STREAMS AND THEIR WORK 181 



Fifth, waterfalls, rapids, and lakes have been elimi- 

 nated from the course of old streams by erosion. 



Study the streams of your vicinity to determine to 

 what stage of development they belong. As you travel 

 from one part of the country to another study the 

 streams in a similar manner. 



Accidents to stream development. Streams meet 

 with many accidents during their life history. Few 

 streams, if any, go through their v entire development 

 uninterrupted. Among the important accidents that 

 may happen to stream development are : 



First, the rising of the land. If the land rises just as 

 rapidly as the downward erosion of the stream, it will 

 remain at a constant stage of development. If the 

 land rises faster than the downward cutting, the slope 

 will become steeper and the current will be accel- 

 erated. Such a stream is said to be rejuvenated, as it 

 is taking on the characteristics of youth. 



Second, the subsidence of the land. If the land is 

 subsiding, the slope becomes less steep and the current 

 slower. Erosion in the bed is lessened but erosion of 

 the banks is increased. If the subsidence is below base 

 level, the stream will deposit sediment in its bed in 

 an attempt to build the bed up to base level. When 

 the subsidence of the land is below base level, water 

 from the ocean or lake will flow up the stream and 

 cover the valley, and in this way form a drowned 

 river valley. Many rivers of the New England States, 



