58 



FRESH-WATER BIOLOGY 



Creek and River Communities 



a. Pelagic sub-formations, independent of bottom and shores 



b. Riffle formation (turbulent water formation) 



c. Sand or gravel bottom formations 



d. Sandy bottomed stream sub-formation, shifting bottom sub- 



formation, aquatic desert 



e. Silt or sluggish stream communities 



(i) Sluggish-stream sub-formations 



(2) Pelagic formations 



(3) Bare bottom formations 



(4) Vegetation formations 



■' QlftciALTUl 



Fig. 7- 

 Three stages in the history of a glacial lake. A, An early stage showing bare bottom, and submerged 

 and emerging vegetation; B and C, successive stages in the deposition of i>eat and marl and the migration 

 of the submerged vegetation toward the center; Erosion and bare bottom are indicated near the shore at 

 the right in A and B but are absent in C. The area inside the emerging vegetation is the plankton 

 region, (.^fter Trauseau.) 



III. Large Lake Communities (Shelf ord; Whipple). 

 Pelagic formations 



Eroding rocky shore sub-formations (turbulent water formations) 

 Depositing, shifting-bottom sub-formations 

 Lower shore formations 

 Deep water formations 



