OPINION/COMMENTARY

TMDLs: Tall Tale of Fishes and Silt

If streams are found to be “impaired,” federal law requires that a Total Maximum Daily Load of pollutant – sediment in this case – be calculated and applied to restore the stream to its intended use. So the job here is to calculate a load of sediment (TMDL) going into the stream that is small enough to restore the stream and make the fish come back. An updated erosion model was used to estimate sediment delivery to the streams; land use in the watersheds was catalogued and an erosion factor assigned to each land use. The approach was to compare the impaired streams to those in the basin that had plenty of fish, rated as unimpaired. When sediment delivery for the 31 impaired and 42 unimpaired streams was compared, however, there was no difference. Impaired stream watersheds in the Piedmont part of the Chattahoochee basin delivered 0.74 tons of sediment per acre per year; unimpaired streams delivered 0.77 tons per acre. In the Coastal Plain, the corresponding figures were 0.63 tons for impaired streams and 0.88 tons for unimpaired streams. A measure of sediment suspended in the water, turbidity, also did not differ in the two classes of streams. If as much sediment is going into unimpaired streams as into impaired ones, how can sediment be the cause of impairment, and how can setting of a TMDL for sediment on impaired streams correct the situation?....

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