A YEAR IN SCIENCE 



as well as the Hudson, the Delaware, the St. Law- 

 rence, the Susquehanna, and the Potomac Rivers have 

 drowned valleys. Some of the finest harbors in the 

 world are drowned river valleys. 



Third, the migration of the divide. A divide is a 

 ridge or mountain chain dividing two river systems. 

 If erosion on one side of the divide is more rapid than 

 on the other, the divide will migrate in the direction 

 of the least erosion. This may cause the streams on 

 that side of the divide to disappear. 



Fourth, glaciers. Glaciers sometimes make deposits 

 across stream valleys, causing the formation of lakes or 

 necessitating the stream's taking another course. 



Streams as factors in human activities. From the 

 beginning of history people have chosen to live near 

 streams, probably because they found in them a means 

 not only for producing food, but also for conveying 

 themselves to other sections of the country. We find 

 in the early history of the United States that all of 

 the settlements were made on the coast and, com- 

 monly, near the mouth of a river. As the people trav- 

 eled inland the navigable streams w^ere the centers of 

 greatest population. 



The most of these navigable streams had flood- 

 plains which were, from time to time, enriched by the 

 layers of sediment brought down in the water. The 

 loss caused by destruction of property during these 

 floods is usually equalled or exceeded by the benefit 

 gained from the fertile layer of soil that is deposited. 



