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 
Fic. 24.—The young stream at Glencoe in spring at high water, showing the 
leaf-barren trees. 
Fic. 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- 
a young stage also, we sometimes find the nymphs 
ICAT\ 
% \ 
2 
haps even at such 
