RIVERS are the flowing waters which bring to the sea, and some times to a lake, the waters which are collected within a certain portion of the earth's surface. The country which is thus drained by a river is called its basin, as the river rune in the lowest part of it, and the country rises on all sides with greater or less steepness, in the fashion of the sides of a basin. The margin of such a basin generally lies contiguous to the basin of another river, and thus constitutes the boundary-line of the two basins. From these margins the waters descend on both sides towards their respective basins, which are separated by them, and hence the whole line of these margins is called a watershed, properly the water-parting. [WavErtansaa.] The basins of rivers vary greatly in size. A brook is the name com monly given to rivers of the smallest description. When such a river rises near the sea or near a large river, into one of which it flows after a short course, it consequently drains a very small surface. If the waters should be increased by those of another brook, the name of brook is changed for that of rindet. The basin of a rivulet is therefore more complicated than that of a brook. One or more brooks descend from the margin of the basin, and by uniting their waters with those of the rivulet, increase its volume. When several rivulets unite and so produce a considerable volume of running water, this watercourse takes the name of rirer. But all such rivers do not reach the sea, or even a lake; most of them join other rivers, and thus a large river is produced. This last-mentioned large river is called the principal rirer, and those which increase its waters are called, with respect to it, offends or tributaries, and sometimes laden or branches. In Syria and Arabia and north-eastern Africa, the winter-brooks, or small streams which flow only in the winter or the rainy season, are called wadies, a term frequently met with in the works of travellers in those countries.
The first waters of a river are generally derived from a spring, which breaks out at the foot of a declivity, or on the side of some hill or mountain; and sometimes from a swamp or a lake. This is called the rower of a river. From this source the river descends through the lowest part of its basin until it terminates its course in the sea, a lake, or another river, and this termination Is called the weak of the river. The cavity in which the running water flows is called the led of the river, and the solid land which bounds this bed is called its banks. The Arad-streams of a great river are those, proceeding from their sources, which by their union or confluence produce the first principal stream, with which another principal stream may be afterwards confluent, or which may receive tributaries.
Must large rivers have their origin in very elevated mountains, or on high tablelands, In descending from which a great difference with respect to the rapidity of their:course, and the nature of the country through which they flow, is observed. Aecorelinglv geographer. divide the whole of the course of such riven into three aivisiorer, the upper, middle, and tourer course.
The upper course of such a river lies within a mountain region, and its source is consequently at a great elevation above the sea. The waters run with greater or less velocity, according to the greater or leas-extent of the mountain-region, and the greater or lees rapidity with which the whole region declines towards the country to which the whole course of the river is directed. When the elevation of the monntain.region decreases with great rapidity, the current of the river Is extremely rapid, and presents a quick succession of cataracta and rapids. The force of the current its so great that pieces of rock of con siderable size, which are frequently detached from the overhanging masses, cannot resist it, and are carried down by the cument, until they reach a point where the rapidity of thedercent begins to diminish. The mountains which constitute the banks of the river often rise several thousand feet above it, and their Wees are united by slopes forming an angle, over which the water rune on bare rock', without the least covering of earth. Thus the river does not flow in a valley, but In a cleft or ravine, which cuts deep Into the mountain masses.
Sometimes there is space enough for a path between the river and the mountains, but in many places this space is only obtained by artificial means, as by cutting away a projecting portion of the rock, or by making a tunnel through it. Where either of these means cannot be applies!, the path is continued over the bed of the river by a wooden bridge of greater or less extent, until a place is retched which oars sufficient space for a path on the sides of the rocks. The course of the river is generally in a straight line, but sometimes it makes short, and abrupt bends which form acute angles. In the last-mentioned case it is, almost without exception, obaervel that the mountains which inclose the river have on one side a projection, and on the other a recension, which correspond so exactly that if It were possible to unite the two mountains, the projecting would exactly fit into the receding part. This peculiarity in the formation of the ravines of mountain streams was observed by the Spaniards in the Andes of South America, who called them qacbradas, or broken ; by this term tacitly assuming that the phenomenon had been caused by a violent disruption of the mountains. This description of mountain-streams and their ravines applies particularly to those of the western Alps along their southern declivity, to those rivers which constitute the upper branches of the rivers Doria and Sesia, to the valleys of Anzasca and Vedro on the southern side of the great road of the Simplon, and to the still larger valley of Aosta, through which the road leads from Switzerland to Italy over the mountainpass of the St. Bernard. The military road of the Romans was made through the Val d'Aosta; but in these parts it was only practicable for beasts of burden. Such deep ravines not only occur between the gigantic elevations of a high range, but like wise in the elevated table-lands. According to Dr. Beke, the rivers of Abyssinia, nearly all effluents of the Nile, or of its great tributaries, in the early part of their course, flow over the level surface of the table land, being little more than muddy brooks, nearly Without water in the dry season, but overflowing their banks so as almost entirely to inundate the plain country during the rains. They escape from the plateau by precipitous falls of SO or 100 feet or more, into fissures in the rocky surface, at first only a few yards in breadth, but gradually opening to the extent of several miles, down which the stream hurries in a succession of falls and rapids, so as to descend several thousand feet in the course of a few miles. In these deep-cut valleys, the rivers thus soon roach a depressiou of 3000 or 4000 feet below the general level of the table-land. The larger valleys are of considerable width ; that of the Abed, or Nile of Bruce, the upper course of the Bahr.el Azrek, or Illue River [NILE, in GEOG. Div.]. for example, is at least twenty-five miles between the extreme' points where it breaks away from the table-land on either side. " And as the country is wild and irregular," remarks Dr. Bel-e, "it is easy for a traveller, who has not taken a comprehensive view of the entire region, and who finds himself shut up in such a valley with a mass of broken country surrounding him, to believe that in ascending from the river he is crossing a mountain chain ; whereas, in reality, he has merely reached the con tinuation of the table-land which he had left on the opposite side of the river." The great rivers of India present corresponding pheno mena to those of the East African table-land, but on a more immense scale. [Ptarsts.] Humboldt observes that the plain of Quito, which is nearly 10,000 feet above the sea, is intersected by ravines, which in some places sink so deep that their bottoms are hardly more than 2000 feet above the sea-level ; and lie adds that some of them are so narrow as not to contain the smallest cultivable space. Similar ravines intersect the table-lands of Guatemala and Anahuac, where they are called naiades:, a term, however, which does not imply the existence of rivers in them or relate to aat circumstance, but merely to the abrupt descent of land.