HOW RIVERS AND VALLEYS MAY BE FORMED 367 



Very often -a river flows with such a speed that it deposits 

 as much as it carries away. The Missouri River does this in 

 one part of its course. 

 Sometimes a river flow- 

 ing through a level region 

 deposits sediment so 

 quickly that its channel FIG. 234. A flood plain formed in a narrow 

 fills up rapidly. The 



river then spreads itself over a larger area and may break up 

 into numerous shallow streams. The Platte River, Nebraska, 

 is an illustration of such a condition. Banks which are cut 

 into and worn down by rivers, and sediment which is spread 

 over valleys by rivers form low plains. Such low plains are 

 called flood plains (Fig. 234), because in times of flood they are 

 apt to be covered with water. The Mississippi River has 

 formed in the lower part of its course a flood plain 60 miles 

 wide. Flood plains are flat fertile lands which are easy to till 

 and to harvest. But be- 

 cause they are sometimes 

 flooded they are uncertain 



homes unless embankments 







are built along the rivers to 

 protect them. 



How rivers and valleys 

 may be formed. The earth 

 is constantly changing. In 

 some places streams dry up, 

 in other places new streams 

 form, and everywhere old 

 FIG. 235. A mountain ravine. streams change their courses 



and alter the lands through 



which they flow. Great changes such as the growth of a full- 

 fledged river and valley take place slowly, requiring millions 



