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Typically , it occurs in mid-reaches of streams in swift waters with gravel to rubble bottom . You can divide the class in half , and assign one-half to brainstorm positive comments and the other half assign to write negative comments . It typically inhabits riffles in lower reaches of medium to large streams with moderate to rapid current and gravel or rubble bottoms . None of these areas can be pursued in complete isolation from the others . Be prepared for some students to finish ahead of schedule , and for others to need extra study time . There is no minimum or maximum age requirement students must be to satisfactorily complete the course . Remember that none of the steps need be lengthy . Use actual equipment with the chart for maximum effectiveness . As a volunteer instructor , you will have to hold courses that fit your own schedule .