Michigan Environmental Education Curriculum
Stream Monitoring

Imagine shaking up a glass jar of muddy water- it is hard for light to shine through the glass because of all the dirt particles suspended in the water. If you keep the jar still for a while, however, the dirt particles will settle out and you will be able to see light coming through the glass.

Turbidity is a measure of how clear the water is. High turbidity means that there are a lot of particles suspended in the water and light cannot get through. Low turbidity means that there are fewer particles in the water and it is more clear.

Turbidity in a stream may increase from:

  • soil erosion
  • higher levels of algae
  • sediment stirred up from the stream bed

Different values of turbidity in water, measured in NTUs.

Turbidity is measured in nephelometric turbidity units, or NTUs. Nephelometric simply means how cloudy the water is. The picture above shows different NTU values for samples of water.

Drinking water must have less than 0.5 NTU, more clear then the beaker of water shown above with a 10 underneath. Imagine that a river may have a turbidity of 1500 NTU and must be cleaned to less than 0.5 NTU in order for people to be able to drink it.

Some examples of the problems caused by high turbidity include:

  • When there are more particles in the water, temperature increases. Warmer water holds less dissolved oxygen.
  • If water has high turbidity, less light reaches the aquatic plants that need sunlight for photosynthesis.
  • Sediment particles can clog fish gills or settle out of the water and bury fish eggs.
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