This volume, ""Risk-Based Decisionmaking in Water Resources IX"", is a collection of papers presented at the Ninth United Engineering Foundation Conference held in Santa Barbara, California on October 15-20, 2000. Participants from government, industry, and universities share their knowledge and experience in the rapidly growing field of risk-based decisionmaking in water resources. The topics include new theoretical and methodological frontiers and the economic dimensions of risk analysis, legislative initiatives and risk assessment, maintenance and rehabilitation, reliability of physical infrastructure, flood frequency and climate non-stationarity, and risk of extreme events.