FALLS AND RAPIDS 329 



very distinctly marked, as on mountain ridges, or it may 

 be difficult to determine, as in a flat country, but if the 

 drainage is well established, it will be apparent. 



If the drainage is not well established, areas may be 

 found which at one time drain in one direction and at an- 

 other time in another. A singular example of the shift- 

 ing flow of a drainage area is found in Yellowstone Park 

 where Two-ocean Creek shifts from one side to the other 

 of a fan it has built, and at one time delivers its drainage 

 into the Atlantic Ocean and at another time into the 

 Pacific. 



Near the dividing line between two drainage areas, 

 swamps sometimes occur, which have streams flowing 

 from them in two directions so that part of their water 

 goes to one stream and part to another. But as these 

 swamps become better and better drained, each stream 

 will carry off its definite part of the water. Divides are 

 irregular in their height, so that roads and railways in 

 passing from one drainage basin to another usually seek 

 out the lowest part of the divide. In mountain regions 

 these low places are called passes. 



Divides do not always remain in the same place, as the 

 river on one side may from some cause become able to 

 carry off the drainage more easily than the river on the 

 other side. It will thus push back its headwaters and 

 shift the divide back until the divide becomes adjusted to 

 the abilities of the two rivers. 



157. Falls and Rapids. In many streams the flow of 

 water is interrupted by falls and rapids. Sometimes 

 the course of a stream is crossed by a great break in the 

 earth's crust, one side of which has been raised above the 

 other. This makes a. fall, or, if the stream is able to cut 

 down fast enough, a rapid. Falls or rapids of this kind 

 have been produced in the Colorado River. 



