Orientation of Clamshells as a Velocity Indicator in a Canal

Abstract

Orientation of isolated shells of Corbicula fluminea on the surface of clam-bearing sediments in the Delta-Mendota Canal indicates that water velocities within the “boundary zone” at the surface of the sediments are lower on the inside radius of canal bends as compared with water velocities on the outside radius of the bends or on tangents. In both cases, however, velocities are strong enough to overturn a notable percentage of half shells on a soft substrate into a “stable” (concave downward) position. Hence, the study indicates the existence of the near-bottom currents capable of moving particles as large as clam shells. Most of canal bottom sediments are composed of much smaller particles. Deposition of such sediments is therefore not a simple mechanical process, but involves filter feeding of Corbicula.