Aesthetics: The presence of unnatural scum or foam or changes in water color, taste, or order can prevent a waterbody from being used as a drinking water source, recreational area, or limit viability as an aquatic habitat.

Ammonia: Inorganic form of nitrogen. Under specific conditions of temperature and pH, the un-ionized component of ammonia can be toxic to aquatic life. The un-ionized component of ammonia increases with pH and temperature.

Contaminated Sediments: Many of the sediments in our rivers, lakes, and oceans have been contaminated by pollutants. Some of these pollutants are directly discharged by industrial plants and municipal sewage treatment plants, others come from polluted runoff in urban and agricultural areas, and some are the result of historical contamination. Contaminated sediments can threaten creatures in the benthic environment, exposing worms, crustaceans and insects to hazardous concentrations of toxic chemicals. Some kinds of toxic sediments kill benthic organisms, reducing the food available to larger animals such as fish. Some contaminants in the sediment are taken up by benthic organisms in a process called bioaccumulation. When larger animals feed on these contaminated organisms, the toxins are taken into their bodies, moving up the food chain in increasing concentrations in a process known as biomagnification. As a result, fish and shellfish, waterfowl, and freshwater and marine mammals may accumulate hazardous concentrations of toxic chemicals. Contaminated sediments do not always remain at the bottom of a water body. Anything that stirs up the water, such as dredging, can resuspend sediments. Resuspension may mean that all of the animals in the water, and not just the bottom-dwelling organisms, will be directly exposed to toxic contaminants.

Debris: Trash, litter, debris, and other types of solid waste from human activities can impair the recreation value and habitat quality of a waterbody. In some cases, debris is directly responsible for wildlife deaths due to entanglement in discarded plastics.

Dioxin: is an extremely toxic compound to most organisims. Due to its very low water solubility, most of the dioxin occurring in water will adhere to sediments and suspended silts. It is generally resistant to microbial breakdown, and tends to accumulate in aquatic life, from algae to fish. Dioxin is not produced or used commercially in the U.S., but is formed mainly during combustion of a variety of chlorinated organic compounds. Dioxin is released to the environment in emissions from the incineration of municipal refuse and certain chemical wastes, in exhaust from automobiles powered by leaded gasoline, in emissions from wood burning in the presence of chlorine, in accidental fires involving transformers containing PCBs and chlorinated benzenes, and from the improper disposal of certain chlorinated chemical wastes.

Low Dissolved Oxygen / Organic Enrichment: Dissolved oxygen is a basic requirement for a healthy aquatic ecosystem. Most fish and beneficial aquatic insects "breathe" oxygen dissolved in the water column. Some fish and aquatic organisms (such as carp and sludge worms) are adapted to low oxygen conditions, but most desirable fish species (such as trout and salmon) suffer if dissolved oxygen concentrations fall below 3 to 4 mg/L (3 to 4 milligrams of oxygen dissolved in 1 liter of water, or 3 to 4 parts of oxygen per million parts of water). Larvae and juvenile fish are more sensitive and require even higher concentrations of dissolved oxygen. Many fish and other aquatic organisms can recover from short periods of low dissolved oxygen availability. Prolonged episodes of depressed dissolved oxygen concentrations of 2 mg/L or less can result in "dead" waterbodies. Oxygen concentrations in the water column fluctuate under natural conditions, but severe depletion usually results from human activities that introduce large quantities of biodegradable organic materials into surface waters. In polluted waters, bacterial degradation of organic materials can result in a net decline in oxygen concentrations in the water. Oxygen depletion can also result from chemical reactions place a chemical oxygen demand on receiving waters. Other factors (such as temperature and salinity) influence the amount of oxygen dissolved in water. Prolonged hot weather will depress oxygen concentrations and may cause fish kills even in clean waters because warm water cannot hold as much oxygen as cold water.

Exotic Species: When introduced to an aquatic environment, non-native species of plants and animals begin to compete for resources with native species. Many of these exotic species are not bound by the ecological constraints which limit the growth of their local counterparts (i.e, no predators). When this occurs, exotic species can flourish and crowd out local species. This can not only adversely affect native species, but can disrupt the balance of entire ecosystems.

