THE WORK OF STREAMS 139 



water table, ground water will enter it as seepage and springs 

 from the sides, and flow away as a stream. When the bottom 

 of a valley is below the wet-weather level of the water table, 

 but above the dry-weather level, it contains an intermittent 

 stream, but when the bottom of the valley is eroded below the 

 water table at its lowest level, the stream is permanent, and 

 the enlargement of the valley proceeds without interruption. 

 Figure 126 shows many gullies starting on an unprotected sur- 

 face in a relatively dry region, while Figure 127 shows a 

 mountain ravine in a humid region. 



Valley deepening. A stream lowers its channel, and so 

 deepens its valley, by removing material loosened by weather- 



FIG. 128. A stream undercutting its bank and widening its valley. 

 Central Illinois. (Crane.) 



ing or by its own corrasion. But there is a limit below which 

 a stream cannot degrade its valley flat. This is the level of the 

 lake, sea, or other valley to which it leads. Furthermore, it 

 can cut to this level only at its mouth, from which the valley 

 bottom rises upstream, very gently in the case of large 

 rivers, and more rapidly where the stream is small. For some 

 distance above their mouths, streams may, however, cut their 

 channels slightly below the level of the sea or lake into which 

 they flow. The lowest level to which a stream can cut is 



