the summit of the Andes, which takes place only when the solar influence is in action. In Hindûstan, and some parts of Africa, rivers exist, which, though they flow night and day, are, from the accession of snow-water, the greatest by day. Those rivers also which fall into the sea have their level daily varied by the tidal wave for some distance from their mouths, the extent through which the influence of the tides is felt being modified by the breadth and shape of their channels and the force of their current. The wider and more direct the bed of a stream communicating with the ocean, and the slower its motion the farther the tide will penetrate; whereas a narrow and sinuous course, and a great velocity, offer obstructions to its progress. The tide of the Atlantic is perceived four hundred miles along the course of the Amazon, and that of the German Ocean extends about seventy miles up the Thames. Important facilities are afforded to the navigation of many rivers by this circumstance, for they are only accessible to vessels of large burden at high water. The rapid of Richelieu, on the St. Lawrence, where the river contracts, and has its course obstructed by rocks, impedes the navigation between Montreal and Quebec, except at high tide, when the water rises fifteen or eighteen feet, and the rapid entirely disappears. A semi-annual or annual rise alone distinguishes the rivers of inter-tropical regions, and of countries bordering on the torrid zone. The semi-annual rise is a feature of those rivers which drain high mountain ranges, and proceeds from the two independent causes, of the melting of the snows in spring or summer, and the great seasonal rains to which such districts are subject. The rivers which have only one annual rise are influenced by the latter cause alone, or by the two acting coincidently, and producing a grand periodical flood. The Tigris rises twice in the year – first, and most remarkably, in April, in consequence of the melting of the snows in the mountains of Armenia; and secondly, in November, through an accession from the periodical rains. The Mississippi likewise is subject to two rises in the year – one about January, occasioned by the periodical rains that fall toward the lower part of its course; but the grand flood commences in March, and continues till June, proceeding from the melting of the ice in the upper part of the continent, where the Missouri and other tributary streams have their origin. A very striking spectacle is exhibited by this river in the season of inundation. It rises from forty to fifty feet in some parts of its course, and is from thirty to a hundred miles wide, all overshaded with forest, except the interior stripe consisting of its bed. The water stands among the trees from ten to fifteen feet in height, and the appearance is exactly that of a forest rising from a lake, with its waters in rapid motion. For the protection of the cultivated lands, and to prevent their conversion into permanent swamps, an embankment, called the Levee, has been raised, which extends two hundred miles on the eastern shore of the river, and three hundred on the western. In Asia, the Ganges, Indus, and Euphrates exhibit inundations upon a similarly great scale. The Euphrates slightly increases in January, but the grand flood begins soon after the middle of March. It attains its height about the 20th of May, after which it falls rapidly till June. The decrease then proceeds gradually until the middle of November, when the stream is at its lowest. The rise of the water at Anah, above the site of