304 ANIMALS IN INLAND WATERS 



Popella guerni is found in the Volga delta; and in the mouth of the 

 Amazon the copepods Weismanella and Pseudodiaptomus gracilis 

 occur. These are marine components of river plankton. 



Stream subdivisions. — The divisions of a flowing stream accord- 

 ing to the amount of water, as it diminishes from mouth to source, 

 which correspond to the popular names river, creek, brook, rivulet, 

 spring, have little to do with the composition of its fauna. Division into 

 lower river (with a minimum of erosion and a maximum of deposit) , 

 middle river (with a balance between erosion and deposit and a more 

 noticeable side erosion), and upper river (with a maximum of deep 

 erosion and a minimum of deposit) is also frequently inapplicable. An 

 important factor for animal life is the velocity of the current, and, in 

 correlation with this, the nature of the substratum, the temperature of 

 the water, and its supply of oxygen. But the fall of a river does not 

 increase steadily from the mouth to the source. The course of the 

 Rhine, for instance, has three regions of very rapid current, between 

 which there are regions of much more gradual fall. The Amazon is a 

 stream of minimum fall through most of its course; Manaus, 1400 km. 

 from the mouth, lies only 26 m. above sea level; the average fall in 

 this region is 19 m. per 1000 km. In the great rapids in mid-course 

 of many large rivers, such as the Congo or the Essequibo, there are 

 animals which have adaptive features much like those of the inhabit- 

 ants of small mountain streams. The muddy Spytkowianka, a source 

 river of the Skava, a Galician tributary of the Vistula, contains fish 

 which are otherwise characteristic of the lower courses of rivers, and 

 only farther downstream is the fall of the Skava sufficient for trout 

 and grayling. 



The fall of the water mainly determines the physical nature of 

 the substratum in streams. Transportable parts of the substratum are 

 carried along; sand and mud can be deposited only in protected quiet 

 coves; otherwise the bottoms of rapidly running rivers are covered 

 with coarse rock which is constantly in motion. The slower the stream 

 the finer the fragments which it carries, until finally only very fine 

 particles can be moved and only fine gravel is deposited ; this is suc- 

 ceeded by sand and lastly by mud bottom. As the organic detritus is 

 lightest and consequently the last to sink to the bottom, quantities of 

 nutritive sediment from the whole river basin collect in the regions of 

 the slowest current, usually near the mouth of a river. This is a de- 

 termining factor for the bottom fauna. All life is crushed among con- 

 stantly moving rocks; only on permanent cliffs, on especially large 

 blocks, or in sheltered places, can animals gain a foothold in a mov- 

 ing water and rock environment. Moving ice in spring and the ice 



