Low pH levels accelerate the release of heavy metals from sediments on the stream bottom. The heavy metals can accumulate on the gills of fish, reducing their chance of survival. This is the measure of the relative cloudiness of water. Turbidity is caused by suspended solid matter scattering light as it passes through water. Suspended solids include clay, silt, plankton, industrial waste, and sewage.
Soil erosion introduces soil and mineral particles to surface water.
Stream bed sediments can be stirred up by organisms feeding off the bottom. Particles remain suspended by water currents for some time. Urban runoff introduces a wide variety of particles to stream water. Algal growth from added nutrients and sunlight can also increase turbidity. As the amount of suspended solids increases, photosynthesis decreases, fish gills become clogged, and eggs are smothered.