Eighteen species of native unionid mussels (Unionidae) were recovered from sediments dredged from behind a coffer dam built during the construction of the Melvin Price Lock and Dam on the Upper Mississippi River. For three species (Amblema plicata, Anodonta grandis, and Obliquaria reflexa), shells in the dredged material were significantly smaller than those of the same species collected at about the same time from other sites in Pool 26 of the Illinois and Mississippi Rivers. The concentration of juvenile specimens in the dredged material is thought to represent a de novo development associated with silt accumulation behind the coffer dam.