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. Material settling into spaces between rocks makes these microhabitats unsuitable for the macroinvertebrates living there. Surface water temperature also rises as suspended particles near the surface absorb heat from the sunlight, which in turn affects dissolved oxygen levels. Another concern of suspended sediments is that attached nutrients, metals, and pesticides can be carried throughout the water system.