In every turbulent flowing system, marginal effects develop in the 

 boundary layers. Close to the substratum, movement of the water gradually 

 slows due to friction, and a boundary layer is formed in which the flow is 

 strongly retarded, until, close to the substratum, it is stagnant (Jaag 

 and Ambuhl 1964). The thickness of this boundary layer depends, among 

 other things, on the velocity of the current above and the shape and 

 roughness of the substratum. Extremely flattened organisms (e.g., Epeorus, 

 Rhithrogena) make use of the boundary layer to avoid the current. 



Many species that live in flowing water (e.g., most Plecoptera) can be 

 maintained only in such water, since they either possess no ventilating organs 

 or have changed or lost the function of those organs in the course of their 

 evolutionary development. They are extremely sensitive to still water and 

 quickly die in it. 



Macrodistribution of aquatic invertebrates can be explained with increasing 

 difficulty as habitat gradually changes moving downstream. Cummins (1975a) 

 described food as the ultimate determinant of macroinvertebrate distribution 

 and abundance in nondisturbed running waters. The current regime, velocity, and 

 turbulence set the limits on the range of sediment particle sizes present 

 as well as controlling such features as the growth of periphyton and macroDhytes 

 and accumulation of particulate detritus. The size of particles present decreases 

 in a downstream direction (Macan 1974, Hynes 1970), resulting in community 

 variation in primary producers, macroinvertebrates, and fish. These community 

 changes may be generally placed into three categories or habitat subsystems: 

 (1) erosional zone, (2) intermediate zone, and (3) depositional zone. Each 

 zone has a characteristic physical-chemical makeup and a characteristic fauna. 



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