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percent of the mean annual water yield is passed, on 

 the average . 



WINTER INSTREAM FLOW POLICY AND RATIONALE 



The winter instream flow approach of the MDFWP has been to 

 recommend a level of instream flow protection that severely 

 restricted or eliminated winter withdrawals in most years. 

 This approach has been recently revised to prohibit winter 

 water depletions altogether. The justification for 

 protecting winter flows is primarily based on the fact that 

 winter is the period most detrimental to trout survival in 

 mountain streams that are subjected to icing and other 

 severe weather conditions. For these streams, the harsh 

 winter environment ultimately limits the numbers and pounds 

 of trout that can be maintained indefinitely by the aquatic 

 habitat. Winter flow depletions would only serve to 

 aggravate an already stressful situation, leading to even 

 greater winter losses and the possible devastation of the 

 fish populations. 



The fact that the flows in Montana's mountain streams are 

 lowest in the winter further justifies the policy of 

 prohibiting winter depletions. The assumption that more 

 water provides space for more fish has let to the well 

 accepted conclusion that the period of lowest stream flows 

 is most limiting to fish. The coupling of the low flow 



