INTERMITTENT STREAMS 



87 



easily weathered and eroded, containing bowlders, gravel, and occasional 

 strata of hard rock. 



2. THE INTERMITTENT STREAM COMMUNITIES 



(Stations 4-8; Tables XVII, XVIII) 

 There are two types of these — intermittent rapids and pool 

 communities. 



An Intermittent Stream 



Fig. 24.— The young stream at Glencoe in spring at high water, showing the 

 leaf-barren trees. 



Fig. 25. — The same in summer, showing the stream entirely dry. 



a) Temporary rapids consocies (Figs. 24, 25). — Small gullies in 

 which water runs only when it is raining do not have any aquatic 

 residents. As soon as such a gully has cut a channel deep enough to 

 stand below ground- water level during a few days or weeks of the rainy 

 season, aquatic insects make their appearance. The species which is 

 usually found in the smallest trickle of water is the larva of the black fly, 

 Simulium (Figs. 27-32). As the stream grows a little larger, and per- 

 haps even at such a young stage also, we sometimes find the nymphs 



