Table 2. Average depths for five riffle cross-sections in the Tobacco River, 

 Montana, at selected flows of interest. Average depths were derived 

 using the WETP computer program. 



Average Depth (ft) 



Flow (cfs) Riffle Riffle Riffle Riffle Riffle 

 cs #1 cs #2 cs //3 cs #4 cs #5 



Too 



110 



120 i 



The minimum depth criteria developed in Oregon could also be used in 

 conjunction with the WAGD option of the WETP program to derive passage 

 recommendations. For this evaluation, criteria are developed requiring at 

 least a certain percentage of the top width of a cross-section to have water 

 depths greater than or equal to the minimum needed for fish passage. In 

 Oregon, at least 25% of the top width and a continuous portion equaling at 

 least 10% of the top width are used (Thompson, 1972). The flow that satisfies 

 these criteria for all cross-sections is recommended. 



Coose Nesting Requirement 



The maintenance of adequate flows around islands selected by Canada geese for 

 nesting is necessary to insure that the nests are protected from mammalian 

 predators. Under low flow conditions, these predators have easy access to the 

 islands and can significantly reduce goose production. The security of the 

 islands is a primary factor in their selection as nest sites by geese. This 

 security is provided by adequate side channel flows, which are a function of 

 depth, width, and velocity. Since wetted perimeter is a function of both 

 width and depth, its relationship to discharge is believed to be the best 

 Indicator of the minimum flows that are needed to maintain secure nesting 

 islands. 



The wetted perimeter/inflection point method is applied to the shallowest area 

 of the side channel bordering each nesting island. A wetted perimeter-side 

 channel discharge curve is generated for each cross-section and the inflection 

 point determined. A curve correlating the side channel flow to the total 

 river flow is also derived during the field season. From these curves, the 

 total river discharge that would provide the inflection point flow in each 

 side channel is determined. The final recommendation is derived by averaging 

 the recommendations for each island or choosing the river flow that would 

 maintain at least the inflection point flow around all the islands being 

 sampled in the study area. The latter method is preferred. 



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