previously thought. Based on the newly estimated 100-year flood discharge, the levees protecting Sacramento no longer provide protection against the 100-year flood. The revised flow frequency relationships have immediate policy implications (e.g., "decertification" of levees by the Federal Emergency Management Agency, resulting in building restrictions and higher flood insurance rates) and also reduce the estimated level of protection provided by the flood control alternatives that are currently being considered for Sacramento.

Perhaps not surprisingly, recalculation of the flow frequency relationships has proven controversial. Occurrence of the 1997 flood has also brought into question many issues of technical methodology that bear on decisions about flood risk management in Sacramento.

Shortly after their release, the results of the USACE flood frequency analysis prompted a number of questions, comments, and criticisms from representatives of local, state, and federal government agencies, public interest groups, private citizens, as well as from the Corps itself. In response, the USACE requested the assistance of the NRC to extend the work of the former Committee on Flood Control Alternatives in the American River Basin. This report is a product of NRC's Committee on American River Flood Frequencies, which was organized to assist the USACE by providing an independent scientific assessment of flood frequency relationships for the American River at Sacramento.

Data Sources And Non-Stationarity

A variety of data types can be used in estimating flood quantiles or exceedance probabilities for the American River. These include systematic streamflow and precipitation data, historical and paleoflood data, and regional hydrometeorological information on extreme events. Flood frequency analysis traditionally has been based on systematic streamflow or precipitation records, where use of the latter requires the application of precipitation runoff modeling.

Flood frequency analysis is commonly based on the assumption that flood flows are independent and identically distributed random variables. In reality, the probability distribution of floods can change in time (i.e., exhibit non-stationarity) as a result of local human activities, such as land use changes or reservoir operations, or regional or global climate change. As noted in NRC (1998a), there are many intrinsic modes of climatic variability at decadal to centennial time scales that may be independent of global warming effects or may confound them. Thus non-stationarity in the American River flood frequency due to climatic factors cannot be unambiguously attributed to changes in atmospheric composition over the last century. For example, there are relatively few gaged streams on watersheds that have not been affected by human activities. Unfortunately, there are also relatively few cases where human impacts on flood magnitude and frequency have been carefully documented. There is evidence of significant changes in land use and surface attributes of the American River basin over the last two centuries.

Furthermore, the assumption that floods are independent and identically distributed in time is at odds with the recognition that climate naturally varies at all scales, and that climate additionally may be responding to human activities, such as

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