The increase from the melting of snow in summer most frequently occurs during the rainy season, so that it is somewhat difficult to determine, with anything like accuracy, the share of each in producing the floods; but in some rivers, as the Ganges and Brahma /Rant, the increase from this cause is distinctly observable, as it occurs sonic time after the rains have commenced, while in the case of the Indus it is the principal source of flood. When the increase from melted snow does not occur during the rainy season, we have the phenomenon of flooding occurring twice a year, as in the ease of the Tigris, Euphrates, Mississippi, and others; but in most of these cases the grand flood is that due to the melting of the snow or ice about the source. In illustration of the enormous vari ation in the volume of rivers subject to periodical rise and fall, we shall give a few instances in which the minimum and maximum delivery per second have been ascer tained: The advantages of this periodical flooding in bringing down abundance of rich fertile silt—the Nile bringing down, it is said, no less than 140 millions of tons, and the Irra wady 110 millions of tons annually—are too well known to need exposition here. Islands are thus frequently formed, especially at a river's mouth (see DELTA). Permanent and capacious lakes in a river's course have a modifying effect, owing to their acting as reser voirs, as is seen in the St. Lawrence; while the Red river (N.) and others in the same tract, inundate the districts surrounding their banks for miles. In tropical countries, owing to the powerful action of the sun, all rivers whose source is in the regions of per petual snow, experience a daily augmentation of their volume; while mine in Peru and Chili,' being fed only by snow-water, are dried up regularly during the night.
The course of a river is necessarily the line of lowest level from its starting-point, and as most rivers have their sources high up a mountain slope, the velocity of their current is much greater at the commencement. The courses of rivers seem to he partially regu lated by geological conditions of the country, as in the case of the San Francisco of Bra zil, which forms with the niost perfect accuracy the boundary-line between the granitic and the tertiary and alluvial formations in that conittry; aud many instances are known of rivers changing their course from the action of earthquakes, as well as from the silting up of the old bed. The inclination of a river's course is also connected with the geologi cal character of the country; in primary and transition formations. the streams are bold and rapid, with deep channels, frequent waterfalls and rapids, and pure waters, while secondary and alluvial districts present slow and powerful currents, sloping banks, wind ing courses, and tinted waters: the incline of a river is, however, in general very gentle —the average inclination of the Amazon throughout its whole course being estimated at little more than 6 in. per m., that of the lower Nile less than 7 in., and of the lower Gau ges about 4 in. per mile. The average slope of the Mississippi throughout its whole length is more than 17 in. per m., while the Rhone is, with the exception of some much
smaller rivers and torrents, the Most rapid river in the world, its fall from Geneva to Lyons being SO in. per tn., and 32 in. from Lyons to its mouth.
The velocity of rivers does not depend wholly on their slope; much is owing to their depth and volume (the latter hieing fully proved by the fact that the beds of many rivers remain unaltered in size and slope after their streams have received considerable acces sions, owing to the greater rapidity with which the water runs off); while bends in the course. jutting peaks of rock or other obstacles, whether at the sides or bottom, and even the friction of the aqueous particles. which, though slight, is produetive of perceptible effect, arc retarding agencies, In consequence, the water of a river flows with different velocities at different parts of its bed; it mores slower at the bottom than at the surface, and at the sides than the middle. The line of quickest velocity is it line drawn along the center of the current, and in cases where this line is free front sudden bends or sharp turns, it also represents the deepest part of the channel. The average velocity of a river may be estimated approximately by finding the surface-velocity in the center of the cur rent by means of a boat which swims just below the surface, and taking four-fifths of this quantity as a mean. If the mean velocity in feet per minute he multiplied by thearea of the transverse section of the stream in sq. feet, the protioict is the amount of water discharged in,cubic feet per minute. According to sir Charles LyeII, a velocity of 40 ft. per minute will sweep along coarse sand; one of ft. fine gravel; one of 120 ft. rounded pebbles; one of 180 ft. (a little more than 2 m. per hour), ,angular stones the size of an egg. The rate of its progression, and a general description of the erosive action of rivers, will be found under WATERFALLS. " Rivers are the irrigators of the earth's surface, adding alike to the beauty of the landscape and the fertility of the soil; they carry off impurities and every sort of waste debris; and when of sufficient volume, they form the most available of all channels of communication with the interior of continents . . . . They have ever been things of vitality and beauty to the poet, silent monitors to the moralist, and agents of comfort and civilization to all mankind." By far the portion of them find their way to the ocean, either directly or by means of semilacus trine seas; but others, as the Volga, Sir-Daria (Jaxartes), Amu-Daria (Oxus), and Kur (Araxes), pour their waters into inland seas; while many in the interior of Asia and Africa—as the Murghab in-Turkestan, and the Gir in the s. of Morocco—" lose them selves in the sands," partly, doubtless, owing to the porous nature of their bed, but much more to the excessive evaporation which goes on in those regions. The following are a few of the chief rivers in each continent, with the lengths of their courses in English statute m., and their drainagcareas in English geographical sq.m. (the Thames is given as a standard of compariSon):