Current statistical methods may be unable to accurately predict recurrence intervals of rare, large‐magnitude floods, especially in semiarid regions having positively skewed annual flood distributions, great hydrologic variability, and widely spaced gaging stations. Current approaches rely on historical data, but catastrophic floods may have recurrence intervals far greater than the length of historical records. In the lower Pecos and Devils Rivers of southwestern Texas, paleoflood discharge and frequency estimates are extended over 10,000 years by the study of slack‐water flood sediments. Slack‐water deposits are typically fine‐grained sand and silt that accumulate during floods in areas where current velocity is reduced, i.e., in back‐flooded tributary mouths, channel expansions, downstream from bedrock spurs and/or slump blocks, and in shallow caves along bedrock walls. Radiocarbon dating of organic detritus in slack‐water deposits establishes the flood chronology while paleoflood discharges can be estimated by slope‐area techniques. Paleoflood information extracted from slack‐water sediments can greatly extend flood records. These floods may be weighted like historical data in log Pearson type 3 calculations of flood frequency. Our morphostratigraphic approach combines recorded data with geomorphic evidence to derive estimates of flood frequency. This technique offers an inexpensive and rapid way to assess catastrophic flood risk.
ASJC Scopus subject areas
- Water Science and Technology