Reach #1 encompasses a 51-mile section .between the river's 

 mouth (river mile 0) and Divide, Montana (river mile 51) . 

 Much of this reach is typical of a river crossing an erodible 

 floodplain. The river meanders through cottonwood-lined banks 

 and in many places breaks up into more than one channel. The 

 channel width generally exceeds 125 feet and gradient aver- 

 ages 14 ft/mile. The bottom substrate consists primarily of 

 cobble and gravel interspersed with boulders. 



The average discharge in reach #1 from 1924-1977, as 

 measured at the USGS gage near Melrose (river mile 31) , was 

 1,157 cf s . Extremes for the period of record since the failure 

 of the Wise River Dam in 1927 have been a minimum of 49 cfs 

 and a maximum of 14,300 cfs. The high water or snow runoff 

 period normally extends from mid-April to mid- July, with peak 

 flows occurring in early June. The lowest flows generally 

 occur during the irrigation season in late August or September. 

 Flows remain relatively low until the onset of runoff the fol- 

 lowing year. 



Brown trout, rainbow trout and mountain whitefish are the 

 dominant sport fish in reach #1. Other species present in- 

 clude brook trout, cutthroat trout, arctic grayling, longnose 

 dace, mottled sculpin, white sucker, mountain sucker, long- 

 nose sucker, burbot, and carp. 



Cross-sectional measurements in reach #1 of the Big Hole 

 River were made in a 993-ft subreach located at river mile 

 36. Six cross-sections were placed in a riffle-pool se- 

 quence (Figures 16, 17 and 18). 



24 



