102 RUNNING WATER 



which we have already found to occur in spring-waters, whence 

 they are, for the most part, derived by the rivers. River- water 

 is, however, usually more dilute than that of springs, because of 

 the rain which falls into it, or pours in from the banks. In very 

 dry regions, where this additional rain supply is at a minimum, 

 and where the streams are concentrated by continual evapora- 

 tion, they are frequently undrinkable, on account of the quantity 

 of matters in solution which they contain. Examples of this 

 are the salt and so-called "alkali" (a very comprehensive term) 

 streams of the arid West, which contain a great variety of dis- 

 solved minerals. 



The quantity of material which rivers are continually sweeping 

 into the sea, is enormously great. Every year the Mississippi car- 

 ries into the Gulf of Mexico nearly 7,500,000,000 cubic feet of 

 solid sediment, either in suspension or pushed along the bottom, 

 an amount sufficient to cover one square mile to a depth of 268 

 feet. In addition to this is the quantity brought down in solution, 

 which is estimated at 2,850,000,000 cubic feet annually. 



Different rivers vary much in the proportion of suspended and 

 dissolved materials which they carry and discharge into the sea ; a 

 roughly approximate average makes the amount of material removed 

 equal to about 11,400 cubic feet (600 tons) of annual waste for 

 every square mile of the land surface of the globe ; that is, wider 

 existing conditions of slope, temperature, rainfall, etc. How great 

 a difference in the result a change in these factors may produce, 

 will be seen from a comparison of the Mississippi and the Ganges. 

 The amount of suspended matter discharged by the former repre- 

 sents a lowering of the surface of the entire drainage area at the 

 rate of one foot in 4920 years, while in the case of the Ganges it 

 is one foot in 1880 years, or more than twice as fast. The amount 

 of material carried by the Amazon has not been determined, but 

 there can be little doubt that it is far greater than that discharged 

 by the Mississippi. The area drained by the Amazon is less than 

 twice as large as the drainage basin of the Mississippi, and- yet it 

 brings to the sea five times as much water as does the great river 

 of North America. 



