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brief snow runoff period when a large percentage of the 

 annual water yield passes through the stream, and a lengthy 

 nonrunoff or low flow period, characterized by relatively 

 stable base flows maintained primarily by ground water 

 outflow. 



Regional differences are evident when the monthly water 

 availability for Montana's mountain streams is compared to 

 that for streams in other parts of the U.S., such as the 

 East coast (Figure 1). The monthly values in this figure 

 are an average for five unregulated streams in each of the 

 two regions. 



For Montana's streams, approximately 76 percent of the mean 

 annual water yield is passed during the snow runoff months 

 of May, June and July, leaving only 24 percent to be passed 

 during the remaining nine months. In contrast, 

 approximately 56 percent of the annual water yield for East 

 coast streams passes during the three high flow months of 

 March, April, and May, leaving 44 percent for the remaining 

 nine months . 



In Montana, the period of lowest flows occurs during the 

 winter from November through March. During each of these 

 five months, from 1.4 to 2.1 percent of the annual water 

 yield is passed, on the average. In the East, the period of 

 lowest flows occurs during summer and early fall from July 



