Duration of turbidity and suspended sediment transport in salmonid-bearing streams, north coastal California

Klein, Randy

Excerpts from the Abstract: "Turbidity in northcoast streams and rivers is largely a function of the amount of sediment carried in suspension by flowing water. It is an integral part of the hydrological, geomorphical, and biological processes of streams and rivers in the northcoast of California and elsewhere. However, water quality and beneficial uses of water can suffer as a result of excessive turbidity, both in terms of increased magnitude and duration of turbidity over background levels. This study focuses on 'chronic turbidity', or the tendency for some streams to remain turbid for a relatively large proportion of the winter runoff period. In this study, I assembled turbidity data for eight continuous turbidity and stage recording stations located on small streams in the northcoast region [Caspar Creek, Freshwater Creek, Jacoby Creek, Horse Linto Creek, Prairie Creek, Godwood Creek, and Little Jones Creek]. Data from individual streams spanning three water years (WY2000-2002) were processed to calculate lengths of time turbidity was higher than several thresholds. Turbidity exceedence analyses, similar to conventional flow exceedence analyses, were also performed, allowing comparison of turbidity levels at various exceedence probabilities. The lowest (and most freqently appearing in the literature on salmonid impacts) threshold was 25 NTU, although inconsistencies in the threshold of impacts to salmonids exist in the literature. To complement the turbidity data, GIS analyses of land and land use characteristics for basin areas upstream of each gaging station were performed. Both natural and human-affected characteristics were summarized. Differences between the study streams in duration of turbidity at several levels (the lowest being 25 NTU) spanned up to two orders of magnitude in some cases. A broad range of turbidities at the 1% and 10% exceedence probabilities was also observed between the study streams. These differences are considered to be far too great to be explained solely by natural variability (geology, climate), thus land use is concluded to be a dominant factor. Although limited by the number of sites assessed (eight), land use variables, particularly road density and annual rate of timber harvest, appeared to be the dominant controls on the gross differences in chronic turbidity observed among the study streams." Recommendations are given for consideration. Appendices include colored maps and turbidigraphs for the EPA study basins, and statistical outputs.

Duration of turbidity and suspended sediment transport in salmonid-bearing streams, north coastal California

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