Clark Fork Basin 



Missoula Aquifer . By measures of existing use and aquifer 

 capability, the Missoula Aquifer is the most significant ground 

 water system within the mainstem Clark Fork. Existing with- 

 drawals are on the order of 61,000 AF/year, and an annual 

 recharge of more than 87,700 AF was estimated for 1986. More 

 importantly, the unusually favorable hydraulic characteristics of 

 the aquifer material imply that very large increases in ground 

 water withdrawals could be supported by the aquifer, as long as 

 the Clark Fork is available as a source of natural and/or 

 induced aquifer recharge (Clark 1986; Missoula City-County Health 

 Department 1987) . Because this relationship implies responses in 

 Clark Fork flows to ground water withdrawal, such increases in 

 ground water use could be incompatible with instream flow 

 objectives or existing water rights in the Clark Fork system. 



Upper Clark Fork . The aquifers of the Deer Lodge Valley and 

 Silver Bow Creek are described in Chapters 1 and 3. These 

 aquifers have a demonstrated record of supporting large well 

 yields, at least locally. The existing high-yield wells serve as 

 municipal, irrigation, industrial, and commercial water supplies. 

 Relatively abundant recharge suggests that the aquifers could 

 support higher levels of ground water development, ignoring for 

 the moment any water quality concerns. Ground water leaves the 

 upper Clark Fork through evapotranspiration or through discharge 

 to gaining reaches of the Clark Fork. 



Bitterroot Valley . Valley-fill sediments of the Bitterroot 

 Valley cover a relatively thin mantle of Quaternary-aged alluvial 

 gravels (generally on the order of 50 feet in thickness) , which 

 overlie at least several hundred feet of Tertiary-aged sediment 

 of varying composition. The Quaternary gravels are generally 

 permeable and can yield several hundred gpm to wells, depending 

 on their saturated thickness. Bitterroot Valley aquifers 

 generally receive recharge from irrigation losses and losing 

 reaches of tributary streams; ground water flows toward the 

 Bitterroot, which receives ground water discharge along most of 

 its lowland reach (McMu^rtrey et al. 1972). Ground water uses 

 from the Quaternary gravels include irrigation, municipal, and 

 some industrial withdrawals. Less productive aquifers on the 

 valley margins supply generally low well yields to an ever- 

 increasing number of residential ground water users. In a number 

 of areas, aquifers underlying elevated benches are heavily 

 dependent on irrigation return flows and ditch seepage for 

 recharge. Changing land uses and abandonment of some irrigation 

 systems leave these high-elevation aquifers subject to lowered 

 water tables and local water supply shortages. 



4-19 



