SxMutq aitctatuM 



MACROINVERTEBRATE DISTRIBUTION 



A checklist of the macroinvertebrates found in the Tongue and Yellowstone 

 rivers is presented in table 2. This list is as complete as possible and 

 utilizes all published sources available, as well as data gathered during this 

 study. Distributional records were taken from Stadnyk (1971), Gaufin et al . 

 (1972), and Thurston et al . (1975). 



For specimens for which a precise species identification was not possible, 

 the most probable species (considering the most recent available distribution 

 data) is listed in parentheses. In the order Diptera, several genera are 

 listed under the family Chironomidae; this is the only place these genera will 

 appear in this report because of unconfirmed identifications. Identifications 

 of this group are difficult both to make and to confirm. 



YELLOWSTONE RIVER 



Mayflies 



The distribution of all mayflies (Ephemeroptera) known to occur in the 

 Yellowstone River (37 species variously distributed) is presented in figure 19. 

 Four species were collected throughout the study area, and a fifth species 

 {Ephemerella inevmis) was missing only from the lower two sampling stations. 



In this figure and in several others, stations 7-12 are shaded and 

 represent the probable location of the transition zone between the salmonid 

 and nonsalmonid zones. This transition zone also corresponds to the inter- 

 mediate zone between the erosional and depositional habitat subsystems outlined 

 by Cummins (1975b) for large rivers. 



The number of mayfly species found at each station is illustrated in 

 figure 20. Station 5 yielded the largest number of species (19) and stations 

 19 and 20 the fewest with 10 species. No pattern of mayfly distribution is 

 apparent throughout the transition zone. Longitudinally, the community 

 exhibits a gradual shift from mountain fauna to prairie fauna more adapted to 

 slower flow, warmer temperatures, and a silty substratum, but the number of 

 species is reasonably constant along the entire river. 



A mature Heptagenia elegantula nymph is shown in figure 21. 



31 



