Water Quality Parameters in Freshwater Systems


Evaluating the physical and biological life of a stream is vital to evaluating its health, but equally important is determining the chemical water quality.  Concentrations of substances in water depend on many factors and vary naturally in different parts of a river, depending on things like geology, climate, soil & vegetation in the area.  These concentrations can also vary throughout the year or even throughout the day.  Because of this variance, it is important to determine baseline water quality for a site before making any assumptions about a cause for change.  

The chemical water quality of a stream or river is good if naturally occurring substances are present in concentrations appropriate for that particular ecosystem and the life it supports.  Problems can occur when human activities alter the concentrations of naturally occurring substances or introduce foreign substances that may be toxic to life in the stream or river.

The three most basic parameters of water quality are pH, dissolved oxygen (DO) and temperature. 


pH is a measurement of how acidic or basic the water in the river is - the pH scale ranges from 0 (strongly acidic) to 14 (strongly basic). Aquatic organisms have a optimal range of pH 6.5 to 8.2 where they thrive. Generally speaking, the quality of life diminishes as the pH becomes more than 9.0 or less than 5.0. Most organisms have adapted to life in water of a specific pH and it could be lethal if it changes too much or too rapidly.

Dissolved Oxygen

Dissolved Oxygen (DO) is just what it sounds like - oxygen dissolved in water. Dissolved oxygen can come from surrounding air, aeration of water tumbling over falls or rapids, and as a waste product from photosynthesis. It is essential for the basic metabolic processes of plants and animals living in the river. How much DO an aquatic organism needs can vary depending on its physical state, its species, water temperature, and more.  Generally speaking, streams with DO greater than 8.0 mg/l are considered healthy systems.

Salmonids are among the most sensitive organisms to changes in pH. A pH of 5.0-6.5 will result in reduced salmonid egg production and is lethal to salmonid eggs and alevin.  Prolonged exposure to a pH of 9.0 is harmful to salmonids as well.  

Interested in comparing pH levels to common substances?  Check this out:

ph range

                                                          Photo source

teaching tool

Learn how to have your students make their own litmus paper for testing pH, plus other science fun!

Aquatic organisms prefer to live in the conditions to which they are best adapted, and some are able to get by with much less than others.  Again, salmonids are an example of one that requires high levels of oxygen.  

The Alaska Water Quality standards state that in waters used by anadromous or resident fish, DO must be greater than 7 mg/l, and in no case may it be lower than 5 mg/l or greater than 17 mg/l. 

Streams with high dissolved oxygen are generally clear and cold streams with enough riffles to provide mixing of oxygen into the water. 


Fisherman often look for salmon near woody debris or deep pools by small waterfalls.  Take a look underwater at one of those areas, and now that you know about their preference for high DO, you may see why its such a popular spot! 

Video 2 of our Fisheries Video Series also shows a good example of this preference. Video & Photo credit goes to Katrina Mueller

Cold water allows oxygen to dissolve more easily, so as temperature increases, DO decreases.  As discussed, salmonids require high oxygen and thus can only live in environments with both cool temperatures and high DO levels.


Water temperature controls the rate of metabolic activites, reproductive activities and life cycles.  Most aquatic organisms are cold blooded, and assume temperatures similar to the surrounding water.  Deviations from temperatures these organisms are adapted to may cause problems.  

According to Alaska Water Quality Standards, as related to the growth of aquatic life:

Temperature may not exceed 20 C at any time and the following maximum temperatures may not be exceeded, where applicable: 

Migration routes for salmon 15 C 

Spawning areas  for salmon 13 C 

Rearing areas for salmon 15 C 

Egg & fry incubation for salmon 13 C

Other parameters that are commonly measured are conductivity, turbidity, and total dissolved solids.

These parameters are often times measured with the use of a YSI meter, pictured below.  Turbidity can be measured with a Secchi disk, an often times homemade device.


Conductivity is a measure of how well water can pass an electrical current.  It is an indirect measure of the presence of inorganic dissolved solids, which are essential ingredients for aquatic life.  These dissolved solids regulate the flow of water in and out of an organisms cells. Sometimes there can be  "too much of a good thing”. A high concentration of dissolved solids can cause water balance problems and decreased oxygen levels.



Turbidity is a measure of the cloudiness of water. It is caused by suspended particles which scatters light passing through the water. The more particles, the muddier and cloudier the water becomes, thus reducing its clarity and making it more turbid. 


Total Dissolved Solids

Total solids is a measure of the suspended and dissolved solids in a body of water.  It is related to both conductivity and turbidity.  


Find out about what we do with these measurements