Holocene Paleoflood Hydrology of the Lower Deschutes River, Oregon

Flood deposits at four sites along the lower Deschutes River, Oregon, were analyzed to determine magnitude and frequency of late Holocene flooding. Deposit stratigraphy was combined with hydraulic modeling at two sites to determine ranges of likely discharges for individual deposits. Combining these results with gaged flood data provides improved flood frequency estimates at the Axford site. The completeness and age spans of preserved flood chronologies differed among the four sites, but results were consistent for the largest floods of the last 5000 years. Single floods exceeded 2860-3800 mVs -4600 cal yr BP, 1060-1810 mVs -1300 cal yr BP, and 1210-2000 m3/s \u3c290 cal yr BP (corresponding to the historic flood of 1861). No floods have exceeded 2860-3770 mVs since the flood of -4600 cal yr BP. Incorporating these results into a flood frequency analysis based on maximum likelihood estimators gives slightly higher flood quantile estimates and narrower confidence limits compared with analysis of gage data alone. Discharge and 2(5 uncertainty for the 100-yr flood calculated using combined paleoflood and gaged records is 1120 +310/-240 mVs, compared with 930 +650/-250 m3/s from analysis of only gaged floods. This revised estimate for the 100-yr flood is slightly greater than our estimate of 1060 m3/s for the February 1996 flood at Axford, a finding consistent with historical records of two floods comparable to the 1996 flood in the last 140 years and with stratigraphic records of several like floods during the last ~1000 years