Paleofloods and a dam-failure flood on the Virgin River, Utah and Arizona

Flood deposits on the Virgin River in southern Utah and northern Arizona preserve a record of the magnitude and frequency of the largest natural floods that have occurred on the river since AD 900. In the Virgin River gorge, several of the natural paleoflood discharges were comparable with, or even greater than, the flood from a catastrophic dam failure in 1989 that produced an estimated peak discharge of 1730 m³ s^-1 in this reach of the Virgin River. To estimate the paleoflood discharges, the heights of slackwater flood deposits in the lower bedrock gorge of the Virgin River were compared with water-surface elevations calculated with the HEC-2 step-backwater hydraulic model. The timing of the paleofloods was determined by the ages of layers rich in human artifacts that are interbedded with the flood deposits. The estimates of both the 1989 flood and the paleofloods were carried out in the Virgin River gorge, about 60 km downstream of the dam break site. No natural floods of this magnitude have occurred within the relatively short period of the gaged record. This study illustrates how incorporating rare, extreme events from the long-term paleoflood record improves the database for flood frequency analysis. The 1989 flood serves as an analog to the effects of rare natural floods with similar magnitudes along the lower Virgin River.