BROOK, CREEK, AND RIVER 277 



the lower stream or river society, and finally the estuary society, 

 found in the sluggish, backed-up waters near the mouths of 

 some streams. The inflowing streams, tributaries to the main 

 river, are likely to display quite unlike inhabitants, those 

 entering well down toward the mouth showing marked contrasts 

 to those entering up toward the source, for the animals and 

 plants present in the main stream, whence they migrate into the 

 tributaries, are so different in the various parts of the course. 



Even in the same region of the stream the species present 

 will vary according to varying depth and rapidity of the current. 

 Along shore the water is shallow and so relatively well aerated, 

 while in the deeper portions of the stream the volume below a 

 given area of surface is much greater and the concentration of 

 carbon dioxide and other gases of decomposition is higher, other 

 things being equal. So a zonation of plants and animals from 

 shore outward may be expected. For the same reason there is a 

 zonation from the surface downward since the oxygen content 

 of the surface layers is high as compared with the bottom 

 layers. This evidently will be most marked in the quiet 

 stretches. In the rapids where the water is constantly stirred 

 from top to bottom there will be no such marked difference. 

 So in the sluggish stretches of the river the oxygen content 

 will be low, that of carbon dioxide high,- while in the rapids 

 where the water is well aerated by its turmoil the reverse will 

 be true. Active forms demanding much oxygen can live in the 

 latter situations but not in the former. 



The character of the bottom affects the animal and plant 

 population. As a rule the stream has its bottom covered with 

 rocks or coarse gravel in its most rapid stretches, a sandy bottom 

 where it is less rapid but still flowing quite vigorously, and a mud 

 bottom in its quiet stretches. In part, the differences in plant and 

 animal life on these different bottoms are due to the varying rates 

 of the current, but in part they are directly connected with the 

 character of the bottom itself. Some plants and animals, for 

 instance, are adapted by holdfasts to attach themselves to a rock 



