Section II began at river mile 33 and extended 10,000 ft down- 

 stream. Summer flow is reduced from Section^. -I, but is main- 

 tained even during low water years. Section III began at river 

 mile 24 and extended 8,000 ft downstream. Summer flow is much 

 reduced compared to Section II and in some years ceases en- 

 tirely. During the non-irrigation months of November through 

 June, flows in the three sections are similar. 



Gaging sites were established at the study sections in 

 July, 1976. Flows were measured with a Gurley-type AA current 

 meter and the stage-discharge relationship for each site de- 

 termined. Flows were recorded weekly. 



Standing crops of trout by species were estimated in Sec- 

 tion I in September 19 76 and September 19 77; in Section II 

 in September 1977; and in Section III in September 1976 

 (Table 10) . Standing crops of rainbow trout could not be 

 estimated in Sections II and III due to their low numbers. 

 April estimates were also made but are not included in this 

 paper. All estimates are presented and discussed in Vincent 



and Nelson (1978) . 



- ■ i» 



Summer flows in Section I in 19 77 were reduced when com- 

 pared to those in 1976. The minimiom summer flow measured in . 

 1977 (393 cfs) was 75% of the minimum measured in 1976 

 (52 3 cfs) . ,^, ^ ..,, - . -^ ■ 



During the study, the population of brown trout was rela- 

 tively stable in Section I. The estimated number and biomass - 

 of age 11+ and older brown trout in September 19 77, which 

 followed a low water year (1977), were 98 and 105%, respec- "^ 

 tively, of those in September 1976, which followed three 

 successive above average water years (1974, 1975, and 1976). 

 Mean annual flows for the Gallatin River during the 1974, 

 19 75 and 19 76 water years, as measured at the USGS gage near 

 Gallatin Gateway, were the three highest for a 44-year period 

 of record, while the mean annual flow during the 1977 water 

 year was one of the lowest with a rank of 33. 



The population of rainbow trout, the dominant trout 

 species in Section I, decreased during the study. The esti- 

 mated number and biomass of age 11+ and older rainbow trout 

 in September 19 77 were 66 and 73%, respectively, of those 

 in September 19 76. The estimated number and biomass of 

 all age-groups of rainbow trout were reduced when compared 

 to those in September 19 76 with age-group 11+ showing the 

 greatest reduction in niamber (49%) and biomass (49%) and 

 age III+ the least (5% for number and 6% for biomass) . 



39 



