Native mussels of the order Unionoida are the most imperiled major taxonomic group of aquatic animals in the United States. Presently, 70 mussel species are listed as federally threatened or endangered and 19 others as candidates for listing (USFWS website), and many workers believe that more than half of the North American species are in jeopardy. The factors that have led to this situation are manifold. However, in Missouri and elsewhere, the construction of dams and reservoirs over the past century has had perhaps the most serious negative impact on the biota of rivers and on mussels in particular. The downstream effects of dams on unionids could possibly be moderated if they were better understood. Each of the four rivers surveyed in the present study has been impounded by at least one major dam, and we hope that the data presented in this report will help in assessing the biological impacts of these structures, in addition to the principle goal of documenting the distribution of species of concern.



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Missouri Department of Conservation, Endangered Species Grant No. E-1-36.
Grant Period: 7/1/2001 – 6/30/2004.

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