Fish Consumption Advisory: Consumption advisories are issued by EPA or the states to protect people from the health risks of consuming contaminated fish and wildlife. They do this by issuing consumption advisories for the general population as well as for sensitive subpopulations. These advisories inform the public that high concentrations of chemical contaminants have been found in local fish and wildlife and include recommendations to limit or avoid consumption of certain fish and wildlife species from specific waterbodies.

Flow Alterations: Changing the quantity of water flowing through a waterbody can alter aquatic ecosystems so that they no longer support native species. Dams, for example, may block the natural migration route of fish species. Flow alterations also provide less water for dillution of natural and human introduced pollutants.

Impaired Biological Community: Natural, undisturbed aquatic ecosystems provide habitat for a broad variety of biota, exhibiting taxonomic richness and complex trophic structure. Such robust aquatic communities can be impaired when a water resource is adversely affected by human activities.

Inorganics: Inorganic chemicals such as trace elements, flouride, cyanide, flourene, chlorine, or hydrogen sulfide.

Mercury: Mercury is a naturally occurring element that can be toxic when consumed by animals and humans. Sources of mercury include weathering of the earth's crust, the burning of garbage and fuels, and industrial emissions.

Metals: Metals occur naturally in the environment, but human activities (such as industrial processes and mining) can dramatically alter their distribution. When metals are released into the environment in higher than natural concentrations they can be highly toxic and cause major disruptions of aquatic ecosystems and decrease a waterbody's suitability for industrial and domestic uses.

Impairment cause not reported: States did not report the cause of a waterbody's impairment to EPA.

Noxious Aquatic Plants: Plant species whose characteristics or presence in sufficient number, biomass, or areal extent may be reasonably expected to prevent, or unreasonably interfere with, a designated use of those waters.

Nutrients: All plants require nutrients for growth. In aquatic environments, nutrient availability usually limits plant growth. Nitrogen and phosphorus generally are present at background or natural levels below 0.3 and 0.05 mg/L, respectively. When these nutrients are introduced into a stream, lake, or estuary at higher rates, aquatic plant productivity may increase dramatically. This process, referred to as cultural eutrophication, may adversely affect the suitability of the water for other uses. Increased aquatic plant productivity results in the addition to the system of more organic material, which eventually dies and decays. The decaying organic matter produces unpleasant odors and depletes the oxygen supply required by aquatic organisms. Excess plant growth may also interfere with recreational activities such as swimming and boating. Depleted oxygen levels, especially in colder bottom waters where dead organic matter tends to accumulate, can reduce the quality of fish habitat and encourage the propagation of fish that are adapted to less oxygen or to warmer surface waters. Highly enriched waters will stimulate algae production, with consequent increased turbidity and color. Increased turbidity results in less sunlight penetration and reductions in submerged aquatic vegetation. Since this vegetation provides habitat for small or juvenile fish, its loss has severe consequences for the food chain.

Organics: Toxic organic chemicals are synthetic compounds that contain carbon. These synthesized compounds often persist and accumulate in the environment because they do not readily break down in natural ecosystems. some of these compounds are recognized or suspected carcinogens or reproductive toxicants.

Other: A cause of impairment which does not fit in any other cause category.

Other Habitat Alterations: The degradation, loss, or alteration of aquatic habitat due to physical degradation, riparian alteration, channel modification, or hinderance of fish passage or migration.

PCBs: Polychlorinated biphenyls (PCBs) are a family of man-made chemicals that contain 209 individual compounds with varying levels of toxicity. Some are recognized carcinogens. Eating contaminated fish is a major source of PCB exposure for humans because PCBs bioaccumulate in some species of fish found in contaminated waters. PCBs were widely used as coolants and lubricants in transformers, capacitors, and other electrical equipment until they were banned in 1977. Although PCBs are no longer manufactured, exposure still occurs as a result of historical contamination and the decommissioning of older transformers and capacitors, which have lifetimes of 30 years or more.

