Protecting Public Health and the Environment.

Common Water Quality Measures

Some of the most common measures of water quality are listed below, with an explanation of why they are important to the health or utility of a water body.


Temperature is a physical property of water that has a profound effect on organisms that live or reproduce in the water, particularly Idaho's native coldwater fish such as salmon, Bull Trout, and steelhead and some amphibians (frogs and salamanders). When water temperature becomes too high, salmon and trout suffer a variety of ill effects, ranging from decreased spawning success, to increased susceptibility to disease and toxins, to death. Water temperature also reduces the solubility of oxygen on which aquatic life depends and increases the toxicity of ammonia. Water temperature may enhance sensitivity to other toxic substances as well. For these reasons, it is important to protect the state's water from unnecessary warming. Idaho's temperature criteria are numeric.


Turbidity is a measure of the clarity of the water. As measured, it is a physical property, but a primary cause of turbidity is fine sediment suspended in the water column. Lack of clarity is often an aesthetic concern, particularly in lakes, but is also of consequence to aquatic life as it interferes with light penetration into the water and thus productivity of plants at the base of the food chain—less light often means fewer plants and animals, including fish. Turbidity can also interfere with some types of recreational use, such as diving.

Bacteria (Fecal Coliform, E. coli, and Enterococci)

Bacteria are measured to determine the relative risk of getting sick from recreating in or on Idaho’s surface waters. These bacteria originate from the wastes of warm-blooded animals; thus, their presence indicates possible pathogens may be reaching a body of water. Sources include inadequately treated sewage, improperly managed animal waste from livestock, pets in urban areas, aquatic birds and mammals, or failing septic systems. Idaho's bacteria criterion is numeric.

Dissolved Oxygen

Oxygen dissolved in water is necessary for aquatic animal life, just as oxygen in air is necessary for human life. The concentration of dissolved oxygen (DO) is a single, easy-to-measure characteristic of water that correlates with the health of aquatic life in a water body. Low DO is often related to an excess of nutrients in water. Large quantities of nutrients in water can cause excessive growth of vegetation. This excessive vegetation, upon decay, can cause low DO. Other forms of organic matter, such as those present in sewage or food waste, can also lead to low DO. For this reason, biological oxygen demand in water (i.e., how much oxygen it would take to decompose all the organic matter present) is a common measure. Idaho's dissolved oxygen criteria are numeric.


Water pH indicates the balance between hydrogen ions, which are acidic, and hydroxide ions, which are basic. A perfect balance of the two is at a pH of 7. Most aquatic organisms prefer a pH of 6.5 to 9. Like temperature, water pH is a fundamental controlling property that affects many other chemical constituents (e.g., dominant form of ammonia and solubility of metals) as well as important biological processes such as the level of permeability of fish gills and amphibian skins (i.e., how well gases can flow through the gills/skin, allowing the organism to breathe). Idaho’s pH criterion is a numeric value that limits the range of pH to no less than 6.5 and no greater than 9.


Nutrients in water include various forms of the chemical elements nitrogen and phosphorus—the same materials we apply as fertilizer to our lawns, gardens, and crops to foster plant growth. They have the same effect in water as they do on land, encouraging the growth of aquatic plants such as algae (floating or attached to rocks) and rooted macrophytes (e.g., water lilies).

Without nutrients, water would be sterile and not support aquatic life. Adding nutrients can be acceptable, even beneficial at times, as they increase the productivity of a water body. However, the process of nutrient enrichment in aquatic systems, such that the productivity of the system is no longer limited by the availability of nutrients is called eutrophication. This is a natural process but may be accelerated by human activities. If too many nutrients enter our waters due to human uses, the growth of aquatic plants becomes excessive. This process is known as cultural eutrophication.

Excessive plant growth can change the clarity and desirability of the water. More importantly, the plants eventually will die and their decay uses up oxygen dissolved in the water. Excessive aquatic plant growth is a leading cause in the depletion of oxygen needed for aquatic life. The root cause, however, is too much nitrogen or phosphorus. Because lakes and reservoirs accumulate many things, including nutrients, they are the type of surface water most prone to cultural eutrophication.

Idaho's criterion for nutrients is narrative; DEQ is working on a national EPA initiative to develop numeric nutrient criteria. The most common measures of nutrients are nitrate-nitrite nitrogen and total phosphorus, but measuring other forms such as total inorganic nitrogen, organic nitrogen, or soluble reactive phosphorus can also help define the problem.


In flowing waters, a dynamic balance exists between the supply of sediment from natural erosion and the energy of the moving water that carries and redistributes the sediment load. Furthermore, this balance varies from place to place within the stream channel. Sediment balance determines the very character of many streams and their suitability for various forms of aquatic life.

