Watershed assessment using stream turbidity based on suspended fine solid generation and inorganic-organic composition

To understand changes in the sources and generation of suspended fine solids associated with human development in a drainage basin, it is essential to understand the effects of human impacts. This study examined the runoff characteristics of suspended fine solids in drainage basins with differing types of land use, and established watershed assessment methods and measures for effective watershed management. The ratios of organic fine solid (OFS) to total fine solid (TFS) concentrations (OFS concentration / TFS concentration) were measured in watersheds representing three types of land use (i.e., forestlands, selectively cut forests, and cultivated lands). In the forested catchments, TFS concentrations were 10 mg LE-1 under normal discharge conditions, and maximum TFS concentrations were 10**2 mg LE-1 under high-discharge conditions caused by rainfall events. The TFS concentrations from the selective-cutting catchment often exceeded 2,000 mg LE-1 in the first year after logging. However, TFS concentrations decreased in the second year after logging. In the agricultural watershed, TFS concentrations at the mouth of the main stream were 10**3 mg LE-1 under high-discharge conditions caused by snowmelt and rainfall events. The amounts of TFSs discharged from the three tributaries increased with the proportion of cultivated land area. The OFS ratio was over 30% for the forested catchments and about 10% for the selectively cut catchment and for agricultural watersheds. The sources of TFSs were bank erosion or material transportation from hillslopes near the stream channel in the forested catchments, sediment deposition below skid trails adjacent to the stream after logging in the selectively cut catchment, and delivery from cultivated land in the agricultural watershed. These results indicate that although TFS transport occurs in forested catchments, TFS concentrations increase as the area disturbed by human activities increases. Human disturbances also change the ratio of inorganic and organic contents (e.g., OFS content is lower after disturbance), causing greater fluctuations in inorganic fine solid concentrations, which can negatively affect river biota, than in organic solid fluctuations. The results of this study led to the creation of two indices: an index of fine solid concentrations that quantitatively shows TFS runoff characteristics, and a composition-ratio index using the ratio of inorganic and organic fine solid concentrations during high-discharge periods. A ratio-order diagram incorporating both indices was also created and used to adapt management plans (i.e., drainage basin preservation, surveillance, and measures) to the specific drainage basin types and to propose measures for preventing stream turbidity.