Evidence for a large middle Holocene flood event in the Pacific southwestern United States (Lake Elsinore, California)

While drought represents a serious threat to the Pacific southwestern United States, floods represent an equally formidable threat. This risk is so significant that the U.S. Geological Survey created the ARkStorm Project. This project aims to prepare California for a future storm(s) on the scale of the disastrous A.D. 1861–1862 events. Unfortunately, our knowledge of premeasurement floods in the Pacific southwestern United States is sparse. To date, the best paleoflood record consists of flood layers in the Santa Barbara Basin, spanning the past 9000 calendar yr B.P. (cal yr B.P.). As an alternative to marine archives, the lakes of the Pacific southwestern United States represent untapped resources for possible premeasurement flood reconstructions. Here, we present evidence for a flood between ca. 4860 and 4820 cal yr B.P. using sediment from Lake Elsinore core LEGC03-4. Core LEGC03-4 is predominantly clayey silt with occasional sandy silt units of variable centimeter-scale thickness. Here, we focus on a specific core section between 350 and 315 cm, where an ~11-cm-thick “unusual” sediment unit (330–319 cm) is well preserved and complete. The core section was analyzed for a variety of physical and chemical properties, including magnetic susceptibility, loss-on-ignition (LOI) at 550 °C and 950 °C, grain size, Corg:Ntotal ratios, and δ13C(bulk organic matter). The unit is characterized by an erosional basal contact and microflame structures. It is normally graded, with laminae occurring in the upper section of the unit. It contains predominantly terrestrial organic matter, and the upper boundary is gradational. It is coeval with the fourth highest sand peak in a previously dated central basin core. Consequently, it is our conclusion that the unusual sediment unit represents a turbidite associated with a large flood-producing precipitation event with a maximum limiting age between 4860 and 4820 cal yr B.P.