hydrologic system. The level of development considered accept- 

 able in a given aquifer system should depend both upon local 

 considerations of ground water availability and surface water 

 sources that recharge the aquifers and that ultimately receive 

 ground water discharge from the aquifers. Because all aquifers 

 receive some recharge from precipitation, only other recharge 

 factors are discussed here. 



Lowland reaches of most smaller streams in the basin 

 contain alluvial deposits that transmit ground water. The 

 hydraulic characteristics of these deposits range from marginal 

 to very favorable in terms of water yield to wells. They are 

 typically limited in extent, and large well yields usually 

 indicate nearby recharge from surface water bodies. Their 

 location in tributary valleys frequently limits the use of such 

 aquifers to supplying domestic and stock needs, although small- 

 scale irrigation withdrawals are occasionally possible. Local 

 industrial operations, especially mines, derive process water 

 from some of these aquifers and present a potential for increased 

 withdrawals in some areas. 



Secondary permeability (fracture and joint systems) controls 

 ground water flow in most of the consolidated rocks occurring in 

 the Clark Fork Basin. Precambrian-aged Belt series rocks, which 

 are widespread in the basin, generally yield only small quan- 

 tities of water to wells. Exceptions occur in areas where major 

 fault systems provide relatively transmissive flow paths, 

 typically along the margins of important structural basins. In 

 these areas, well yields are occasionally adequate for community 

 supplies and even modest irrigation. Despite their large areas 

 of exposure throughout the region, these aquifer systems are at 

 some risk for local overdevelopment, particularly in areas of 

 increasing residential density, because of their storage and 

 recharge limitations. < 



Bedrock aquifers featuring deep ground water circulation 

 often express themselves as the thermal springs that are 

 scattered throughout the basin. Some of these present the 

 possibility of additional commercial development of geothermal 

 water. 



The important high-yield aquifers of the Clark Fork region 

 occupy the major structural/topographic basins and are composed 

 of unconsolidated to semi-consolidated sands and gravel deposited 

 by fluvial and glacial processes. They vary substantially in 

 hydraulic characteristics, their mode of interaction with surface 

 water bodies, and their relative degree of development. 



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