RIVER 63 RIVER Some also percolates deeply into the rocks, ultimately emerging as deep-seat- ed springs at a great distance. When a river flows toward a region of great evaporation and small rainfall, such as exists in the interior of each of the great continents, evaporation removes more water than is supplied by 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 contributions from distant rain- fall 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. The annual inundations of the Nile are due to the monsoon rainfall on the great mountains of Abyssinia. The Ori- noco is another instance of seasonal rains producing tremendous inundations, over 40,000 square miles of Llanos be- ing said to be laid under water by the summer rains. The Amazon is an in- stance of a river which is always more or less in flood as the various tribu- taries attain their greatest height at different seasons. The Ganges overflows its banks in summer when the monsoon rainfall is reinforced by the melting of snow on the Himalayas. Where the sea- sons of maximum rainfall and of snow- melting are different, as in the Missis- sippi, the Tigris, and Euphrates, there are two regular floods in the year. The danger of flooded rivers arises from the suddenness with which the wa- ter rises and overflows narrow valleys or even plains. Frightful devastation fol- lows the bursting of glacier obstruction lakes in mountain valleys. The most se- rious 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 comes down; the current of the Theiss is sometimes re- versed for many miles. Great rivers which have embanked their course above the level of the plain are the most dan- gerous of all when flooded. The damage caused by the bursting of the levees on the lower Mississippi necessitates a great expenditure in strengthening the em- bankments, and the most disastrous in- undations recorded in history have fol- lowed the bursting of the banks of the Hoang-ho and its consequent changes of course. River water is spoken of as fresh, but it always contains a certain amount of solid matter in solution, varying from two grains in the gallon or less in rivers draining hard crystalline rocks to 50 grains in the gallon or more in lime- stone districts. The temperature of rivers, as a rule, follows that of the air, but is subject to variations on account of the effect of rain. The great rivers of Europe and Asia, such as the Rhine, Danube, Volga, In- dus, Ganges, Brahmaputra, Yang-tse- kiang, afford access to the sea to enor- mous populations. The Amazon, with its plain track extending for nearly 3,000 miles, is in many ways less like a river than a fresh inland sea ; but the Missis- sippi and St. Lawrence, though less ex- tensive, are of greater value for carry- ing sea traffic to inland places. In their torrential -and upper valley tracks rivers are of use chiefly for transporting tim- ber and driving machinery. It is inter- esting to note that in Switzerland, Nor- way, and Sweden, where there is no coal, there exist exceptional facilities for the use of water power on account of nu- merous mountain torrents. In hot coun- tries rivers are of the utmost service in irrigating agricultural land; the Zeraf- shan and Murghab are entirely consumed in that service, and since the completion in 1890 of the barrage on the Nile no water escapes to the Mediterranean in the low Nile months except along irri- gation canals. The largest rivers of the world, with their length in miles, are: Amazon, 4,000; Nile, 3,766; Yangtse, 3,400; Yenisei, 3,300; Mississippi, 3,lG0; Missouri, 3,000; Congo, 3,000; Lena, 2,800; Niger, 2,900; Ob, 2,300; Hoang-ho, 2,600; Amur, 2,500; Volga, 2,300; Mackenzie, 2,525; La Plata, 2,300; Yukon, 2,300; St. Lawrence, 2,150; Rio del Norte, 1,800; Sao Francisco, 1,200; Danube, 1,725; Euphrates, 1,700; Indus, 1,700; Brahma- putra, 1,680; Zambesi, 1,600; Ganges, 1,500; Mekong, 2,500; Amu Daria, 1,500; Ohio, 950. The pollution of rivers has of late years, in consequence of the extension of manufactures, caused serious concern. No person has a right to poison or pol- lute a stream, and if he do so any of the persons whose lands abut on the stream lower down may bring an action to recover damages. At common law, indeed, in every question of river pollu- tion, the real question of fact is whether there has been any material increase of pollution beyond that which is natural to the particular stream, or beyond that which has existed there for the prescrip- tive period. Questions of river pollution are eminently fitted for submission to a jury, and are generally disposed of in that way.
