20 



nitrates and/or the decomposition of organic matter in the deeper parts 

 of the reservoir. 



Pre- and postimpoundment flows at the Barretts gage, located 

 approximately 15 miles below the dam, are compared in Figure 1. From 

 October through March, Clark Canyon Reservoir stores water, therefore, 

 inflows exceed outflows. However, average monthly discharges (October 

 through March) during preimpoundment years were not greater than those 

 after impoundment. The similarity of average monthly discharges (October 

 through March) is a result of above normal inflows in postimpoundment 

 years. Although pre- and postimpoundment average monthly discharges 

 were similar (October through March), the variability of average monthly 

 discharges was greatest after impoundment. The major impact of the 

 reservoir on the upper reaches of the river was to extend the high flow 

 period an additional five months from April through September. This 

 extension occurs at the expense of October through March flows. 



Pre- and postimpoundment flows at the near Twin Bridges gage, 

 located approximately 52 miles below the dam, are compared in Figure 2. 

 The major effect of the reservoir on the lower reaches of the river was 

 to increase flows during the preimpoundment low flow months of May, 

 July, August and September. November and late spring remained high 

 flow periods. 



The influence of Clark Canyon Reservoir on water temperatures in 

 the Beaverhead River is shown in Figure 3. The Grant and Blaine 

 stations are located on the Beaverhead River approximately 0.25 and 

 52 miles, respectively, below the dam. The Red Rock River, which is 

 the major tributary to the reservoir, reflects the temperature of the 



