176 THE WORK OF RUNNING WATER 



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. 



Thousands of years ago, during the Glacial Period, 

 Lake Michigan drained into the Mississippi system. In 

 recent geological times it has drained into the St. Lawrence 

 system. Chicago, by dredging a drainage canal along an 



DIVIDES BETWEEN STREAMS 



The ridge in the center of the picture separates two streams flowing in 

 opposite directions. 



ancient outlet, has restored part of the drainage of the lake 

 to the Mississippi system. 



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. 



River Development. The rain which falls upon a 

 flat country runs off very slowly, a large part of it soaking 



