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Relationship of the Nature of Suspended Clay Minerals to Hydrologic Conditions¹

ABSTRACT

The clay mineralogy of sediments in suspended transport and their relationship to soils of a small agricultural watershed in western Oregon were evaluated. The clay fraction (<2 µm) was separated from suspended sediment and watershed surface soil samples by centrifugation in distilled water. Half of the clay fraction of each sample was Mg²⁺-saturated and the other half was K⁺-saturated. Slides of both the Mg²⁺- and K⁺-saturated fractions were prepared and subjected to a sequence of characterization treatments. After each treatment the samples were analyzed using copper k-alpha radiation on a Norelco x-ray diffractometer.

It was initially anticipated that the characteristics of sediments would change with progression of the rainfall season as downcutting began to erode material from the upper B horizon. However, no general seasonal trend in the nature of suspended clays was observed. Rather, the character of the entrained clay minerals was dependent on the type of hydrologic event. During high-energy hydrologic conditions, the clay mineralogy of suspended sediments was similar to that of the upper horizons of the watershed soils. In low-energy hydrologic conditions, the suspended sediments were enriched with well-organized kaolinite and smectite and depleted of chloritic intergrades. Selective dispersion combined with preferential settling of "nondispersed" materials are thought to be the mechanisms involved with this phenomenon.

Key Words: sediment mineralogy • sediment characteristics

¹ Contribution of Dep. of Soil Science, Oregon State University, Corvallis, OR 97331. Oregon Agric. Exp. Stn. Tech. Paper no. 6156.

² Former Graduate Research Assistant and Professor of Soil Science, respectively, Dep. of Soil Science, Oregon State University. Senior author is currently Adjunct Instructor, Dep. of Agronomy, Iowa State University, Ames, IA 50011.