Environmental change detectors lichen - Lichen are sensitive to the concentration of sulphur dioxide. The number of lichen in a particular location indicates how clean the air is.
If raw sewage is released into the river, bacterial pollution in water increases and uses up oxygen.
Some invertebrates such as mayfly larvae is found in the river which are sensitive to dissolved oxygen. If mayfly larvae is found in the river the river is clean.
Rat - tailed maggots and sludgeworms indicate a very high level of water pollution.