PALEOFLOOD RECORD RECONSTRUCTION AT AN ARCHAEOLOGICAL SITE ON THE OWYHEE RIVER, SOUTHEASTERN OREGON

The magnitude and frequency of late-Holocene floods on the Owyhee River in southeastern Oregon were reconstructed from fine-grained flood deposits within an archaeological site in the river canyon. The Owyhee River, located in the northern Great Basin, originates in northeastern Nevada, flows into Idaho and Oregon where it follows the Oregon-Idaho border until it drains into the Snake River in Idaho.

Examination of the stratigraphy at the study site has shown that six to nine large floods have occurred in the last 2700 years. These flood units were identified in both riverbank profiles and throughout archaeological trench walls. Artifacts found to date in the profiles and trench walls are consistent with ages assigned by radiocarbon analyses. Paleoflood deposits were described at two additional locations within a 5 km reach downstream of the main site. The site farthest downstream contains 17-22 flood deposits ranging in age from 7800 yrs BP to the flood deposited in 1993. The other site, approximately 2 km downstream from the main site, contains artifacts that are eroding out of the boundary between a sequence of burrowed clay and silt sediments and overlying sandy flood deposits. This site indicates 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 main study site upstream includes human artifacts interbedded with the flood deposits. Pending age analysis will reveal whether or not the frequency of the floods varied within that period.

The paleoflood record also provided a means to assess the recurrence probability of a particularly large historic flood of 55,700 ft³s^-1 (1577m³s^-1) that occurred in 1993. The stratigraphic evidence from this project indicates that it was the largest flood in at least the last 2700 years. Preliminary data collected in recent years suggest that 1993 flood may have been one of the largest to have occurred on the Owyhee River since the mid-Holocene.