Description:
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.