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The more that is known of the influence of the nature of the contributing watershed upon sedimentation, the more will it be possible to protect reservoirs from early loss of capacity. The particular purpose of this study was to learn to what extent the mineralogical composition of the watershed soils determines the rate of sedimentation and the mineralogy of the sediments in reservoirs. Even though the soils of the 13 watersheds in Indiana and Illinois were similar in clay composition, significant relationships were found. There were only small differences between the composition of the soil clay of the watershed and that of the resulting sediment clay. The amounts of vermiculite, mica, kaolinite, and amorphous alumina in the sediment clays could be accounted for by the amounts in the soil clays. In most cases montmorillonite occurred in larger proportions in the sediment clays, while in case of quartz this relationship was reversed. The estimation of sedimentation rates from geomorphological data, including soil texture, (Paulet, Kohnke, & Lund, 1972) could not be improved by adding mineralogical parameters. This does not necessarily mean that clay composition had no effect upon sedimentation rates, but rather that the soil clays were too similar to demonstrate a definite trend.

The value of a water storage reservoir can decrease rapidly if its capacity is reduced by settling of eroded and suspended soil. Many factors determine the rate of this “siltation.” The purpose of this paper is twofold: to analyze to what extent the influence of the characteristics of the soils of the contributing watershed upon rate and type of sedimentation of reservoirs can be assessed quantitatively and to determine how these effects are correlated with other geomorphological factors. This study was made on 14 reservoir‐watershed combinations in Indiana and Illinois. The majority of the soils were silt loams. Under the conditions of this study, as the mean particle size decreases and the more uniform their particle size, the greater were the sedimentation rates. Sediment volume weight was found to be a function of particle size and organic matter content. The larger the mean particle size and the smaller the organic matter content of the sediment, the greater was the bulk density. The variation of sediment particle size and sedimentation rates could be evaluated quantitatively as functions of geomorphological and pedologic watershed characteristics.