This table was compiled by EPA from information submitted in the states' 1998 and 2000 305(b) reports and represents the number of waterbodies for which the listed stressors or categories of stressors were cited as a cause of impairment.

The sediment referred to here is clean sediment/silt, not toxics-laden bottom sediments. Nutrients are phosphorus and/or nitrogen. "Other habitat alterations" means dams, channelization, bank destabilization, and removal of riparian vegetation, but usually not flow alteration. Organics refers to synthetic organics, not naturally occurring organic materials. Noxious aquatic plants includes blooms of blue-green algae and invasive species such as hydrilla.

The two most common causes of impairment, nutrients (nitrogen and phosphorus) and clean sediments, are parameters for which EPA and most states do not currently have numeric WQC. EPA is in the process of issuing criteria guidance for nutrients. Visit the EPA Office of Science and Technology's (OST) nutrient criteria homepage.

Not all categories of stressors are mutually exclusive. For example, impaired biologic community is a condition that could result from any number of stressors (e.g., flow alteration, pH, temperature, and/or metals) listed in the table, but it could also mean impairments resulting from the introduction of exotic species. Fish consumption advisories would overlap with pesticides, metals, and/or organics.

IMPORTANT NOTE: The precise numbers presented in these tables should not be assigned a great deal of significance. Even the exact order in which the different stressors are listed should not be considered definitive. What can be said with considerable confidence is that the three most frequently encountered causes of impairment are nutrients, pathogens and sediments. By contrast "toxic chemicals" such as metals, pesticides, synthetic organics, and ammonia are not as frequently encountered. (This is not to say that toxics need not be addressed in those waterbodies where they are a problem.)