LAND WATERS STREAMS 



189 



level. That part of the delta above the surface of the water in 

 which it is built, is like a flat alluvial fan. 



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. 162). The delta 

 has been built quite across the gulf near its upper end, shutting 

 off the head. In the arid climate of the region, this shut-off head 

 has become a nearly dry basin, the lowest part of which is about 

 300 feet below sea-level. The Skagit River, in Washington, has 

 built out its delta so as to surround what were high islands in 



Fig. 160. Diagrammatic profile and section of a delta. 



Fig. 161. Diagrammatic profile and section of an alluvial fan. 



Puget Sound, thus joining them to the mainland. The deltas of 

 the Mississippi (Fig. 157), the Nile and the Hoang-Ho Rivers are 

 among the large and well-known deltas. The united delta of the 

 Ganges and Brahmaputra is also a great one, having an area (above 

 water) of some 50,000 square miles. The Po has built a delta 

 14 miles beyond the former port of Adria, which gave its name to 

 the Adriatic Sea. The Rhone River (France) has advanced its 

 delta some 15 miles in as many centuries. 



The effect of delta-building is to increase the area of the land; 

 )ut it is to be noted that the processes which lead to delta-building 

 reduce the volume of the land-masses, even though they increase 

 their area. 



