1*k{hic& o^ (Atfrfvi withdMXwaU 



It is difficult to predict the effects of flow reduction on the 

 invertebrate fauna because of the large number of species involved and the 

 inability to discuss the environmental requirements and tolerances of a group 

 as large as the Ephemeroptera or Trichoptera. Even within genera there are 

 large variations in tolerance. The need to know environmental requirements of 

 a species is complicated in- the west because few western species have been 

 intensively examined. Roback (1974) lists the habitat requirements of many 

 aquatic insects in terms of chemical concentrations, but few western species 

 are listed. Because of these problems, the following evaluation of effects 

 of reduced flows will be general. 



The three levels of development projected for the Yellowstone Impact 

 Study (see Report No. 1 in this series) were not considered in this impact 

 assessment because of the lack of specific invertebrate data and because 

 this invertebrate study was completed before the final projections were 

 available. 



CHEMICAL 



Attempts to explain the distribution of species in terms of chemical 

 differences have not had much success except where conditions are extreme 

 (Macan 1974). At present in the Yellowstone River, dissolved oxygen 

 concentrations are sufficiently high to sustain invertebrates and fish. 

 Dissolved oxygen could influence invertebrate communities if reduced flows 

 are so low that the BOD of domestic sewage or decaying organisms taxes the 

 reaeration capacity of the river. 



With reduced flows, increased concentrations of nutrients could result 

 in an increase in peripnyton growth, especially of the present dominant alga 

 Cladophora. A large mat of Cladophora would increase the diversity of 

 benthic habitats, probably resulting in a larger standing crop of benthic 

 organisms and a shift in benthic species composition (Percival and 

 Whitehead 1929). 



SILT 



The Yellowstone River carries large amounts of suspended material, mostly 

 inorganic in nature. There is sufficient current to remove much of this 

 material, and silt deposits are not frequent along the river. The high 

 spring runoff is one factor that keeps the river flushed of inorganic sediment. 



95 



