The Streamside Forest Removes Sediment and Sediment-Attached Phosphorus by Filtration
The streamside forest functions
as a FILTER by removing sediment and other suspended solids from surface runoff.
Sediment is probably the most common and most easily recognized of the nonpoint
source pollutants. Cropland erosion accounts for about 38% of the approximately
1.5 billion tons of sediment that reach the nation's waters each year. Pasture
and range erosion accounts for another 26%.
Photo right: Agricultural runoff can carry sediment, nutrients and pesticides to surface waters.
Sediment is the most easily recognized of the nonpoint
source pollutants.
Sediment suspended in the water can reduce or block the penetration of sunlight, adversely affecting the growth and reproduction of beneficial aquatic plants.
Sediment deposited on the stream bottom can interfere with the feeding and reproduction of bottom dwelling fish and aquatic insects, weakening the food chain. Large deposits of sediment can overfill stream channels and floodplains, greatly increasing the potential for flooding.
Several mechanisms of sediment removal are at work in the streamside forest. Some sediment settles out as the speed of the flow is reduced by the many obstructions encountered in the forest litter. Additional sediment is filtered out by the porous soil structure, vegetation and organic litter as the runoff flows over and into the floor of the streamside forest.
Phosphorus is also reduced by the filtering action of the streamside forest because about 85% of available phosphorus is bonded to the small soil particles comprising the sediment. Approximately 4% of the phosphorus is attached to soil particles too small to be filtered by these processes resulting in a removal of about 80% of phosphorus by the riparian forest filter. The minor amount of ammonium which is bound to sediment can be filtered out in the same way.
However, dissolved phosphorus and nitrate must be removed by either microbial or biochemical transformation processes.
Photo right: Stream bank erosion also contributes to stream sediment load.
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