Statement of Problem: Tumbling Creek Cavesnail (TCC) is restricted to a single cave stream in Tumbling Creek Cave, Taney County, Missouri. Population surveys conducted since 1996 have documented recent sharp decreases in snail populations, with only one individual snail found in 13 surveys of the core habitat area after October 2000. The cause(s) for this decline is not known, but potential causes include sedimentation and/or deterioration of water quality due to human activities in the recharge area of the cave stream (McKenzie, 2003) Instrumentation has been installed to improve capabilities for monitoring water quality in the cave stream, and passive sampling devices (semipermeable membrane devices or SPMDs) have been deployed in the stream to determine concentrations of hydrophobic organic contaminants (e.g., polynuclear aromatic hydrocarbons, and persistent organochlorine compounds) in stream water. However, these ongoing efforts are not adequate to characterize potential contaminant threats to the TCC. In particular, they do not address concerns about impacts of sedimentation in the cave stream, which may involve both an increased quantity of sediment deposited in the stream and degraded sediment quality. A study of contaminant and sediment impacts on the TCC was approved for funding during FY2004 under the Science Support Program, for support of research priorities of the Fish and Wildlife Service. This study plan describes Phase 1 of this study, which is intended to be funded in two phases: Phase 1. This phase of the study was funded for FY2004-2005 at a level of $25-k. Phase 1 will characterize sediment quality and quantity in the cave stream. Sediment quality will be assessed based on characterization of persistent chemical contaminants and toxicity to benthic invertebrates. Chemical analyses will include analyses of components such as toxic metals that are not sampled by SPMDs. Chronic toxicity tests will evaluate adverse effects of sediment exposure on survival, growth, and reproduction of benthic invertebrates. Fine stream-bed sediments will be collected in the cave stream, in areas where the TCC does not currently occur, and in surface streams draining the recharge area for Tumbling Creek, in an attempt to identify sources areas for any contaminants of concern. The impacts of sedimentation on habitat of the TCC will be addressed by monitoring suspended sediment during low- and high-flow periods over the course of a year and by quantifying rates of sediment deposition. Phase 2. Phase 2 of this study will be completed during FY2005-6. Based on potential contaminants of concern are identified during Phase 1, risks posed by these contaminants to the TCC will be determined based on toxicity tests with surrogate species. A suitable surrogate would be a closely-related species of snail, preferably a cave-dwelling species that has a large and stable population. Performance of toxicity tests with a surrogate species will require development of suitable methods for laboratory culture and for toxicity testing.