ABSTRACT

The role of clams and other "filter feeders" in sedimentation probably far exceeds the generally recognized accumulation of their skeletal remnants. In the course of their feeding, these organisms constantly remove suspended inorganic and organic particles from water, combine them with organic binder, and deposit them as excreta. The "sticky" excreta (and shells) trap sand and other particles and contribute to sedimentation.

The amount of clastic sediments deposited in this manner is significant, for example, in the 113-mile-long Delta-Mendota Canal in the western San Joaquin Valley, California. The sediments consist of 20-35 percent "fines" which were probably precipitated as excreta, 35-50 percent sand, and 30 percent clamshells. Highly eutrophic, turbid deltaic water which is pumped into the canal supports an abundant, varied biota. Asiatic clams (Corbicula fluminea) are particularly numerous. The average number of live clams per square foot of sediments is in the order of 1000. Laboratory pumping rate of a one-year-old Corbicula appears to be about 20 cc/hour.

There is a general decline of clam infestation in the downstream direction, away from the Delta: In 1965-66, biomasses of Corbicula declined from about 140 (pool 1) to 2.5 (pool 18) tons per linear mile. Average sedimentation rates decline from about 292 yd³/mi/yr in pools 1 through 5, to 9 yd³/mi/yr in pools 15 through 19. Apparently smaller pumping-deposition of suspended particles by clams in downstream reaches results in correspondingly lower sedimentation rates.