Brown, Timothy. 2002. Short-term total suspended-solid concentrations resulting from stream crossing obliteration in the Clearwater National Forest, Idaho. M.S.

This was a pilot study to investigate the short-term effects of stream crossing obliteration (removal) on downstream water quality in small, headwater streams. Study sites were located on the Idaho Batholith in the Clearwater National Forest in north central Idaho. Paired grab-samples were taken above and below stream crossings just before, during, and after stream crossing obliteration. Samples were analyzed in a laboratory for total suspended-solid (TSS) concentrations and nephelometric turbidity according to standard, widely accepted methods. Paired- and one-sample t-tests were used to compare upstream and downstream sediment yields resulting from various levels of mitigation. The relationship between TSS concentrations and nephelometric turbidity was examined using simple linear regression analysis.

The objectives of this study were to: (1) determine the significance of short-term increases in TSS concentrations and turbidity; (2) quantitatively describe changes in suspended-sediment yield; (3) compare the effectiveness of different sediment trap configurations; (4) document currently accepted stream crossing obliteration methods; and (5) evaluate the compliance of current practices with adopted water quality standards from which projects of this type are exempted.

Total suspended-solid concentrations upstream of sites ranged from 1.0 to 5.7 milligrams per liter (mg/L). Downstream concentrations were highly variable ranging from 2.9 to approximately 68,500 mg/L. Upstream suspended-sediment yields ranged from 0.1 to 0.8 kilograms (0.3 to 1.7 lbs), while downstream yields ranged from 0.8 to 95.4 kilograms (1.9 to 210.3 lbs). Yields were normalized by the duration each site was monitored for statistical comparison.

The linear relationship between TSS concentrations and nephelometric turbidity was strong, with a Pearson correlation coefficient of 0.918. Nephelometric turbidity was estimated for samples based on the resulting regression equation (MSE = 0.132; r2 = 0.842). Adopted water quality standards were exceeded a majority of the time when one or no sediment trap was used, but only occasionally when two were used.

Downstream, normalized yields were statistically greater when all sites were considered (p = 0.082), but statistically indistinguishable when the yields of just mitigated or highly mitigated sites were compared (0.119 < p < 0.148) at a 0.10 level of significance.

As expected, the highest concentrations and sediment yields were observed downstream of sites with little or no mitigation. The highest concentrations observed were frequently associated with the sediment trap removal process. The use of two sediment traps reduced peak concentrations an order of magnitude from approximately 68,500 mg/L to 3,000 mg/L, and suspended-sediment yields 10 to 30 times. Thus, the use of at least two sediment traps is recommended during this type of work.

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