117 



RIVERS. 



RIVERS. 



118 



collects and keeps suspended in its course from one lake to the other 

 is deposited in these lakes. Thus this large river brings no debris 

 and earthy matter, or very little, to its wide (estuary, which can- 

 not therefore be changed into a delta by the accumulation of such 

 matter. 



Most large rivers, as already observed, admit of this division of 

 their course into three parts, an upper, middle, and lower course; 

 but the exceptions are far from being rare. It sometimes happens 

 that the characteristic features by which the middle course is distin- 

 guished, occur in the upper course. This takes place when a river 

 originates on an elevated table-land, and traverses a considerable part 

 of it. Thus the Indus, the Sutlej (an affluent of the Indus), and the 

 Sampoo, rise on the elevated table-land of Tibet, and drain a portion of 

 it : in this part of their course they resemble the middle course 

 of the Khine or Danube. But where they leave the plain and enter 

 the mountain-region of the Himalaya, they resemble the moun- 

 tain-streams of the Alps, except as to the volume of water. When the 

 Indus and the Sutlej have descended into -the plains of the Punjab, 

 they assume the character of the lower Rhine and lower Danube. The 

 Sampno, after leaving the mountain-region, traverses a hilly tract of 

 neat extent, the valley of Asam, before it enters the alluvial plain of 

 Bengal. There are other riven), in which only the characteristic 



-i of the middle and lower course can be recognised : the number 

 rable, and some of them are of the first magnitude. 

 ThiM the V..LM and Mi--i.-i|.|.i. neither of which rises in a mountain- 

 region, but in a hilly tract, in the greater part of their course present 

 the characteristics of the middle course of the Rhine and Danube, 



.vards their mouths they traverse a Urge plain. The number 



n whose whole course lies through a hilly or undulating country 

 is still greater, as is th can with nearly all the riven of England and 

 Southern Scotland. There are also rivers which in their whole course 

 traverse a mountain region, but they are all small ; such are some of 

 the rivers in North Scotland and in Sweden, and nearly all the rivers 

 of Norway, and those on the west coast of South America. 



The number of rivers which do not reach the ocean u not great, if 

 we except those which fall into the Caspian Sea and into the lake of 

 Aral The other rivers without an outlet always terminate their 

 course in a lake. It was formerly supposed that the water of some of 

 them was absorbed by a dry soil, and that they were lost in the sand ; 

 and this opinion still prevails as to some rivers which descend from 

 Mount Atlas southward to the Sahara. But the point remains doubtful. 



- other rivers which have no communication with the sea, some 

 few traverse elevated table-lands, consisting of plains surrounded by 

 nous mountain-ranges, through which the waters cannot find an 

 outlet, and consequently collect in the lower part of the plains, and 

 form lakes large enough to part with all their surplus water by evapo- 

 ration. Such rivers occur in the valley of Tenochtitlan in Mexico. 

 The most remarkable is the Desaguadero, in the valley of Titicaca in 

 Bolivia, which runs about 300 miles, and is luet in a lake or in swamps. 

 Tli- llyarkan or Yerkan, in Chinese Turkistan, is still larger, but its 

 character is imperfectly known. Another kind of such lakes occurs in 

 the plains of Mexico anil of South America, and almost exclusively in 

 those parts which have no rain or Terr little. On the table-land of 

 Mexico the greater number of rivers between 24* and 30 N. Lit. 

