Aquatic Insects and Stream Habitat: River health can also be measured by the kinds of insects and in-stream habitat that are present.
The hydrology of a river system, which is mainly determined by soils, geology, and land use, is a critical physical factor to aquatic life. In streams where groundwater is the principal water source, stable flow patterns occur, characterized by low seasonal and daily fluctuations in discharge. Stable flows promote stable habitat for aquatic life in the form of diverse bottom substrates, secure in-stream cover, and moderate water velocities and temperatures. Unstable flows scour river bottoms and may cause excessive sedimentation which in turn covers up critical in-stream habitat.
Aquatic insects such as mayflies, caddisflies, and stoneflies depend on stable flows, high water quality, and sufficient in-stream habitat for survival. These are insects in the orders Ephemeroptera (the mayflies), Plecoptera (the stoneflies), and Trichoptera (the caddisflies), generally called the EPT orders. These insects are particularly sensitive to factors that reduce oxygen or flow, or increase temperature or otherwise raise metabolic rates in a stream.
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