54 PHYSIOGRAPHY 



Mississippi carries its load in the same way. Streams carry some 

 sediment, even when not in flood. Some of them have so little mud 

 that their waters seem clear, while others, like the Missouri and the 

 Platte, are always turbid. Since most rivers run to the sea, much 

 of the sediment which they carry finally reaches the ocean and is 

 deposited there. 



The amount of material which certain streams carry to the sea 

 has been estimated. For a given river, the estimate is made by 

 calculating the number of gallons or the number of cubic feet of 

 water discharged by it each year, and then determining the average 

 amount of sediment in each gallon or each cubic foot. It has been 

 estimated that the Mississippi River carries to the Gulf of Mexico 

 more than 400 million tons of sediment each year, an average of 

 more than a million tons a day. It would take nearly 900 daily 

 trains of 50 cars each, each car loaded with 25 tons, to carry an 

 equal amount of sand and mud to the Gulf. All the rivers of the 

 earth are perhaps carrying 40 times as much as the Mississippi. 



We have seen that ground-water dissolves rock slowly, and that 

 springs bring some of this dissolved matter to the streams. These 

 dissolved substances are commonly invisible, and, unlike mud, 

 remain in the water even after it has become quiet. The amount 

 of matter carried to the sea in solution each year, by all the rivers 

 of the earth, has been estimated to be about one-third as much 

 as the sediment (mud, etc.) carried by the rivers. 



These general facts show that the rivers are constantly shifting 

 solid matter from land to sea. This is, indeed, their great work. 

 Even the water which falls on the land, but does not flow to the sea, 

 helps to make the rock decay (p. 44), and so prepares it for removal 

 by running water. It may therefore be said, that every drop of 

 water which falls on the land has for its mission the getting of the land 

 into the sea. 



Gathering sediment. As the rain-water flows down the slopes 

 on which it falls, it carries along particles of soil and weathered rock 

 to the streams to which it flows. The amount of sediment which 

 a stream gets in this way is large if the immediate run-off flows over 

 cultivated fields whose slopes are steep. The water which flows 

 over slopes well covered with vegetation, such as pasture land or 



