The macroinvertebrate fauna of the lower Yellowstone is predominantly silt 

 tolerant. Genera known to be silt tolerant include: Isonyahia, Tricorythod.es y 

 Caenis, Traverella, Braahyaercus , Stenonema, Dactylobaetis , and Ephoron 

 (Berner 1959, Jensen 1966). It is not known how much silt the benthic fauna 

 of the lower river can tolerate. Sampling station 20 has the lowest gradient, 

 greatest silt concentrations, and lowest benthic diversity of all sampling 

 stations. If station 20 is used as an example of what could happen at other 

 stations if a high level of development is achieved, the result will be a 

 fauna poorer in numbers and species. 



TEMPERATURE 



Reduced flows, resulting in a shallower river, would probably result in 



higher summer water temperatures. These increased temperatures, besides 



affecting dissolved oxygen levels, would affect invertebrate growth, emergence, 



egg hatching, and metabolism. The net effect would probably be a reduction of 

 the fauna. 



Another factor associated with temperature is ice. In the lower Yellowstone 

 River, a solid ice cover lasts for several months (figure 57). Ice cover at 

 Glendive lasted from late December to April during the winter of 1974-75 and 

 from late November to mid-March during 1975-76. Surface ice can act in several 

 ways to kill invertebrates (Brown et al . 1953). Low flows would permit thicker 

 ice conditions, freezing of large areas of shallow water, and increased 

 gouging and molar action during the time of ice break-up (figure 58). 



CURRENT AND BOTTOM HABITAT 



Bottom samples taken at Glendive and Intake during 1975 revealed that 

 invertebrate densities are directly proportional to current velocity up to 

 velocities of 3.0 ft/sec (no samples were taken at velocities greater than 

 3.0 ft/sec). 



Flow reductions in the Yellowstone would result in reduction in current 

 velocities across the river channel because of its "U" shaped configuration. 

 A general reduction in velocity would result in a faunal reduction because of 

 most species' preference for swift currents. Minshall and Winger (1968) found 

 that a reduction in flow caused a large increase in the percentage of organisms 

 drifting, exposing a greater number of invertebrates to predation 1 by fish 

 which could result in species extinction in a section of stream. 



It is possible to relate invertebrate densities to discharge if mean 

 current velocities across the river at several points are known. The Bureau 

 of Reclamation's Water Surface Profile (WSP) Computer Program (U.S. Department 

 of Interior 1968) utilizes current and depth measurements from several 

 transects to compute area and mean current velocity in several subsections of 

 all transects at any desired discharge. At the Intake station, the WSP Program 

 was used to predict mean current velocities in 15 subsections (shown in figure 

 59) at three discharges (table 23). The mean current velocity was placed in 

 the regression equation obtained from kick samples in November 1975 (sampling 



96 



