Most rivers overflow the low countries which are adjacent to their banks, either at regular semitone of the year or occasionally. This takes place when the supply of water is greater than the bed of the river can contain. In this respect rivers may be divided into three classes : the first curnprelienda the rivers whose Inundations are produced by the melting of snow and ice ; the second comprehends those which are annually swollen by regular rains; and the third those which only occasionally cause inundations.
All large rivers that drain countries of which the mean winter temperature is below 30', are annually subject to great risings when the snow and ice melt. In such countries snow falls for several montium, and as only a small part of It Is dissolved, it accumulates to a great amount. As soon as the frost ceases, the snow begins to melt, and rune off by the smaller rivers, which suddenly swell and carry an unusual supply of water to the principal river, whose volume being thus Increased to three or four times its ordinary magnitude, overflows the adjacent low country. These inundations, though they generally improve the soil, are very injurious to agriculture, by destroying the growing corn, and covering extensive tracts with sand, gravel, and ether manse earthy inatter. In some rivers these inundations last only from two to four weeks; In others two or three months; and in some even five or six months. Where the inundations are long, they are less violent, and cause less damage than when they are short; in the latter case the whole mass of water suddenly deluges the country, while in the former the water rises slowly. This difference in the inundations of rivers is mainly to be attributed to the direction in which they flow. Let us take a river like the Mississippi, which flows from north to south through 17° or 13° of latitude. In winter the basin is covered with snow, and if the whole were melted in a few days, it would produce such a volume of water as would probably cover nearly half the basin. But the melting of the snow is gradual. Whilst the temperature in the northern districts is below the freezing-point, the spring has already made considerable progress in the southern districts, the snow which has there fallen has been dissolved, and the water thus produced has had the requisite time to run off and reach the sea. Thus with the progress of the sun towards the northern tropic, the line of the melting snow proceeds northward, and thus the supply of water runs off gradually, until the snow of the most northern region is dissolved. More than two months elapse between the melt ing of the snow in the northern region and the commencement of the melting in the lower part of the river. The inundations of the Missis
sippi therefore are not extensive, if the great length of that river and of its affluents be considered, but they last from three to four months. A considerable part of the delta of that river is indeed under water for six months, but this must be ascribed to the tract of elevated ground which extends not far from the sea between the Atchafalaya and the La Fourche, and prevents the enormous mass of water which collects in the lowlands near tho first-mentioned branch from running oft' sooner. When a river situated in the northern hemisphere flows from south to north, the melting of the snow of course commences near the upper branches of the river, and proceeds northward. In this case the volume of water which collects at a certain period in the lower course, where the lowlands are generally most extensive, is much greater, and the inundations are much more extensive and attended with more mischief. But atilt they cannot be compared with inundations of those rivers which run from east to west or from west to east. In countries which are drained by such rivers, the whole Imes of snow is dissolved in a few days, especially when a thaw is accompanied by rain, and all the waters thus produced pass through the principal channel in the course of a week or two. In such rivers the volume of water during the inundations is three or four times larger than it is in the middle of the summer or the beginning of autumn, and the inun dations spread to a great distance, and frequently cause great loss of property, and sometimes also of life, especially when the winter has been unusually long and the falls of snow very heavy. [Nrestes, in 0E00. Div.] But the river St. Lawrence forms an exception here also. As its general course is from west to east, one would suppose that a largo extent of country within its basin would be annually subject to inundation, but this discs not appear to be the case in any part of its course. If any portion of it is awollen by the melting of the snow within the basin, the river soon enters one of tho lakes through which Its course lies, and thus the addition of a compa ratively small volume is not sufficient to raise the surface of the lake to any large amount. Thus the same cause which prevents its filling up the wide estuary, prevents the river from overflowing the adjacent country. A diurnal rise and fail characterises the rivers of Switzerland and those of the western Himalaya, where a powerful sun melts the glaciers by day, and the head-streams are frozen by night.