1081 Conservation Biology, Pages 1081–1093 Volume 11, No. 5, October 1997 Contributed Papers Threats to Imperiled Freshwater Fauna BRIAN D. RICHTER,* DAVID P. BRAUN,† MICHAEL A. MENDELSON,‡ AND LAWRENCE L. MASTER *Biohydrology Program, The Nature Conservancy, P.O. Box 430, Hayden, CO 81639, U.S.A., email brichter@tnc.org †Biohydrology Program, The Nature Conservancy, 1815 N. Lynn Street, Arlington, VA 22209, U.S.A., ‡Biohydrology Program, The Nature Conservancy, 2060 Broadway, Suite 230, Boulder, CO 80302, U.S.A.  The Nature Conservancy, 201 Devonshire Street, 5th Floor, Boston, MA 02110, U.S.A. Abstract: Threats to imperiled freshwater fauna in the U.S. were assessed through an experts survey address- ing anthropogenic stressors and their sources. Specifically, causes of historic declines and current limits to re- covery were identified for 135 imperiled freshwater species of fishes, crayfishes, dragonflies and damselflies, mussels, and amphibians. The survey was designed to identify threats with sufficient specificity to inform re- source managers and regulators faced with translating information about predominant biological threats into specific, responsive actions. The findings point to altered sediment loads and nutrient inputs from agri- cultural nonpoint pollution; interference from exotic species; and altered hydrologic regimes associated with impoundment operations as the three leading threats nationwide, accompanied by many lesser but still sig- nificant threats. Variations in threats among regions and among taxa were also evident. Eastern species are most commonly affected by altered sediment loads from agricultural activities, whereas exotic species, habitat removal/damage, and altered hydrologic regimes predominate in the West. Altered sediment loading from ag- ricultural activities and exotic species are dominant problems for both eastern mussels and fishes. However, eastern fishes also appear to be suffering from municipal nonpoint pollution (nutrients and sediments), whereas eastern mussels appear to be more severely affected by altered nutrient impacts from hydroelectric impoundments and agricultural runoff. Our findings suggest that control of nonpoint source pollution associ- ated with agriculture activities should be a very high priority for agricultural producers and governmental support programs. Additonally, the large number of hydropower dams in the U.S. subject to federal re-licens- ing in coming years suggests a significant opportunity to restore natural hydrologic regimes in the affected rivers. Amenazas a la Fauna Dulceacuícola en Riesgo Resumen: Se estimaron amenazas a la fauna dulceacuícola de los Estados Unidos en riesgo mediante un estudio de expertos enfocado en estresores antropogénicos y sus fuentes. Se identificaron específicamente las causas de disminuciones históricas y los límites actuales para la recuperación de 135 especies dulceacuícikolas de peces, langostinos, libélulas, mejillones y anfibios en riesgo. El estudio fué diseñado para identificar amenazas con suficiente especificidad como para informar a los manejadores de recursos y reguladores que encaran la traducción de información sobre amenazas biológicas predominantes en acciones específicas y sensibles. Los resultados apuntan hacia cargas de sedimentos y entrada de nutrientes alterados por fuentes agriculturales sin puntos de contaminación; interferencia de especies exóticas y regímenes hidrológicos al- terados asociados a operaciones de retención, como las amenazas más importantes a nivel nacional, acom- pañadas por muchas otras menores pero aún significativas amenazas. Tambien fueron evidentes variaciones entre regiones y entre taxas. Las especies del este son mas comunmente afectadas por cargas de sedimentos de actividades agriculturales, mientras que las especies exóticas, la remoción/daño del hábitat y alteración de regimen hidrológico predominaron en el oeste. Cargas de sedimentos alteradas por actividades agriculturales Paper submitted July 8, 1996; revised manuscript accepted December 31, 1996.