STUDY AREA 



Madison River 



The Madison River originates in Yellowstone National Park 

 at the junction of the Firehole and Gibbon rivers and flows in 

 a northerly direction for 149 miles to Three Forks, Montana 

 where it joins the Jefferson and Gallatin rivers to form the 

 Missouri River (Figure 1) . There are two man-maie impoundments 

 on the river; Hebgen Reservoir, located 1.5 miles downstream 

 from the park boundary, and Ennis Reservoir, located 58 miles 

 downstream from Hebgen Reservoir. From its source in the 

 park, the Madison flows across a high conifer forested plateau 

 (7,000 ft and higher in elevation) to Hebgen Reservoir. Upon 

 leaving Hebgen Reservoir, the river flows about 1.5 miles 

 through a narrow canyon to Quake Lake, a natural lake formed 

 by an earth slide during a major earthquake on August 17, 

 1959. Below Quake Lake the river enters the upper Madison 

 River valley where it flows about 51 miles before entering 

 Ennis Reservoir. After leaving Ennis Reservoir, the Madison 

 enters a narrow gorge (Bear Trap Canyon) where it flows about 

 14 miles before entering the lower Madison River valley for 

 the final 26 miles to its junction with the Jefferson and 

 Gallatin rivers. 



The Madison River drains approximately 2,500 square miles. 

 About 70% of the drainage is covered with coniferous forests. 

 The riparian zone of the wide, open upper and lower Madison 

 River valleys is vegetated with willow, alder, cottonwood, 

 and an occasional conifer. Agricultural lands in the upper 

 and lower valleys are primarily used for cattle grazing and 

 hay production. 



Flows in the Madison River are regulated by Hebgen Res- 

 ervoir. Hebgen Reservoir, built in 1915 by the Montana Power 

 Company, stores water for downstream hydro-electric genera- 

 tion. Water storage usually occurs during the snow runoff 

 period of mid-May through early July. Stored water is re- 

 leased to downstream reservoirs during the fall (October- 

 December) . Fall releases usually range from 1,500 to 2,200 

 cfs at Hebgen Dam. 



Ennis Reservoir, built in 1908 by the Montana Power 

 Company, has a rather stable water level with little storage 

 capacity of its own. Its primary function is to create a 

 head for the hydro-electric facility immediately below Ennis 

 Dam. Outflows from Ennis Reservoir are mainly regulated by 

 Hebgen Dam. 



