RIVER. Water falling on the land in the form of rain, or resulting from melting snow, or rising to the surface in springs, flows over the surface to a lower level. Where two slopes of land dip together the surface drainage col lects to form a stream, and when evap oration is not very rapid several such streams ultimately unite and the volume of water they carry flows to the sea or to a salt lake. Small streams are termed runnels, rivulets, rills, brooks, becks, or burns; large streams are termed rivers, but the word has no precise reference to the magnitude of the stream to which it is applied.
The beginning of a stream—whether brook or river—is called its source, and may be a spring issuing from under ground, a lake or marsh in which rain fall accumulates, melting snow, or sim ply the gathering tricklings from falling rain. The path of a stream is its course, and is the line of lowest level from the source to the end, which if occurring in a lake or the sea is termed its mouth. The connected streams which unite in one river form a river system. The series of convergent slopes down which a river system flows—the land which it drains—forming its basin or catchment area, and the name watershed is also sometimes erroneously applied to it. The names watershed, waterparting, and di vide are used to designate the boundary line separating adjacent basins. A wa tershed is always the meeting-place of the highest part of divergent slopes, and from the characteristic form of conti nents the main watershed of a continent is almost always the crest of a range of mountains. In many cases, however, the diverging slopes meet in a low plain the summit of which may be occupied by a great marsh whence rivers creep away in opposite directions. The basins of all the rivers draining into the same ocean are called collectively the drainage area of that ocean. The main river to which the others are said to be tributary gives its name to the whole river system. It is often difficult to decide which of sev eral converging streams is entitled to carry the name of the main river to its source. Some geographers give this dis tinction to the longest, others to that with the highest source, and others to that with the most direct course. The course of a typical river has been di vided into three parts, though these are not represented in all cases. The tor
rential or mountain track is the steepest, its gradient usually exceeding 50 feet in a mile, and the velocity of its current being very great. The valley or middle track has a gradient which is rarely greater than 10 feet and often less than 2 feet in a mile. The plain track near est the mouth of a river has a gradient of only a few inches in a mile. Rivers such as the Amazon, Mississippi, Gan ges, Volga, and the long rivers of Si beria, in which the plain track is of very great length, are the most valuable for navigation, the limit of easy navigabil ity being a gradient of about 1 foot in a mile.
The velocity of a river is proportional to the slope of the bed, but it also bears a relation to the depth of the channel and the volume of water flowing in it. On account of friction on the bottom and sides of the channel retarding the stream, the water flows fastest on the surface and in the middle. The carry ing power of a river for suspended solid particles and for stones and gravel pushed along the bed depends on the velocity alone. The following table shows how rapidly the carrying power falls off as the velocity diminishes. , 0.170 mile per hour will just begin to work on fine clay.
0.340 " " " lift fine sand.
0.454 " " " lift sand as coarse as linseed.
0.682 " " " sweep along fine gravel.
1.364 miles " " roll along rounded pebbles one inch in diameter.
2.045 " " " sweep along slip pery angular stones as large as an egg.
Rivers in flood, even in the plain track, sometimes attain a velocity of over 5 miles an hour, and torrents may even flow as fast as 20 miles an hour. The course of a river is gradually carved out and shaped by the flow of the water. The sediment and stones carried along are powerful erosive agents in the tor rential and valley tracks, and the char acter of the valleys or gorges produced depends largely on the geological struc ture of the region. The course of a river is frequently determined by lines of faults, but perhaps more often it ap pears to be independent of the nature of the strata. Some great rivers, nota bly the Volga, press against the right bank, cutting it into a steep cliff, while the left bank is left as a very gentle slope. This is explained by the directive influence of the earth's rotation.