2 FRESH-WATER BIOLOGY 



The more or less actively flowing waters appear in the form of 

 springs or rivulets, then increase and unite to make brooks, creeks, 

 and rivers. The transition is ordinarily gradual and size has only 

 a secondary influence on the biological character of the stream. 

 The rate of flow, and the physical and chemical character of the 

 soil over and through which water drains into a stream and by 

 which its banks and beds are formed are the chief factors in de- 

 termining its life. 



From the tiniest rivulet to the mightiest river one may 'find 

 every possible intermediate stage, and between the swiftest moun- 

 tain torrent and the most sluggish lowland stream there exists 

 every intermediate gradation. Biologically considered, the torrent 

 imposes on the development of life within its waters evident me- 

 chanical limitations which are not present in the slow-flowing 

 streams. Ordinarily the biological wealth of a stream varies in- 

 versely with its rate of flow, and anything which stops or checks 

 the flow makes conditions more favorable for the development of 

 life. Flowing waters are thinly inhabited and also present con- 

 siderable difficulties to the student.; hence they are relatively un- 

 explored territory. 



Waters of the static type, characterized by lack of flow, form an 

 equally continuous series from the great lakes or inland seas pro- 

 gressing by insensible gradations through lake, pond, and pool to 

 the morass or swamp. In the first group size permits more wind 

 action; it also provides greater stability in level as well as in 

 thermal and chemical conditions. Possessing only limited com- 

 munication with the ocean these bodies of water constitute biolog- 

 ical units of great definiteness. The lake is a microcosm; a minute 

 replica of the ocean, it responds more quickly to changes in its en- 

 vironment, is simpler to grasp and easier to study. Yet it is 

 withal the most complicated of inland environments (Shelf ord). 



The distinction between water bodies of different size is often 

 indefinite. Puddle, pond, and lake form in fact a continuous 

 series. Yet in a strict sense lakes are characterized by a central 

 region deep enough to exceed the limits of growth of the flora in 

 the shore zone. Ponds are shallow lakes, usually insignificant in 

 area, yet still of relative permanence. They constitute distinct 



