The lower river extends from below the confluence with 

 the Flathead to Lake Pend Oreille in Idaho (Figure 1-5) . 

 Important tributaries in this section include the Thompson, 

 Bull, and Vermilion rivers and Rock and Prospect creeks. 

 This segment differs greatly in that 60 of the approximately 

 100 miles of river are impounded by the Thompson Falls, Noxon 

 Rapids, and Cabinet Gorge dams. When the Clark Fork reaches 

 the Idaho border, it is Montana's largest river, with an 

 average annual discharge of 22,360 cfs (United States 

 Geological Survey 1987) . Streamflows in this segment are 

 governed by weather, geology, and irrigation, and to a great 

 degree by reservoir and dam operation. 



Waters in the lower segment are also classified B-1. 

 Many of the water quality problems of the lower river segment 

 stem from the flow regime of the reservoirs. 



Water quality problems in all sections of the Clark Fork 

 and in some of the tributaries are discussed in detail in 

 Chapter 3 . 



GROUND WATER 



Information on ground water is limited in some parts of 

 the Clark Fork Basin. However, in many areas, ground water 

 is widely available and represents a valuable resource. It 

 is used mainly for domestic purposes and to a lesser extent 

 for livestock, irrigation, public and municipal, and in- 

 dustrial purposes (Casne et al. 1975; Nunnallee and Botz 

 1976) . 



In the Deer Lodge Valley (headwaters to Garrison) , the 

 majority of ground water occurs in pore spaces between grains 

 of Quaternary and Tertiary sediments, with a smaller amount 

 occurring in fractured bedrock. Generally, water in the 

 Quaternary rocks is unconfined, while water in Tertiary 

 sediments is confined. The water table is only about 5-10 

 feet below the surface in the floodplain alluvium adjacent to 

 the Clark Fork, whereas it may be from 10-150 feet below the 

 surface in alluvial fans and terraces (Konizeski et al. 

 1968) . 



The ground water resources in the Deer Lodge Valley are 

 recharged by precipitation and snowmelt runoff, infiltrating 

 irrigation water, and tributary streams that lose water to 

 the ground water system. Normally, the Clark Fork gains 

 water from the aquifer system, although during runoff, it 

 usually rises high enough to provide some temporary recharge 

 to the ground water. Ground water discharge from the Deer 

 Lodge Valley occurs via evapotranspiration; effluent seepage 



1-3 



