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<br /><<10,00.19 <br /> <br />How Much Sediment is Harmful? <br /> <br />Given the goal of Robust Stream Health, one definition of "harmful" would be <br />the loss of one or more Stream Health classes. There is abundant scientific <br />literature concerning biological effects from sedimentation that suggest many <br />usable methods. One method used by the Forest Service is based on Tarzwell <br />Substrate Ratios. The in-depth analyses of different substrates by Tarzwell <br />(1937, 1938), Cordone and Kelley (1961), and Hynes (1970) are of particular <br />value and provide the foundation for its use to estimate Stream Health, <br /> <br />Hynes concluded the larger the stones, the more complex the substratum, and <br />the more diverse the fauna. Sand is relatively poor, while silty sand is <br />somewhat better, and muddy substrate is pretty good. For stony substrates, <br />the controll ing factors are bed stabil ity and the space open for colonization. <br /> <br />Cordone and Kelley, Hynes, and Tarzwell all conclude that just a small amount <br />of sand or sil t shifting in and around the gravel on the bottom can el iminate <br />much of the area suitable for the attachment or hiding of the aquatic insects <br />and drastically reduce the total production of these forms. Small amounts of <br />sand, not discernible by casual glance but evident only on close inspection of <br />the bottom materials, can bring about these Significant changes. Hynes also <br />draws attention to the loss and change of fauna on stony substrates when they <br />are exposed to surface deposits of sand, silt, or clayey silt; this is true <br />even if the deposit is only there for part of the year (Hynes p209). <br /> <br />Cordone and Kelley conclude an inadequate food supply is often the most <br />limiting factor and the cause of poor fishing quality. This makes the study <br />of bottom fauna one of the more significant approaches that can be used in the <br />detection and measurement of sediment problems. <br /> <br />Tarzwell Substrate Ratios <br />The Tarzwell Substrate Ratio (TSR) graph displays fish food productivity for <br />different types of stream bottom substrates. Clarence Tarzwell found that <br />clean sand was the least productive of all the substrates and therefore made a <br />good divisor from which to build a system of ratios. This approach, used by <br />EPA in their 1983 hazard evaluation training, infers that cobbles, with a TSR <br />of 32, is 32 times more productive than the same unit area of clean sand. <br /> <br />Notice the three groups: sand and silt dominated substrates, rocky materials <br />with minor sand or silt influences, and substrates where plants provide major <br />productivity. Recalling Photo 8, sedimentation triggered large productivity <br />losses: top left, the gravel and small cobbles have a 32 TSR, while the right <br />side, with a thick silt and fine sand layer, has an 8 TSR. To go from 32 down <br />to 8 is a 75% drop in productivity. In the middle and lower photos, sediment <br />(TSR 12) has replaced plant beds (TSR 250) and aquatic mosses (TSR 140). <br /> <br />Text Page 9 <br /> <br /> <br /> <br />.':: <br /> <br />- <br />," <br /> <br />;v,: <br /> <br />,llt'~"J <br />