RIVER 



737 



margin. These are increased in height by each 

 successive flood, and, the river-bed being simultane- 

 ously silted up, broad muddy rivers like the Missis- 

 sippi, Po, and Hoang-ho come in time to flow along 

 the top of a gently sloping natural embankment, 

 the sides of which are termed levees in Louisiana. 

 Professor Lappareut, calculating from Dr Murray's 

 data regarding the amount of sediment carried 

 <lown by rivers, finds that they would suffice to 

 wear the entire surface of the land down to sea- 

 level in four million years. The entrances of rivers 

 into lakes or the sea are usually marked by great 

 banks of deposit (see DELTA), or by bars of gravel 

 or sand. In some cases, however, such as the 

 River Plate, the Thames, and Tay, the mixture of 

 river and sea water is gradual, and the sandbanks 

 are spread over a very large area, but not built 

 up into a delta at any one place. Professor 

 Osborne Reynolds has shown, by a remarkable 

 series of experiments, that the form of the sand- 

 banks is due to the outline of the coasts of the 

 estuary and to the tides. In a. few instances, such 

 as the Forth, rivers enter deep arms of the sea in 

 which neither banks nor ban* are formed. The 

 Congo sweeps directly into the ocean, throwing 

 down great banks of deposit along the continental 

 slope to right and left, but leaving a deep canon- 

 like gully for the bed of the stream itself ; a similar 

 condition occurs where the Rhone enters the Lake 

 of Geneva. 



The ultimate source of all rivers is the condensa- 

 tion of water-vapour from the atmosphere in the 

 form of rain, snow, and even dew. If the land 

 were composed of impermeable rocks all the rain- 

 water not lost by evaporation would run off 

 directly over the surface, and rivers would only 

 flow during and immediately after showers. A 

 large part of the rainfall, however, soaks into 

 the soil, which retains it as in a sponge, especi- 

 ally if the land be marshy, and allows it to flow 

 off gradually as superficial springs. Some also 

 percolates deeply into the rocks, ultimately emerg- 

 ing as deep-seated springs at a great distance. 

 The indirect and permanent supply of water to 

 rivers by springs and by the outflow of lakes is 

 independent of local rainfall at the time, and serves 

 to maintain the volume of the river at a certain 

 minimum during the dry seasons. When a river 

 flows toward a region of great evaporation and 

 small rainfall, such as exists in the interior of i-aoh 

 of the great continents, eva]K>ration removes more 

 water than is supplied bv the remote tributaries, 

 and the stream may fail to fill the hollow it 

 enters, and therefore cannot overflow into the sea. 

 This is the case with the Oxus entering the Aral 

 Sea, and the Volga entering the Caspian. It may 

 be that evaporation is so far in excess of contribu- 

 tions from distant rainfall or snow-melting that 

 the river dries up as it flows, and its last remnant 

 is absorbed in the desert sand. This is the fate of 

 the Murghab, the Heri-rud, the Zerafshan, and 

 many other rivers of central Asia. 



Contrasted with these cases are those in which 

 the periodical or occasional increments of direct 

 inflow increase the volume so much as to cause a 

 great rise of level or even extensive inundations. 

 The annnal inundations of the Nile are due to 

 the monsoon rainfall on the great mountains of 

 Abyssinia, which increases the discharge at Assouan 

 to fifteen times the amount of the river at its lowest. 

 The Orinoco is another instance of seasonal rains pro- 

 ducing tremendous inundations, over 40,000 square 

 miles of the Llanos being said to be laid under 

 water by the summer rains. The Amazon is an 

 instance of a river which is always more or less 

 in flood as the various tributaries attain their 

 greatest height nt different seasons. In June, 

 when the highest level occurs in the main river, 

 411 



20 or 30 miles of forest on each side of its banks 

 are laid under water for hundreds of miles. The 

 Ganges overflows its banks in summer when the 

 monsoon rainfall is reinforced by the melting of 

 snow on the Himalayas. Where the seasons of 

 maximum rainfall and of snow-melting are different, 

 as in the Mississippi, the Tigris, and Euphrates, 

 there are two regular floods in the year. 



The danger of flooded rivers arises from the 

 suddenness with which the water rises and overflows 

 narrow valleys or even plains. Frightful devasta- 

 tion follows the bursting of glacier obstruction 

 lakes in mountain-valleys (see LAKE). The 

 great rivers of Siberia remain frozen at their 

 mouths long after the ice and snow have been 

 melted in the interior, and broad strips on their 

 margins are necessarily laid under water by the 

 natural outflow being stopped. The most serious 

 floods in the Danube and Theiss have resulted from 

 the constriction of the channel at the Iron Gates, 

 which prevents the flood water from passing away 

 as rapidly as it conies down ; the current of tiie 

 Theiss is sometimes reversed for many miles. The 

 widening of the channel has been repeatedly at- 

 tempted as a remedy by increasing the outlet ; and 

 an elaborate system for regulating the river here, 

 to be completed in 1895, was begun in 1890. 

 In other cases, such as the tributaries of the Loire, 

 and the southern rivers of the Argentine Republic, 

 the melting snow swells the torrential track, 

 and, on account of the abrupt change of level 

 and the flatness of the plain, the lower part 

 of the rivers cannot carry away the immense 

 volume of water rapidly enough, and floods result. 

 In some instances torrential rivers have been 

 successfully diverted into lakes, which regulate 

 their outflow, preventing either dangerously high 

 or extremely low water. Great rivers which have 

 embanked their coarse above the level of the plnin 

 are the most dangerous of all when flooded. The 

 damage caused by the bursting of the levees on the 

 lower Mississippi necessitates a great expenditure 

 in strengthening the embankments, and the most 

 disastrous inundations recorded in history have 

 followed the bursting of the banks of the Hoang-ho 

 (q.v.) and its consequent changes of course. 



River-water is spoken of as fresh, but it always 

 contains a certain amount of solid matter in solu- 

 tion, varying from two grains in the gallon or less 

 in rivers draining hard crystalline rocks to fifty 

 grains in the gallon or more in limestone districts. 

 The nature of the salts dissolved naturally differs 

 according to the geological character of the country 

 traversed, but all samples of river- water differ from 

 sea- water in containing a much smaller proportion 

 of chlorides, and a very much larger proportion of 

 cai-lnmates and of silica. 



The temperature of rivers as a rule follows that 

 of the air, but is subject to variations on account 

 of the effect of rain. During sudden floods in 

 summer the temperature of the water may fall 

 many degrees in a few hours as the melted snow 

 or hail precipitated on the lofty mountains is 

 carried toward the sea. 



The great rivers of Europe and Asia, such as the 

 Rhine, Danube, Volga, Indus, Ganges, Brahma- 

 putra, Yang-tsze-kiang, afford access to the sea to 

 enormous populations. The Amazon, with its plain 

 track extending for nearly 3000 miles, is in many 

 ways less like a river than a fresh inland sea ; but 

 the Mississippi and St Lawrence, although less 

 extensive, are of greater value for carrying sea 

 traffic to inland places. In their torrential and 

 upper valley tracks rivers are of use chiefly for 

 transporting timber and driving machinery. It 

 is interesting to note that in Switzerland, Norway, 

 and Sweden, where there is no coal, there exist 

 exceptional facilities for the use of water-power on 



