PALEOFLOOD HYDROLOGIC RESEARCH IN THE SOUTHWESTERN UNITED STATES

Paleoflood hydrology is a cross-discipline between geomorphology and hydrology that uses geologic evidence to estimate discharges for historic and (or) prehistoric floods. Analysis of fine-grained flood deposits has been used to estimate the magnitude and data of occurrence of floods on rivers and streams in the southwestern United States. These flood deposits and other geologic evidence of floods, termed paleostage indicators, are emplaced along the margins of channels from streamflows with high concentrations of suspended sediment. Dates for floods are determined from historic records, analysis of scarred or damaged trees, relative ages of soils developed on flood deposits, and radiocarbon dating of organic material entrained in flood deposits. Flood discharges can be estimated from paleostage indicators by using the step-backwater method in which the channel is either assumed or demonstrated to be stable. A comparison of the techniques that have been used reveals inconsistencies caused by a rapidly evolving discipline, and standardized procedures are needed for the analysis of flood deposits. The three case studies presented illustrate the use of paleohydrologic data with gaging data for estimating flood frequency using maximum-likelihood techniques. Maximum-likelihood techniques for fitting probability distributions can explicitly account for the uncertainties inherent in paleoflood data and provide greater flexibility in the use of paleoflood records with gaging records in flood-frequency analysis thereby yielding improved estimates of design floods with large recurrence intervals.