Pesticides: Pesticides are synthetic chemicals developed to control insect and plant pests. Pesticides disperse into the environment after application, and can cuase contamination of surface water and ground water. Some pesticides can persist in an aquatic ecosystem for years and bioaccumulate in aquatic foodchains. Many of the potential effects of pesticides on humans and aquatic ecosystems are difficult to evaluate because of inadequate information on effects of low-level mixtures, transformation products, and seasonal exposure. Examples of pesticides with adverse impacts on waterbodies include DDT, DDE, alachlor, malathion, diazinon, chlordane, and carbofuran.

pH: The standard measure of the concentration of hydrogen ions is pH. A pH value of 7 represents a neutral condition. A low pH value (less than 5) indicates acidic conditions; a high pH (greater than 9) indicates alkaline conditions. Many biological processes, such as reproduction, cannot function in acidic or alkaline waters. Acidic conditions also aggravate toxic contamination problems because sediments release toxicants in acidic waters. Common sources of acidity include mine drainage, runoff from mine tailings, and atmospheric deposition.

Pathogens: Some waterborne bacteria, viruses, and protozoa can cause human illnesses, ranging from typhoid and dysentery to minor skin diseases. These pathogens may enter waters through a number of routes, including inadequately treated sewage, storm water drains, septic systems, runoff from livestock pens, and sewage dumped overboard from recreational boats. Because it is impossible to test waters for every possible disease causing organism, regulatory agencies usually measure e. coli indicator bacteria (which are found in great numbers in the stomachs of warm blooded animals). The presence of indicator bacteria suggests that the waterbody may be contaminated with untreated sewage and that other, more dangerous organisms may also be present. Bacterial criteria are frequently used to determine if waters are safe for contact recreation or shellfish harvesting.

Radiation: Designated beneficial uses are not obtainable due to radioactivity in waterbodies. When present in levels above natural levels, the elements cesium, uranium, thallium, strontium, radium, and heavy metals can contribute to water quality impairment.

Salinity/Total Dissolved Solids/Chlorides: Salinity can render water undrinkable, unuseable for agricultural purposes, or incapable of supporting aquatic life. Irrigation return flows are a frequent cause of excess salinity, since agricultural runoff dissolves and concentrates mineral salts found in soils. Urban and industrial uses can also cause salinity problem. Treated municipal sewage adds about 35 kg of inorganic salts per year per person served. Wastewater from iron and steel manufacturing, cement making, and other industries can add 200-500 mg per liter of salt and other dissolved solids.

Sediments: Sedimentation refers to soil particles that enter the water column from eroding land. Depending on climate, geology, and vegetation, watersheds experience a natural sediment load. Sedimentation is considered a pollutant when it exceeds this natural level and has a detrimental effect on water quality. Rain washes silt and other soil particles off of plowed fields, construction sites, logging sites, urban areas, and strip-mined lands into waterbodies. Sedimentation and siltation can severely alter aquatic communities. Sediment may clog and abrade fish gills or suffocate eggs and aquatic insect larvae on the bottom. Suspended silt may interfere with recreational activities and aesthetic enjoyment of waterbodies by reducing water clarity. Nutrients and toxic chemicals may attach to sediment particles on land and ride the particles into surface waters where the pollutants may settle with the sediment or detach and become soluble in the water column.

Temperature: Changes in the temperature of a waterbody that lower its value as habitat or increase the adverse effects of other impairment causes such as low dissolved oxygen.

Toxics: Chemicals present in the waterbody that adversely effect biota.

Unknown: The cause of the impairment was not able to be determined.

Wetland Loss: Wetlands include swamps, marshes, bogs, and similar areas. In their natural condition, wetlands provide many benefits, including food and habitat for fish and wildlife, water quality improvement, flood protection, shoreline erosion control, ground water exchange, as well as natural products for human use and opportunities for recreation, education, and research. Wetlands help maintain and improve water quality by intercepting surface water runoff before it reaches open water, removing or retaining nutrients, processing chemical and organic wastes and reducing sediment loads to receiving waters. Wetlands also function like natural basins, storing floodwater that overflows riverbanks and protecting adjacent and downstream property from flood damage.