Why measure water quality?

Although water quality varies naturally with location and time, human activities such as logging, agriculture, mining and settlement can degrade water quality. For example, when natural vegetation is removed from the stream bank sediment, nutrients, and contaminants can wash into streams and influence stream temperature, turbidity, and other water quality parameters. When roads and pavement replace vegetation water is not absorbed into the ground and filtered before it enters waterways and the impermeable surfaces allow contaminates to easily wash into streams or wetlands.

Water quality is based on many parameters that can influence water in different ways. Here is a list of the main water quality parameters that Central Westcoast Forest Society measures during restoration projects.

Dissolved oxygen (DO)

Oxygen is essential for aquatic life. As water flows downstream, it is rolled around and mixed by rocks or drops in gradient. As this mixing occurs, oxygen from above the surface is drawn into the water. The measure of dissolved oxygen tells how much oxygen is available in the water for fish and other aquatic organisms to breathe and the amount of DO in water can affect the number and kind of animals found there. Healthy water generally has high levels of DO. Salmon require at least 6 parts per million (ppm) of dissolved oxygen to breathe comfortably. Several factors can affect how much DO is in water, such as, temperature, the amount and speed of flowing water, the plants and algae that produce oxygen during the day and take it back in the night, pollution, and the composition of the stream bottom. For instance, slow flowing water has little surface turbulence so very little oxygen mixes into the water and when organic matter decomposes in water it can also consume oxygen as well as warmer water holds less oxygen than cold water. Planting streamside vegetation and trees or maintaining stream bank vegetation helps keep temperatures cool and oxygen levels high for fish and other species.


Water temperature determines the kinds of animals that can survive in a stream. Water temperatures higher than 25°C are lethal to some species of fish such as coho salmon. Optimum temperatures for most salmon are in the range of 3°C to 15°C. This temperature range will allow salmon to be at their peak metabolic rate for growth. Water temperatures also influence incubation rates and the time that eggs will hatch. Generally warmer waters will speed up the growth rates of salmon eggs, alevin, and fry.


The pH scale measures the relative acidity or alkalinity of any substance by determining the concentration of hydrogen in the water. PH ranges from very acidic (at pH 0) to very basic (at pH 14), with 7 being neutral. Pure water has a neutral pH of 7. Most aquatic organisms are sensitive to changes in pH and prefer a pH between 6.0 to 8.5. Changes in pH can affect how chemicals dissolve in the water and high acidity can be deadly to fish. Some species of salmon here on the coast have become specially adapted to lower pH (more acidic) waters but they still can only tolerate a gradual variation in the water pH.