A Record of Late Holocene Paleomegafloods From Oxbow Lakes in the Sacramento Valley, California

A number of recent studies show that California has been subject to periodic megafloods over the past two millennia. In this paper I show that a paleoflood record is preserved in oxbow lake sediments in the Sacramento Valley. I report on the sediment stratigraphy of cores recovered from Little Packer Lake and Razor Slough, two oxbow lakes located in the middle reach of the Sacramento River in Glenn County, California. An age model for the cores was established using AMS radiocarbon dates and the first appearances of non-native pollen types. The chronology indicates that Little Packer Lake formed as a neck cut-off oxbow around 800 years ago, and Razor Slough shortly afterwards. The mean sedimentation rates for both lakes is about 1 mm/yr. The cores from the two lakes show basically the same sequence of major flood events. Sediment samples taken from the cores were analyzed for magnetic susceptibility, pollen content, organic content, and bulk density. Segments of the lake cores were x-rayed. X-radiography revealed distinct layers of flood deposited sediment. X-rays, organic content and bulk density determinations show that flood deposits differ significantly from non-flood sediments. As a check on the hypothesis that the differences in sediment properties are ineed the result of flooding, I compared the lake record with the known history of major floods in the Sacramento Valley over the past 150 years from published records. The two records are very similar. In addition, the timing of some of the earlier flood events in the oxbow lake record is close to that reported for other prehistoric floods in central California. One implication of this study is that the oxbow lake sediments can provide reliable and replicable records of prehistoric flooding. While the records reported here cover the last 800 years, a more extensive coring effort in the same area could potentially extend the record back several thousand years.