Temperature impacts nutrient availability, oxygen solubility, and decomposition rates. Water temperature is a keystone parameter of coldwater fish species conservation; if water temperatures aren’t below a certain level, several other key parameters may also negatively impact the ability of coldwater fish species to survive and reproduce. The optimum temperature for trout to flourish and grow is between 56° to 70° F. At temperatures above 70° F, trout become sluggish and inactive.


pH is the measurement of the concentration of available hydrogen and hydroxyl ions in a body of water and is measured on a scale of 1 to 14 with 7 being neutral, values less than 7 being acidic (lots of free hydrogen ions), and values greater than 7 being basic (lots of free hydroxyl ions).

Most fish species prefer pH levels between 6.5 and 9.0 and are negatively impacted when levels fall below 5.0 or rise above 9.6. When pH levels fall below 6.0, fish become vulnerable to fungal infections and heavy metals and begin to die at levels below 4.0. Trout will not reproduce in an acidic environment (waters with a pH of 6 or lower).


The amount of oxygen dissolved in a stream is crucial for the health of gill-breathing aquatic organisms. If oxygen concentrations drop below certain levels, the entire aquatic ecosystem can become stressed and, in extreme cases, suffocate in a process called eutrophication. The following table shows the impact of different levels of dissolved oxygen, measured in parts per million (ppm).

0-2 ppm: not enough oxygen to support life.

2-4 ppm: only a few fish and aquatic insects can survive.

4-7 ppm: good for many aquatic animals, low for cold water fish

7-11 ppm: very good for most stream fish


Total dissolved solids (TDS) are common components of many effluents, but are typically not well characterized in terms of either chemical constituents or toxicity. Total dissolved solids represents an integrative measure of the concentrations of common ions (e.g., sodium, potassium, calcium, magnesium, chloride, sulfate, and bicarbonate) in freshwaters.


Conductivity is a measure of the capability water in a stream to pass an electric current. This is an indicator of the concentration of dissolved electrolyte ions in the water. It doesn't identify the specific ions in the water. However, significant increases in conductivity may be an indicator that polluting discharges have entered the water.

Every creek has a baseline conductivity depending on the local geology and soils. Higher conductivity will result from the presence of various ions including nitrate, phosphate, and sodium. Distilled water has a conductivity ranging from 0.5 to 3 µS/cm, while most streams range between 50 to 1500 µS/cm. Freshwater streams ideally should have a conductivity between 150 to 500 µS/cm to support diverse aquatic life.


Turbidity is a measure of how particles suspended in water affect water clarity. It is an important indicator of suspended sediment and erosion levels. Typically it will increase sharply during and after a rainfall, which causes sediment to be carried into the creek. Elevated turbidity will also raise water temperature, lower dissolved oxygen, prevent light from reaching aquatic plants which reduces their ability to photosynthesize, and harm fish gills and eggs. Acceptable turbidity levels would be < 10 fnu for trout waters and < 25 fnu for non-trout waters.


Plants use nitrate to build protein, and animals that eat plants also use organic nitrogen to build protein. When plants and animals die or excrete waste, this nitrogen is released into the environment as NH4+ (ammonium). Nitrate enters streams from natural sources like decomposing plants and animal waste as well as human sources like sewage or fertilizer.

Nitrate is measured in ppm. Natural levels of nitrate are usually less than 1 ppm. Concentrations over 10 ppm will have an effect on the freshwater aquatic environment. 10 ppm is also the maximum concentration allowed in human drinking water by the U.S. Public Health Service. For a sensitive fish such as salmon the recommended concentration is 0.06 ppm.


Phosphorus in small quantities is essential for plant growth and metabolic reactions in animals and plants. Phosphate-induced algal blooms may initially increase dissolved oxygen via photosynthesis, but after these blooms die more oxygen is consumed by bacteria aiding their decomposition. Sources of phosphate include animal wastes, sewage, detergent, fertilizer, disturbed land, and road salts used in the winter.

Phosphates do not pose a human or health risk except in very high concentrations. Larger streams may react to phosphate only at levels approaching 0.1 ppm, while small streams may react to levels of PO4-3 at levels of 0.01 ppm or less. In general, concentrations over 0.05 will likely have an impact while concentrations greater than 0.1 ppm will certainly have impact on a river.