 terminate in lakes ; and in the states which compose the Argentine 

 vers of this kind are numerous between 28* and 34* S. lat., 

 west of '.I W. long. As very little rain falls in some of these coun- 

 tries, and in at all, the rivers are supplied with water by 

 the rains which fall at certain seasons on the mountains in which they 

 originate, and by the springs which exist there. But as the supply of 



>< very moderate, it does not give force sufficient to the currents 



to carry them through those extensive tract* which separate them 



sea. It is remarkable that some of these rivers and all the 



lakes in which they terminate, are salt in South America ; and it ix 



; ' that this u also the case with most of those on the Mexican 



Most riven overflow the low countries which are adjacent to their 

 banks, either at regular season* of the year or occasionally. This takes 

 place when the supply of water is greater than the bed of the river can 

 contain. In thw respect rivers may be divided into three clauses : the 

 first comprehends the rivers whoee inundation* an produced by the 

 melting of snow and ice ; the second comprehends those which are 

 annually swollen by regular rains ; and the third those which only 



:*rge rivers that drain countries of which the mean winter 

 temperature i* below 30*, are am -t. to great risings when 



the snow and ice melt. In such countries snow falls for several 

 Maths, and as only a small part of it is dissolved, it accumulates to a 

 (treat amount. As soon as the frost ceases, the snow begins to melt, 

 and runs off by the smaller rivers, which suddenly swell and carry an 

 unusual supply of water t" the principal river, whose volume being 

 thus increased to three or four times its ordinary magnitude, overflows 

 the adjacent low country. These inundations, though they generally 



e the soil, are very injurious to agriculture, by destroying the 

 growing corn, and covering extensive tracts with sand, gravel, and 

 other ooarsc earthy matter. In some rivers these inundations last 

 only from two to four weeks ; in others two or three months ; and in 

 some even five or six months. Where the inundations are long, they 



are less violent, and cause less damage than when they are short ; in 

 the latter case the whole mass of water suddenly deluges the country, 

 while in the former the water rises slowly. This difference in the 

 inundations of rivers is mainly to be attributed to the direction in 

 which they flow. Let us take a river like the Mississippi, which flows 

 from north to south through 17" or 18 of latitude. In winter the 

 basin is covered with snow, and if the whole were melted in a few 

 days, it would produce such a volume of water as would probably cover 

 nearly half the basin. But the melting of the snow is gradual. Whilst 

 the temperature in the northern districts is below the freezing-point, 

 the spring has already made considerable progress in the southern 

 districts, the snow which has there fallen has been dissolved, and the 

 water thus produced has had the requisite time to run off and reach 

 the sea. Thus with the progress of the sun towards the northern 

 tropic, the line of the melting snow proceeds northward, and thus the 

 supply of water runs off gradually, until the snow of the most northern 

 region is dissolved. More than two months elapse between the melt- 

 ing of the snow in the northern region and the commencement of the 

 melting in the lower part of the river. The inundations of the Missis- 

 sippi therefore are not extensive, if the great length of that river and 

 of its affluents be considered, but they last from three to four months. 

 A considerable part of the delta of that river is indeed under water for 

 six months, but this must be ascribed to the tract of elevated ground 

 which extends not far from the sea between the Atchafalaya and the 

 La Fourche, and prevents the enormous mass of water which collects 

 in the lowlands near the first-mentioned branch from running off 

 sooner. When a river situated in the northern hemisphere flows from 

 south to north, the melting of the snow of course commences near the 

 upper branches of the river, and proceeds northward. In this case the 

 volume of water which collects at a certain period in the lower course, 

 where the lowlands are generally most extensive, is much greater, and 

 the inundations are much more extensive and attended with more 

 mischief. But still they cannot be compared with inundations of 

 those rivers which run from east to west or from west to east. In 

 countries which are drained by such rivers, the whole mass of snow is 

 dissolved in a few days, especially when a thaw is accompanied by 

 rain, and all the waters thus produced pass through the principal 

 channel in the course of a week or two. In such rivers the volume of 

 water during the inundations is three or four times larger than it is in 

 the middle of the summer or the beginning of autumn, and the inun- 

 dations spread to a great distance, and frequently cause great loss of 

 property, and sometimes also of life, especially when the winter hi; 

 been unusually long and the falls of snow very heavy. [NIKMKX, in 

 Otoo. Div.] But the river St. Lawrence forms an exception bin 

 also. As it* general course is from west to east, one would suppose. 

 that a large extent of country within its basin would )H> annually 

 subject to inundation, but this doe* not appear to be the case in any 

 part of its course. If any portion of it is swollen by the melting of 

 the snow within the basin, the river soon enters one of the lakes 

 through which iU course lies, and thus the addition of a compa- 

 ratively small volume is not sufficient to raise the surface of tin- 

 lake to any largo amount. Thus the same cause which prevents its 

 filling up the wide icstuary, prevents the river from overflowing 

 the adjacent country. A diurnal rise and fall characterises the 

 rivers of Switzerland and those of the western Himalaya, where a 

 powerful sun melts the glaciers by day, ami the head-streams are 

 frozen by night. 



Rivers whoee inundations are produced by regular rains have the 

 greater part of their course either within the tropics or at least between 

 30* N. lat. and 30 3. Int. It is a known fact, that in those regions 

 heavy rain 'falls daily from three to six months in the year. These 

 heavy rains commence when the sun in its program from one tropic to 

 the other approaches the zenith of a country, and they continue till 

 it has passed a certain distance from it. In tho beginning of tli.- 

 wet season, as this part of the year is called in those countries, tin- 

 rains are sometimes so heavy that in the course of a day the 

 country is covered with water a foot deep. The rivers of cour^. 

 begin to increase in their volume of water, and after some time they 

 rise to the level of the banks, and begin to run over. These inunda- 

 tions generally last from two to four months. They are more ro.yidnr 

 than those which are produced by the melting of the snow, and in 

 general do not exceed a certain height. The rural economy of those 

 countries in which they take place is founded on the knowledge of 

 this periodical event, and on the certainty that the inundations will 

 fertilise the fields by depositing on them a fine mud, which enriches 

 more than the best manure artificially collected and applied. 

 The supply of fertilising matter may not be dun in an equal ratio to all 

 the head-streams or all the tributaries of a river. In the important case 

 of the Nile, according to M. Linant, an eminent scientific traveller and 

 geographer, it is by the Atbara, its first great tributary, that most of the 

 black-earth and slime is brought down which manures the lands in Egypt ; 

 from which it has received also the designation of Bahr-el- Aswad, or the 

 Black River. Whenever the inundations do not rise to the usual height, 

 which is sometimes the case, a great part of the country which is not 

 covered with water yields little or nothing, and the consequence ia 

 dearth and famine. When, on the other hand, the inundations rise 

 higher than usual, they are also injurious to rural economy, by 

 reaching those tracts which are set apart for the cultivation of plants, 



