August. 



The mean flow for a 49-year period of record at the USGS gage near the 

 uBstreara boundary of reach #2 was 23.1 m /s. Flows ranged from 3.3 to 27A.6 

 m Is. This gage, which is upstream of all irrigation diversions, reflects 

 the natural flow regime of the river. The high water period normally occurs 

 from late May to late July with peak flows occurring in early June. 



Cross-sectional measurements in reach #2 were made in a 191 m subreach. 

 Seven cross-sections were placed in a riffle-pool sequence. 



Big Hole River 



The free-flowing Big Hole River originates in the Bitterroot Mountains 

 of southwest Montana and flows 251 km before joining the Beaverhead River to 

 form the Jefferson River. The river drains about 6,413 sq km. Throughout 

 its length, cattle ranches and irrigated hay lands occupy much of the river 

 valley. During low water years, the dewatering of the river for the irriga- 

 tion of hay crops can be severe. Water temperatures considered undesirably 

 high for the growth and propagation of salmonids have been recorded in past 

 years when severe dewatering has occurred. 



Reach #1 encloses an 82 km section between the river's mouth and Divide, ^ 

 Montana. Much of this reach is typical of a river crossing an erodible flood- ^ 

 plain. The river meanders through cottonwood lined banks and in many places 

 breaks up into more than one channel. The channel width generally exceeds 

 38 m and gradient averages 2.7 m/km. The bottom substrate consists primarily 

 of cobble and gravel interspersed with boulders. The dominant sport fish in 

 descending order of abundance are mountain whitefish, brown trout and rainbow 

 trout. 



The mean flow for a 54-year period of record at the USGS gage located 

 about mid-reach was 32.8 m Is. Extremes for the period of record since the 

 failure of the Wise River Dam in 1927 have been a minimum of 1.4 m /s and a 

 maximum of 405.2 m /s. 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 following year. 



Cross-sectional measurements in reach //I of the Big Hole River were made 

 in a 303 m subreach. Six cross-sections were placed in a riffle-pool sequence. 



INSTREAM FLOW RECOMMENDATIONS 



Standing Crop and Flow Data 



The long-term standing crop and flow data generated a range of recommendations ^ 



