Created in 1972 in response to the passage of the Clean Water Act and the shameful burning of the Cuyahoga River in 1969, the Ohio EPA made, as its highest priority, the cleanup of Ohio's waterways. In order to do this, the agency first had to establish what constituted clean natural waterways. Over the next 30 years, the EPA began monitoring all major waterways on a regular basis, constructing in the process a series of metrics, or indicators, of water quality using fish and invertebrates as biological indicators of ecosystem health. Fish and invertebrate sampling provides a better indication of aquatic ecosystem health than monitoring chemical and physical parameters of waterways; because biological populations accumulate environmental stresses that may not be present or detectable when chemical or physical tests are carried out. Certain species that are more sensitive to pollution than others may show changes in the face of intermittent or sublethal doses of contaminants, and synergistic/antagonistic effects of several pollutants may be impossible to predict. Therefore, changes in populations in a habitat may provide evidence of problems that would otherwise go undetected.