You are looking at the litmus paper test for a river’s ability to support life.
This is a beatis nymph and it dwells mainly in clean, flowing water.
Pollution is insidious because the real damage it does can’t easily be observed. Aquatic creatures support the fish life in the river - without them, the fish will starve.
Mayfly nymphs are extremely sensitive to changes in water and environmental conditions. Their decline points to a much bigger problem coming down the line.
If we are looking for a template for a waterways we must make them clean, not chemically clean like a swimming pool, but naturally clean and packed with organisms.