RIVER. Rivers are the result of the natural tendency of water, as of all other bodies, to obey the law of gravitation by moving downward to the lowest position it can reach. The supply of water for the formation of rivers. though apparently derived from various sources, as from rain-clouds, springs, lakes, or from the melting of snow, is really due only to atmospheric precipitation; for springs (q.v.) are merely collectionsof rain-water; lakes are collections of rain or spring water in natural hollows, and snow is merely rain in a state of congelation. The rills issuing from springs and from surface-drainage unite during their downward course with other streams, forming rivulets; these, after a fur ther course, unite to form rivers, which receiving fresh accessions in their course from tributaries (subordinate rivers or rivulets) and their feeders (the tributaries of tributaries), sweep onward through ravines, and over precipices, or crawl with almost imperceptible motion across wide, flat plains, till they reach their lowest level in ocean, sea, or lake. The path of a river is called its course; the hollow channel along which it flows, its bed; and the tract of country from which it and its subordinates draw their supplies of water, its basin, or drainage-area. The basin of is river is bounded by an elevated ridge, part of which is generally mountainous, the crest forming the water-shed; and the size of the basin, and the altitude of its water-shed, determine, eateris paribus. the volume of the river. See RAIN. The greater or less degree of uniformity in the volume of a river in the course of a year. is one of its chief physical features, and depends very much on the mode in which its supply of water is obtained. In temperate regions, where the moun tains do not reach the limit of perpetual snow, the rivers depend for their increase wholly on time rains, which, occurring frequently, and at no fixed periods, and discharg ing only comparatively small quantities 6f water at a time, preserve a moderate degree of uniformity in the volume of the rivers—a uniformity which is aided by the circum stance, that in these zones, only about one-third of the rain-fall finds its way directly oven the surface to the rivers; the remaining two-thirds sinking into the ground, and finding its way to spring-reservoirs, or gradually oozing through at a lower level in little rills which continue to flow till the saturated soil becomes drained of its surplus moisture, a process which continues for weeks, and helps greatly to maintain the volume of the river till the next rain-fall. This process, it is evident, is only possible where the temper
ature is mild, the climate moist, evaporation small, and the soil sufficiently porous: and under these circumstances, great fluctuations can only occur from long-continued and excessive rains or droughts. In the hotter tracts of the temperate zones, where little rain falls in summer, we occasionally ffnd small rivers and mountain torrents becoming completely exhausted; such is often the case in Spain, Italy, Greece, and with the Orange, one of the largest rivers of s. Africa. , In tropical and semi-tropical countries, on the other hand. the year is divisible into one dry and one wet season (see Rats); and in consequence, the rivers have also a period ieity of rise and fall, the former taking place first near the source, and, on account of • the great length of course of some of the tropical rivers, and the excessive evaporation to which they are subjected (which has necessarily most effect where the current is slow), not making itself felt in the lower part of their course till a considerable time afterward. Thus, the rise of the Nile occurs in Abyssinia in April, and is not observed at Cairo till about midsummer. The fluctuations of this river were a subject of perpetual wonder ment to the ancient civilized world, and were of course attributed to superhuman agency; but modern travel and investigation have not only laid bare the reason of this phenom enon, but discovered other instances of it, before which shrinks into insignifi cance. The maximum rise of the Nile, which is about 40 ft., floods 2,100 sq.m. of ground; while that of the Orinoco. in Guiana, which is froth 30 to 26 ft., lays 45,000 sq. m. of savannah under water; the Brahumputra at flock] covers the whole of upper Assam to a depth of 10 ft., and the mighty Amazon converts a great portion of its 600,000 sq. in. of silvas into one extensive lake. But the fluctuations in the rise of the flood-waters are surpassed by sonic of the comparatively small rivers of Australia, one of which, the Hawkesbury, has been known to rise 100 ft. above its usual level. This, however, is owing to the river=beds being hemmed in by lofty abrupt cliffs, which resist the free pas sage of a swollen stream.