STREAMS BUILD UP LAND 365 



upon the swiftness with which it runs : when it flows swiftly 

 in hilly, mountainous regions, it can carry a great load ; but when 

 its speed is suddenly checked, as at the base of a hill or ravine, 

 it cannot carry so much, and deposits sediment and debris. 

 Material deposited by streams at the foot of steep slopes 

 accumulates in the form of a cone or fan, and is called an al- 

 luvial fan (Fig. 232). Continual deposits increase the size of the 



fan, which in time ^_____ 



becomes so large 

 that it obstructs the 

 stream which created 

 it, and forces the 

 stream to take a 

 longer, slower route 

 around it. The 

 water which flows 

 around the fan adds 

 to its size by fresh 



deposits along its FIG. 232. An alluvial fan on which trees are growing. 



edge; thus the fan 



grows and sometimes it becomes miles wide and hundreds of 

 feet thick. Alluvial fans 40 miles wide exist in California. 

 They make excellent farm lands and support abundant crops. 



Rivers that flow over steep lands move swiftly and gather a 

 load ; rivers that flow over gently sloping land move less rapidly 

 and deposit sediment. The slope or gradient of a stream gen- 

 erally becomes less and Jess as the river flows onward toward 

 the ocean, and the load that the stream can carry becomes 

 less and less. The Joad that the slowly moving river cannot 

 carry is deposited on the stream bed or is spread over the 

 valley through which it flows. That which is deposited on the 

 bottom fills up the channel, and may form sand bars or even 

 islands in the river (Fig. 233). The sediment brought to the 



