Reconstructing paleoflood hydrology with slackwater deposits: Verde river, arizona

Fine-grained slackwater flood deposits provide a key to the paleoflood history of the Verde River in central Arizona. During large flows, the slackwater deposits accumulate in areas of sharply reduced flow velocity at channel expansions and contractions or in back-flooded tributaries. The upper surfaces of the highest deposits approximate the peak stages of the associated floods. Stratigraphic analysis, radiocarbon and archaeological dating, and correlation between slack-water sites along the study reach revealed multiple floods, including two paleo-floods that pre-date the 80-year observational record of flows on the Verde River. Discharges calculated with the HEC-2 computer flow model extended the record of rare flood events by 1000 years. The largest flow in that time period measured only 5000-5400 cms, which supports the contention of an upper limit on the maximum flood that can be reasonably expected. Revised recurrence intervals based on the extended paleoflood record were used as a basis to compare various methods of flood frequency analysis. A combination of the log-Pearson Type III distribution for recurrence intervals up to 100 years, and a regional envelope curve for less frequent flow events, produced a frequency curve that coincided with both the modern and paleoflood discharges.