An Assessment of the Paleoflood Hydrology Methodology: Analysis of the 1993 Flood on Tonto Creek, Central Arizona

The flood of Januruy 8, 1993 is the largest flood preserved in the paleoflood record for Tonto Creek, and probably was the largest flood in the past three hundred years on Tonto Creek. Deposits left by the flood provided an opportunity to calibrate the results of the previous paleoflood analysis of Tonto Creek completed by O'Connor and others, 1986). In addition, these deposits afforded a rare opportunity to test some of the basic assumptions of the paleoflood methodology, which has been applied on a variety of streams throughout the world. Hydraulic modeling of the largest 1993 flood indicates that a peak discharge of about 1500 cubic meters per second (cms) (50,000 cfs) occurred on Tonto Creek at the study reach. Comparison of peak discharge estimates based on various of types of flood deposits left by 1993 floods indicates that peak discharge estimates based on slackwater sediment elevations alone underestimate maximum flood peaks by about 30 percent relative to peak discharge estimates based on flotsam elevations (House and others, 1995). The 325 year paleoflood record preserved in the study reach indicates that the largest peak discharges on Tonto Creek have occurred since 1941, the period of record of the USGS stream gage on Tonto Creek. Therefore, paleoflood data were used only to extend the length of record. Comparison of the historical peak discharge estimates at the paleoflood study reach and published gage estimates reveal a significant discrepancy between estimated flood magnitudes that cannot be explained by hydrologic routing or watershed area. The peak discharges gauged by the USGS and the length of record established by paleoflood data were used in the MAX program, a threshold exceedance statistical analysis model (Stedinger, 1988), to estimate a 100-year flood magnitude of about 1800 cms (63,000 cfs) at the USGS gage site on Tonto Creek.Documents in the AZGS Document Repository collection are made available by the Arizona Geological Survey (AZGS) and the University Libraries at the University of Arizona. For more information about items in this collection, please contact azgs-info@email.arizona.edu