Turbidity is a measure of the particles in water, particularly how much the material present in the water disrupts the passage of light.  In other words, turbidity is a measure of water clarity, as it is the observable characteristic of the extent that the suspended particles in the water impair the ability to see clearly through it.These suspended particles are generalized as sediment, which include various types of matter including soil particles, algae, plankton, and microbes. These particles are extremely small, and can change the color of the water (Figure 1).
A nephelometer (Figure 2) is used in water treatment plants to measure turbidity, which measures light scattered within a section of water. In lakes, turbidity may be measured by use of a secchi disk (Figure 3), which is released into water until it can no longer be seen, at which point the depth of the water is measured.
Figure 1. A highly turbid water stream.
Figure 2. A nephelometer.
Figure 3. Secchi disk for measuring turbidity.
High turbidity increases water temperature due to the particles absorbing sunlight. Higher temperatures of water result in less oxygen content, leading to hypoxic conditions. Suspended particles also scatter light, preventing it from reaching plants and algae, further reducing oxygen content. These higher temperatures lead to fish using more oxygen due to increased metabolic rates, limiting the oxygen supply even more.
Knowledge of water's turbidity is important and is closely monitored in water treatment facilities. Most particles that contribute to turbidity are not harmful to humans upon ingestion, however their content in water is an indicator to other substances that do cause harm. Measurements of turbidity give an indication of how well the treatment processes are working and allow for optimal selection in treatment technologies.
See water quality degradation for more information on the effects of substances within water.