Different methods of mechanical analysis of subaqueous organic-mineral sediments were evaluated using samples from the Delta-Mendota Canal, California. These samples contained some peaty debris and numerous Asiatic clams ( Corbicula sp.). The use of oven-dry sediments does not correctly represent their original composition because of mechanical breakage during drying and alteration of colloidal particles. Better results were obtained by using wet sediments, without previous drying. Initial dry weights of such samples were obtained by totaling dry weights of fractions. Water is an essential constituent of many particles, for example, peat. In organic-mineral mixtures, oven-dry weight of such particles may not correctly reflect their relative abundance. Representation of composition of such sediments in percents by weight at their field capacity moisture content using material separated without previous drying may be a useful approach. Another promising technique, for sediments which are composed of constituents with different specific gravities or contain closed clam shells is the representation of their composition in percents by volume.