Lesson 1, Topic 6
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Are there many pools and riffles? Copy

September 8, 2020
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Pool to riffle ratio (riffle frequency). Undisturbed streams typically have alternating pool and riffle areas which tends to support the highest species diversity and food sources for fish. Fish wait for benthic insects to float by at the head of a pool or at the tail of a riffle.

Pool RiffleIf the ratio of distance between riffles to stream width is between 5:1 and 7:1, heterogeneity for aquatic insects and fish is optimal, while a ratio of more than 25:1 is considered a poor habitat. Since benthic communities thrive as a result of integrated environmental factors (substrate, food availability, current etc.), and species have preferences for alternative substrate types, it follows that maximum variability in streambed morphology should support higher species diversity (Barbour and Stribling 1991). Riffles are places that support high-quality habitat and a diverse fauna of aquatic insects and fish. Upstream land use activities can profoundly change pool/ riffle relationships, as well as human-caused changes in flood and low-flow discharge (Frissell et al. 1986). The evaluator uses a tape to measure the average distance between riffles and the width of the bankfull channel.

For high gradient streams, calculate the ratio by dividing the average distance between riffles by the average stream width. Measure this for a distance of approximately 150 to 300 feet. For low gradient streams, calculate the ratio by dividing the average distance between river bends by the average river width. You may want to determine this ratio on a map rather than in the field if your river is large and the distance between bends is great. If a stream contains riffles and bends, the dominant feature with the best habitat should be used (Barbour and Stribling).