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" Paleoflood Hydrology, Design of Hydraulic Structures, and Water-Resources Management Robert D Jarrett, US Geological Survey Box 25046, MS 418, Denver, Colorado 80225 USA The economic and environmental importance of major floods emphasizes the need for a better understanding of the hydrometeorologic processes and variability of flooding. Floods are the most destructive of all natural hazards. In the United States, the average annual flood damage is over $2.5 billion and the average annual number of deaths is about 100. Unfortunately, flood losses continue to increase. US Geological Survey researchers have developed an interdisciplinary approach that enhances conventional flood studies with paleoflood information. Paleoflood hydrology is the study of evidence of prehistoric floods that have occurred in about the past ten thousand years. Paleoflood techniques offer a way to lengthen short-term hydrologic data, to reduce the uncertainty in hydrologic analysis, and therefore, to help reduce flood losses. For more than a decade, USGS hydrologists have conducted interdisciplinary research of many contemporary and prehistoric (paleofloods) floods on rivers throughout the Rocky Mountains. The 1976 Big Thompson River (Colorado) flood, which killed 140 people and caused over $35 million in damages, was the type of flood believed to occur frequently at any elevation in the Rocky Mountains. Analyses of streamflow data indicate that extreme floods occur very infrequently than computed by using existing engineering methods in many basins. Researchers also identified that there is an elevation limit to substantial flooding. Peak flows in river basins above about 1.700 m in the Northem Rockies to about 2,400 m in the Southern Rockies are small compared to floods at lower elevations. These results have important implications for the design of hydraulic structures in flood plains, flood-plain management, and the implementation of flood-warning systems. Modifications of the existing spillways of the 162 high-risk dams in Colorado are expected to cost over $200 million. For example, Olympus Dam in Estes Park, Colorado, is located at an elevation of 2,300 m. The spillway was designed in the 1950's for a flood of 637 m3/s. Design requirements developed in the 1980's for spillways in the Rocky Mountains would have required a spillway modification to accommodate a flood of 2,780 m3/s. USGS research showed that floods have not exceeded about 140 m3/s in at least 10,000 years. Thus, the USGS, in collaboration with the Bureau of Reclamation, was able to demonstrate that a spillway modification of about $10 million was unnecessary. The USGS methods have been applied to dams in other parts of the country. By using paleoflood studies, approximately $60 million was saved in unnecessary spillway modification for Bradbury Dam on the Santa Ynez River in California. There are thousands of dams throughout the Rocky Mountain region and similar spillway modifications would cost many tens of billions of dollars. Throughout the Rocky Mountain region (and in other States and in other countries), dam-safety decision-making and spillway-size criteria are being reevaluated. Additional hydrometeorologic and paleoflood research needs are being formulated so that dam-safety design criteria can be revised.