n6 



WORK OF RUNNING WATER 



3. At debouchures. Where a swift stream flows into sea or 

 lake, its current is checked promptly and soon destroyed altogether, 

 and its load is dropped. If not washed away by waves, etc., the 

 deposits of river-borne sediment in such places make deltas. 



A delta has some features in common with an alluvial fan. In 

 both cases the principal deposit is concentrated at the point where 



Fig. 1 19. A delta in a lake. The village is Silva Plana, in the Engadine, 

 Switzerland. (Robin.) 



the velocity is checked. In the case of the delta, however, the cur- 

 rent is checked more completely, and the debris accumulates (at 

 the outset) below the surface of standing water. Though started 

 below water, deposition on the surface of a delta may build it up to, 

 and even above, the water-level. That part of the delta above 



water is like a flat alluvial fan. 

 In profile, the delta differs from 

 the alluvial fan in that its edge 

 has a steep slope (compare Figs. 

 121 and 122). 



Much land has been made by 

 delta-building. Thus the Colo- 

 rado River has built a great delta 

 many square miles (above water) 

 in area at the head of the Gulf of 

 California (Fig. 123). The delta 

 has been built quite across the 

 gulf near its upper end, shutting 

 off its head. In the arid climate of 

 Fig. 120. The delta of the Nile. the region, this shut-off head has 





