Aquatic organisms are subject to pollutants in the stream as it flows by, day and night. Consequently, the health of the organisms reflects the quality of the water they live in. If the pollution levels reach a critical concentration, certain organisms will migrate away, fail to reproduce, or die, eventually leading to the disappearance of those species at the polluted site. Normally, these organisms will return if conditions improve in the system. Each component influences the health of the biological community individually and together.
Toxic chemicals are only a single factor within the water-chemistry component.
The relative importance of one component may change with time and location. At certain times of the year or in different geographic settings, a single factor may exert primary control on the well-being of the biological community. For example, straightening a stream and removing all woody debris drastically alters a stream's geomorphology. This results in loss of natural habitat and shelter for certain organisms. The organisms that require this shelter will disappear from the modified system.