Paleoflood Record Reconstruction at an Archaeological Site on the Owyhee River, Southeastern Oregon

Stephanie Louise Vandal
July 2007

Abstract

The magnitude and frequency of late Holocene floods on the Owyhee River in southeastern Oregon were reconstructed from fine-grained flood deposits at three sites in the river canyon. The stratigraphy at the Birch Creek study site (BCSS) preserves a record of seven to nine large floods from the last 2800 years. Two additional study sites, the Iron Gate and Waterwheel, within a 5-km reach of the BCSS, showed 18-26 floods from the late Holocene to 1993 A.D. and 17-22 floods from 8600 B.P. to 1993 A.D., respectively. Hydrologic Engineering Center-River Analysis System modeling of the 1993 flood and several paleofloods suggests that the 1993 flood (1600 m³/s) was one of the largest floods to have occurred on the Owyhee River in at least the last 1400 years, and perhaps in the last 2800 years.

Artifacts found to date in the profiles and trench walls at the BCSS are consistent with ages suggested by radiocarbon analyses. Artifacts found at the Iron Gate site, approximately 2 km downstream from the BCSS, indicate extensive human use of the site during or after a period when it contained a localized wetland environment, but the overbank flood deposits since that time expose no obvious human occupation sites. In contrast, the BCSS upstream includes human artifacts interbedded with the paleoflood deposits.

Full Thesis (PDF format)