All plant and animals have certain chemical requirements.
Abundant oxygen is essential to all aquatic organisms. They also have preferred temperature and pH ranges where they thrive. Aquatic organisms become stressed when these conditions are not met. Reproduction is often one of the first things to be influenced by non-optimal conditions.
Some chemical measures are very specific and others like conductivity are a more general measure of water quality.
Measurement of bacterial levels is also useful, especially in streams like Catfish Run, where raw sewage may enter the stream from malfunctioning sewers. This organic enrichment increases the biological oxygen demand and decreases the dissolved oxygen available to aquatic organisms.