DERIVING REG0MMENDAT10NS USING WETTED PERTMETER 



When formulating flow recommendations for a waterway, the annual flow cycle Is 

 divided into two separate periods. They consist of a relatively brief runoff 

 or high flow period, when a large percentage of the annual water yield is 

 passed through the system, and a nonrunoff or low flow period, which is 

 characterized hy relatively stable base flows maintained primarily by 

 groundwater outflow. For headwater rivers and streams, the high flow period 

 generally includes the months of May, June and July while the remaining months 

 encompass the low flow period. 



Separate instream flow methods are applied to each period. Further, it is 

 necessary to classify a waterway as a stream or river and to use a somewhat 

 different approach when deriving low flow recommendations for each. A 

 waterway is considered a stream if the mean annual flow is less than 

 approximately 200 cfs. 



Method for the Low Flow Period - Streams 



The wetted perimeter/inflection point method is presently the primary method 

 being used by the MDFWP for deriving low flow recommendations for streams. 

 This method is primarily based on the assumption that the food supply is a 

 major factor influencing a stream's carrying capacity (the numbers and pounds 

 ot fish that can be maintained indefinitely by the aquatic habitat). The 

 principal food of many of the juvenile and adult game fish inhabiting the 

 streams of Montana is aquatic invertebrates, which are primarily produced in 

 stream riffle areas. The method assumes that the game fish carrying capacity 

 is proportional to food production, which in turn is proportional to the 

 wetted perimeter in riffle areas. This method is a slightly modified version 

 of the Washington Method (Collings, 1972 and 1974), which is based on the 

 premise that the rearing of juvenile salmon is proportional to food production 

 and in turn is proportional to the wetted perimeter in riffle areas. The 

 Idaho Method (White and Gochnauer, 1975 and White, 1976) is also based on a 

 similar premise. 



The plot of wetted perimeter versus flow for stream riffle cross-sections 

 generally shows two inflection points, the uppermost being the more prominant . 

 In the example (Figure 3) , these inflection points occur at approximate flows 

 of 8 and 12 cfs. Beyond the upper inflection point, large changes in flow 

 cause only very small changes in wetted perimeter. The area available for 

 food production is considered near optimal beyond this point. At flows below 

 the upper inflection point, the stream begins to pull away from the riffle 

 bottom until, at the lower inflection point, the rate of loss of wetted 

 perimeter begins to rapidly accelerate. Once flows are reduced below the 



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