impact of the reservoir on the flow regime in reach #2 was to extend the 

 high water period an additional four months from April through September. 

 ThiB extension occurs at the expense of October through March flows. 



The mean flow for a 70-year period of record at „ the USGS gage located 

 near the downstream boundary of reach #2 was 12.0 m /s. Flows ranged from 

 2.0 to 77.1 ra /s. The historic peak flows occurred in late May to mid-June. 

 Since 1964, flows at this gage reflect regulation by Clark Canyon Dam. 



Cross-sectional measurements in reach #2 were made in a 180 m subreach. 

 Seven cross-sections were placed in a riffle-pool sequence containing an 

 island. 



Gallatin River 



The free flowing Gallatin River originates at Gallatin Lake in Yellow- 

 stone National Park and flows north for approximately 185 km to Three Forks, 

 Montana where it joins the Madison and Jefferson Rivers to form the Missouri 

 River. The river drains about 4,662 sq km, all above an elevation of 1,219 m. 

 Most of the drainage basin above 1,524 m is covered with coniferous forest 

 and located within Yellowstone National Park and the Gallatin National Forest. 

 The drainage basin below 1,524 m consists primarily of the Gallatin Valley, 

 one of the richest agricultural regions in Montana. 



Reach #2 of the Gallatin River encloses a 55 km section located within 

 the Gallatin Valley between the confluence of the East Gallatin River (river 

 km 19) and the mouth of the Gallatin Canyon (river km 74) . As the river 

 leaves the canyon, flow is confined to a single channel. Mean channel width 

 at this point is about 46 m and gradient averages 4.0 m/km. As the river 

 progresses through the Gallatin Valley, the flow becomes braided into three 

 to four channels with the main channel shifting from year to year. Mean 

 channel width in the lower valley is about 197 m. The dominant sport fish 

 in descending order of abundance are mountain whitefish, brown trout, rain- 

 bow trout and brook trout. 



The streambed at the mouth of the canyon is approximately 20% boulder, 

 70% cobble and 10% gravel and sand. In the lower portion of reach //2, the 

 streambed is approximately 50% cobble and 50% gravel, sand and silt. 



Fish cover in the upper valley consists primarily of overhanging, 

 rooted, bank vegetation and large instream boulders. Cover in the lower 

 valley is composed primarily of cottonwood log jams and debris piles. The 

 large instream boulders of the upper valley are absent in the lower valley. 



Reach #2 is markedly affected by irrigation diversions. As the river 

 progresses through the valley, water is diverted for the irrigation of hay 

 lands during the summer growing season. The degree of flow reduction depends 

 on the annual flow with more severe dewatering occurring in low water years. 

 In some years portions of the river are totally dewatered in late July and 



