Paleohydrology and Flood Frequency in the Virgin River, Zion National Park: Potential for Reconstruction from Landslide-dammed Lake Sediments

The Virgin River drains an area of over 13,000 km2 in southern Utah, northern Arizona and southern Nevada before entering into Lake Mead as a major tributary to the Colorado River. Peak flood discharges occur during spring meltwater runoff with subordinate floods related to El Niño-influenced mid-summer convective storms (USGS gage data). These hydro-climatological conditions and narrow river canyons within the Virgin River catchment have created high flood hazards for communities. River gage records used to reconstruct flood frequency and recurrence intervals are limited to the last few decades. However, flood slackwater deposits (Enzel et al., 1994) and lake sediment records may be able to extend this record over the last several thousand years. A large landslide within Zion Canyon in Zion National Park dammed the North Fork of the Virgin ~ 8000 years ago and created a 60 m deep lake that existed until just after 3600 years ago (Biek et al., 2000). Lake sediments consist of alternating coarse (silty-sand and fine sand) and fine-grained layers (clay, clayey-silt, silt). Initial descriptions suggest that these coarse-fine couplets may represent annual deposition. Moreover, variations in couplet thickness may be related to changes in annual/seasonal discharge and flood frequency. Initial results and observation from these sediments are presented along with relationships to other paleoflood studies in the region