PALEOFLOODS AND A DAM-FAILURE FLOOD ON THE VIRGIN RIVER, UTAH AND ARIZONA

Flood deposits on the Virgin River in southern Utah and northern Arizo na preserve a record of the magnitude and frequency of the largest nat ural floods that have occurred on the river since AD 900. In the Virgi n River gorge, several of the natural paleoflood discharges were compa rable with, or even greater than, the flood from a catastrophic dam fa ilure in 1989 that produced an estimated peak discharge of 1730 m3 s-1 in this reach of the Virgin River. To estimate the paleoflood dischar ges, the heights of slackwater flood deposits in the lower bedrock gor ge of the Virgin River were compared with water-surface elevations cal culated with the HEC-2 step-backwater hydraulic model. The timing of t he paleofloods was determined by the ages of layers rich in human arti facts that are interbedded with the flood deposits. The estimates of b oth the 1989 flood and the paleofloods were carried out in the Virgin RiveT gorge, about 60 km downstream of the dam break site. No natural floods of this magnitude have occurred within the relatively short per iod of the gaged record. This study illustrates how incorporating rare , extreme events from the long-term paleoflood record improves the dat abase for flood frequency analysis. The 1989 flood serves as an analog to the effects of rare natural floods with similar magnitudes along t he lower Virgin River.