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---, Colorado River Action Plan-Mainstem II. restore habitat II.A. restore and manage flooded bottom land habitat II.A.1. 29-1/2 Road gravel pit II.A.l.e. monitor and evaluate success II.A.2. Adobe Creek II.A.2.e. monitor and evaluate success II.A.3. Walter Walker II.A.3.e. monitor and evaluate success II.A.4. develop and implement levee removal II.AA.a. preconstruction contaminants screening Colorado River Action Plan-Gunnison II. restore habitat II.A. restore flooded bottom land habitat II.A.2.a. preconstruction contaminants screening II.A.2.d. evaluation III. Study BackgroundlRationale: Selenium is a metalloid that naturally occurs in aquatic habitats in the Colorado River Basin. Human activities have increased concentrations of selenium in aquatic habitats. Adverse effects of elevated selenium concentrations on fish include reduced growth and survival. Several objectives of the Recovery Implementation Program for Endangered Fish Species in the Upper Colorado River Basin relate to restoration of nursery habitats for razorback sucker. Many potential nursery habitats have elevated selenium levels in water and food organisms. Concern exists about potential effects on larval razorback sucker that may reside in these habitats. In response to this concern, a study entitled "Assessment and Prediction of Effects of Selenium Exposure to Larval Razorback Sucker" was initiated during spring 1998 to quantify the potential for adverse effects by conducting standard early life-stage toxicity tests in which razorback sucker larvae are exposed to gradients of selenium-contaminated water and food organisms. Results of this study will be represented as concentration-response relationships. These relationships will show the response of growth and sUrViv"al as functions of selenium exposure from water and dietary pathways (Figure I). The relationships can be used to predict potential effects of selenium concentrations in nursery habitats on larval razorback sucker. The research proposed in this scope of work is designed to support and extend results of the companion study described above. The purpose of this study is to provide data to evaluate the accuracy of the laboratory-derived predictive equations for selenium toxicity, and to determine if co-contaminants in nursery habitats influence the magnitude of toxicity at these sites. The methods used to expose razorback sucker larvae to selenium in this research will be identical to those in the companion proposal except the water source will be water from potential nursery habitats in the upper Colorado River Basin. Site waters Selenium 2