736 



RIVER 



gbuw winks, factories for steam-engines and other 

 machinery, ami iron ami steel factories. 



Kivcr. \Vater falling on the land in the form 

 of rain, or ri-ulting from melting Know, or rising to 

 the surface in -pnng-. Mows over tin- surface to a 

 lower level. YA'heie two slope- of land dip together 

 the surface drainage collects to form a stream, and 

 when evaporation U not very rapid several such 

 streams ultimately unite and the volume of water 

 they carry Hows to the sea or to a salt lake. Small 

 streams are termed runnels, rivulet.", rills, brooks, 

 becks, or hums ; large streams are termed rivers, 

 but tin 1 word has no precise reference to the magni- 

 tude uf the stream to which it is applied. Dr 

 Johnson gives as definitions : ' Brook, a running 

 water les- I lian a river ; ' and ' River, a land current 

 of water bigger than a l>rook,' and thin fairly illus- 

 trates the use of the winds as popularly applied. 



The beginning of a stream whether brook or 

 river is called ii- source, and may be a spring 

 issuing fiinti underground, a lake or marsh in which 

 rainfall accumulates, melting snow, or simply the 

 gathering tricklings from falling rain. The path 

 of a stream is its course, and is the line of lowe- 1 

 level from the source to the end, which if occurring 

 in a lake or the sea is termed its mouth. The con- 

 nected 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 forms its basin or catchment area, and 

 the name watershed is also sometimes erroneously 

 applied to it. The names watershed, water- 



Jiarting, and divide are used to de-Lnate the 

 Ntundary line separating adjacent basins. A 

 watershed is always the meeting-place of the 

 highest part of divergent slopes, and from the 

 characteristic form of continents the main water- 

 shed of a continent is almost always the crest of a 

 range of mountains. In many coses, however, the 

 diverging slupes meet in a low plain the summit of 

 which may be occupied by a great marsh whence 

 rivers creep away in opposite directions. The 

 lia-ins of all the rivers draining into the same 

 ocean are culled collectively the drainage area of 

 that ocean. The main river to which the others 

 are said to be tributary gives its mime to the whole 

 river-system. It is often dillicult to decide which 

 of several converging streams is entitled to carry 

 the name of the main river to its source. Some 

 geographers give this distinction to the longest, 

 ntlteis to that with the highest source, and others 

 to that with the most direct coui-c. This diversity 

 of opinion is increased when the name of a river 

 leaving a large lake is given to one of several 

 nearly equal streams which enter it. Hence it is 

 that dilleienl computers disagree as to the length 

 nf rivers. The course of a typical river has been 

 divided into three part-, although these are not 

 represented in all cases. The torrential or mountain 

 track is the steepest, its gradient usually exceeding 

 SO feet in a mile, ami the velocity of its current 

 tieing 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 jitain track 

 nearest the month of a river has a gradient of only 

 11 few inches in a mile. Rivers such as the Amazon, 

 Mi i--ippi, (langes, Volga, and the long rivers of 

 Siberia, in which the plain truck is of very great 

 length, are the most valuable for navigation, the 

 limit of easy navigability being a gradient of altoni 

 1 foot in a mile. 



The velocity of n river is proportional to tin- 

 slope of the lied, but it also bears a relation to the 

 depth of the channel and the volume of water 

 Mowing in it. On accountof friction on the liottiim 

 and sides of the channel retarding the stream, the 

 water flows fastest on the surface and in the 

 middle. The carrying power of a river for suspended 



solid particle- and for stones and gravel pushed 

 along the lied depend- mi the velocity alone. The 

 following table shows how rapidly the carrying 

 power falls oil' as the velocity diminishes. 



0170 mile per hour will Jurt begin to work on line clay 



0-340 lift fine und. 



0-464 ii n lid Mini u cacne u Unwed. 



0-flBt n M iwrrp along fine gravel. 



1-364 it n roll along rounded pebble* 1 Inch In 



diaui' 

 S-O4S 99* tweep along slippery angular lUme* 



aa large u 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. Tin- sediment 

 and stones carried along are powerful erosive 

 agents in the torrential and valley tracts, and the 

 character of the valleys or gorges produced depends 

 largely on the geological structure of the region. 

 The course of a river is frequently determined by 

 lines of faults, but perhaps more often it appears to 

 be independent of the nature of the strata. Some 

 great rivers, notably the Volga, pies- again-t the 

 right bank, cutting it into a steep dill, while the 

 left bank is left as a very gentle slope. This is 

 explained by the directive influence of the earths 

 rotation (see EARTH, Vol. IV. p. 165). 



Rivers are of very great importance as agents 

 of change in dynamic geology, the form of 

 valley they excavate being determined partly by 

 the nature of the rocks, partly by the climate. In 

 rainless or arid regions steep- walled Cations (q.v.) 

 are cut to a great depth across high plateau-- ; 

 in rainy regions suliaerial denudation leads to the 

 formation of wide valleys of much gentler slopes. 

 Bars of more durable rook crossing the course of a 

 stream lead to the formation of Waterfalls (q.v.) or 

 rapids from the rapid erosion of the softer strata 

 balow. The river auove the obstruction is reduced 

 to what is termed the base-level of erosion ; the 

 velocity of the current is checked, and wide alluvial 

 deposits are laid down on either side. In course of 

 time the liar of hard rock is completely cut through 

 by a gorge, and the gradient of the stream is ulti- 

 mately rendered uniform. In this way the common 

 features of gorge and meadow are produced again 

 and again along the course of a stream. 'I ho 

 deposits of alluvium form terraces along the valley 

 track of a river, and as the stream cuts its channel 

 deeper they are left at various heights as monu- 

 ments of its erosive power. When u river is fail I v 

 established in its valley it is, geologically speaking, 

 a more permanent feature than lakes or mountains. 

 Upheaval, which acts very slowly, may even elevate 

 a range of mountains across its course, yet all the 

 while the river, cutting its way downward, re- 

 mains at the same absolute level. The I'intah 

 Mountains, as they were upheaved, were divided 

 in this way by the Green River, the chief tribu- 

 tary of the Colorado. In limestone regions the 

 solvent power of river-water on carbonate of 

 lime leads to the formation of Caves (q.v.) 

 and underground rivers, which as a rule emerge 

 from their subterranean channels on lower ground. 

 Sometimes they do not reappear on land. Inn dis 

 charge their fresh water through openings in the 

 lied of the sea. Such submarine ii\cr cut lances 

 are not uncommon along the shores of the Adriatic, 

 of)' the coast of Florida, and in other ealcarcou- 

 regions. When a river advances along a nearly 

 le\el plain toward the sea its carrying power falls 

 oil; gravel, sand, and finally mud are deposited 

 on its margin, and the stream pursues a peculiar 

 winding course. During a Mood the swift and 

 muddy stieam rises, overflows ita banks, and widens 

 out on the level land. The current is at once 

 checked and a long bar of deposit forms along cadi 



