Paleoflood hydrology of the San Juan River, southeastern Utah, USA

Abstract

The extraordinary number of slackwater deposit-paleostage indicators (SWD-PSI) preserved in the bedrock canyon of the San Juan River in southeastern Utah offers a unique opportunity in paleoflood/flood-frequency analysis. The San Juan, a major tributary of the Colorado River and Lake Powell reservoir, has been regulated by Navajo Reservoir in New Mexico since June 1962. The largest recorded flood for the San Juan River near Bluff, Utah (record from 1914-1962) is 1,980 m3/s on September 10, 1927, with a stage of about 10 m at Mexican Hat. The bedrock canyon between Bluff and Mexican Hat contains abundant slackwater deposits and driftwood lines. Stratigraphic sequences have been described at 11 sites, and paleostage indicators such as driftwood lines have been described at several others. Paleoflood discharges typically are calculated by generating numerous water-surface profiles using a 1-D step-backwater model such as HEC-RAS. Abundant paleostages indicators, such as occur in the San Juan River, allow for tighter control in the model, and a more accurate estimate of discharge can be made. The highest-elevation flood evidence in the study reach occurs as driftwood lines that contain sawn wood, indicating the floods are historic in age, and likely correlated with a catastrophic flood which was observed, but not directly measured, in 1911. The discharge of this flood was calculated as 4,200 m3/s. Including the 1911 flood in the flood-frequency analysis increased the estimate of the 100-year flood by 44 percent, which may not be realistic based on the lack of evidence of any larger floods. The threshold of 4,200 m3/s likely extends well beyond the historic time period may represent an upper limit for the magnitude of large floods in the San Juan River. A repeat photography analysis showed that substantial channel narrowing has occurred in the alluvial reach below Bluff, Utah, but that little or no channel narrowing has occurred in the canyon reach. Matched photos show a general increase in channel roughness that should be included in hydraulic models of paleofloods of the San Juan River.