A. A good question, and one that has several facets to it. Let me address the impacts of sedimentation on the different levels of the food chain.

Algae/diatoms: These organisms form the base of the food web, or at least one of the main bases, and is impacted by sedimentation in three ways. First, by its presence in the water column, suspended sediments (turbidity) decrease the amount of sunlight penetrating to the bottom of the stream, thus effectively shading the algae/diatoms and decreasing their potential for production of oxygen and growth. Second, the suspended sediments act as an abrasive, effectively scouring algae/diatoms from the substrate. Third, suspended sediments that settle to the bottom cover the algae/diatoms present, thus cutting them off from both light and oxygenated water and resulting in their death. All of these impacts decrease the amount of algae/diatoms on the bottom, the prime food resource for grazing/scraping insects.

Invertebrates: Immature insects which utilize the surface of stones for their food source (grazers/scrapers) or living site (many filterers) are impacted by sedimentation. The former are impacted as described above, and the latter are impacted by being smothered from sediments that settle on them. Fine sediments are extrememly detrimental to mayflies because they settle on and adhere to their gills, effectively smothering them. Sediments that settle and fill the spaces between stones on the bottom of streams effectively eliminate these sites for habitation by invertebrates which seek out these places for shelter and food.

Trout: Again, there is a dual impact here. First of all, if the sediments settle and fill up the spaces between the gravels that trout prefer for spawning, the fish will simply avoid these places for digging their redds. Secondly, if sedimentation occurs after the fish have spawned, it will fill the spaces between the gravels and prevent the flow of aerated water to developing eggs - smothering and killing them.

That's what sediments do to organisms in the streams; the future, obviously, is pretty bleak. ~ Stream Doctor

The 'Stream Doctor' is a retired professional stream ecologist and author, now living in the West and spending way too much time fly-fishing. You are invited to submit questions relating to anything stream related directly to him for use in this Q & A Feature at streamdoctor@aol.com.