Many human activities disturb the ground and thus accelerate erosion. Concentrated runoff, such as in roadside ditches, can also accelerate erosion. This increases the sediment load in a stream and often degrades a stream's ability to sustain aquatic life. Spawning gravels are particularly vulnerable to degradation by deposits of fine sediment—sand and silt—that fill the spaces between the larger gravel. This deposition reduces living space for eggs and recently hatched fish and, in extreme cases, suffocates the eggs. Modified flows also change the sediment balance, often reducing a stream’s ability to move sediment.

Idaho's criterion for sediment is narrative. Sediment comes in many sizes and can be measured in many ways, and many complexities exist in determining how much sediment is too much. For more information, refer to the Guide to Selection of Sediment Targets for Use in Idaho TMDLs.

Toxic Substances

A toxic substance is any substance, material, or disease-causing agent, that upon exposure, ingestion, inhalation, or assimilation into an organism will cause death, disease, malignancy, genetic mutation, or other abnormalities in affected organisms or their offspring. Many if not most substances that are toxic are also useful to us, before they enter our waters. Much can be done in our day-to-day lives to reduce the release of toxics into our environment. See DEQ’s information on pollution prevention.

Toxic criteria exist for protecting aquatic life and human health. Idaho's water quality standards contain many numeric criteria for toxic substances, but many more toxic substances have no numeric criteria. Thus, a narrative criterion for toxics substances also exists to protect against the adverse effects of a vast array of substances whose toxicity is either unknown or insufficiently quantified to specify numeric criteria.


Ammonia is quite toxic to aquatic life. While some ammonia is naturally occurring, elevated ammonia levels can result from discharge of inadequately treated effluent, such as domestic sewage. Even if ammonia levels are low in the effluent discharged, if the effluent carries too much nitrogen-containing organic waste and dissolved oxygen levels in the water receiving the effluent are too low, ammonia can form instream. Ammonia criteria vary with the pH and temperature of the water.


High concentrations of some metals such as arsenic, cadmium, copper, mercury, and lead pose a threat to aquatic life, domestic water supplies, livestock, and human health. Eating fish contaminated with certain metals, arsenic and mercury in particular, can cause the metals to accumulate in human tissue, posing a significant health threat. Potentially dangerous levels of metals are identified mainly through chemical analysis of water, but sediment and fish tissue analyses are also used.


Mercury is toxic to aquatic life and humans, but its toxicity is primarily a human health concern. Inorganic mercury occurs naturally due to its presence in rocks and soils. It is slowly released through erosion and weathering into surface waters. Most of the mercury in surface waters remains inorganic, but in certain environments (low pH, low dissolved oxygen, and high dissolved organic matter), as are found at the bottoms of lakes, marshes, and wetlands, some of it is converted to a much more toxic organic form—methylmercury. Methylmercury accumulates in muscle tissue of fish that most people prefer to eat and thus is of particular interest from a human health standpoint.

In April 2005, Idaho adopted a fish tissue methylmercury criterion to protect individuals who may eat fish from Idaho surface waters. This criterion of 0.3 milligrams methylmercury per kilogram of fresh weight fish is based on protecting an adult consumer who eats an average of 17.5 grams of fish per day—about one, 8-ounce meal every other week. 


There are many toxic substances that are human-made, carbon-based chemicals. These are collectively called "organics" and include household and agricultural pesticides, many solvents, and other household and industrial chemicals. Polychlorinated biphenyls (PCBs), for example, are industrial chemicals that are toxic and carcinogenic. Although banned in the United States in 1977, PCBs persist in the environment, and they accumulate in fish and human tissues when consumed. Some organic compounds are highly bioaccumulative and may be measured in sediment or tissue as well as water.

Pharmaceuticals and personal care products (PPCPs) are emerging pollutants of concern. PPCPs can affect reproductive and developmental processes in fish and wildlife and the insect life they depend on for food. They include a wide variety of synthetic chemicals, such as antibacterial compounds in soaps, hormones, fragrances, cleaning agents, and both over-the-counter and prescription medications. PPCPs likely enter surface waters primarily through end-use rather than manufacturing, either by excretion or disposal by flushing. Low concentrations of PPCPs have been found in the Boise River, which was part of a 2002 US Geological Survey study on pharmaceuticals, hormones, and other organic wastewater contaminants in surface water. The most common PPCPs found in the Boise River were steroids, nonprescription drugs, and insect repellant.

Staff Contacts

Water Quality Standards Lead
DEQ State Office
Water Quality Division
1410 N. Hilton
Boise, ID 83706
(208) 373-0502

Related Pages

Water Quality Standards

Numeric Water Quality Criteria

Narrative Water Quality Criteria

Criteria for Toxic Substances


Mercury in Surface Water