EXTREME FLOODING DURING THE HOLOCENE FOR SELECTED REGIONS IN THE WESTERN UNITED STATES

Climatic conditions naturally vary over time; however, recently there has been growing recognition that anthropogenic effects is influence climatic variability. Numerous paleoclimatic studies have been used to document mean annual temperature, precipitation, and streamflow fluctuations during the Holocene. Although substantial research and modeling have been done to assess the effects of climate change on hydrology, there is little information to assess the effects of climate variability and stationarity on the magnitude and frequency of extreme flooding during the Holocene. Inferring extreme flood variations from studies of average hydroclimatic conditions or data at single sites are extremely problematic. Paleoflood hydrology is the study of the movement of water and sediment in rivers before the time of hydrologic measurement. Paleoflood data provide a means to assess the effects of climate variability and stationarity issues on the magnitude of large floods during the Holocene.

This presentation compares contemporary flood and Holocene paleoflood data for extreme floods within five distinct hydroclimatic regions in the western United States; the High Plains and Rocky Mountains of Colorado, east slope of the North Cascades, Washington, northwestern Montana, and the desert southwest. Contemporary floods are defined as having occurred during about the past 100 years since systematic monitoring began and include historical floods. For each region, envelope curves of maximum flooding were developed for contemporary and Holocene data with drainage area at a site. These envelope curves of maximum contemporary floods and paleofloods then were used to infer effects of climatic variability on flood magnitude during the Holocene. Generally, the difference between maximum contemporary flood and paleoflood magnitudes in each region are within the measurement error (about 10 to 30 percent) associated with the current methods used to estimate flood magnitude. Thus, differences in climate, human influences, and watershed conditions across the range of Holocene climates appear to have had relatively minor effects on flood magnitude in the five regions investigated. The envelope curves for each region probably reflect an upper bound of flooding during the Holocene for each region.