Paleoflood hydrology of the alluvial Salt River, Tempe, Arizona

Abstract

The paleoflood history of the lower Salt River was documented using slackwater sedimentation techniques. Slackwater sediments are finegrained alluvial deposits which accumulate in zones of ineffective flow along the margins of flood channels. The tops of slackwater deposits can be used as a proxy for the water surface of the flood responsible for their deposition. Slackwater deposits used in this study also included post-abandonment fill in ancient Hohokam Indian irrigation canals. Paleodischarges were estimated using the HEC-II water-surface profile computer model. Cross-section information for the prehistoric Salt River channel was derived from a 1904 contour map of the Salt River Valley. This study is the first application of the slackwater technique to an alluvial river. Flood deposits dating from 1100 years before present to 1976 were analyzed. HEC-II modelling indicates that 27 floods during that time period exceeded the bankful discharge of 175,000 cfs (5000 cms). Two floods exceeded 420,000 cfs (11,900 cms). One of these large floods occurred around A.D. 890. The other occurred within the past 410 years. The largest flood of the historical era was the February, 1891 flood which had a peak discharge of 260,000 cfs (7400 cms). The findings of this study compare favorably with previous studies of the paleohydrology of the Salt River.