FLOOD HISTORY OF THE YAKIMA RIVER, WASHINGTON

The main purpose of this project was to produce a revised flood-frequency record for the Yakima River. The number and timing of floods in the last few hundred years were determined by describing the floodplain stratigraphy at three sites in the Yakima River canyon south of Ellensburg, Washington. Samples from these sites were submitted for radiocarbon and tephra (volcanic ash) analysis, which provided a chronological timeline for the flood deposits. Results from radiocarbon and tephra chronology analyses indicate that 5 floods emplaced overbank deposits at site A2 10-7-01. These results indicate that the flood deposits were made between 180 (± 50) BP and the 1980 Mt. Saint Helens Ash deposit. The large flood of 770 m3 s-1 (27,200 cfs) in February, 1996 emplaced sediments on top of the ash layer, which helps constrain the discharge threshold required to overtop the existing deposits. Determining the magnitude of past floods will be done by surveying channel cross-sections and using the HEC-RAS hydrologic computer model to calculate the discharges associated with the flood deposits. Finally, combining the information on the magnitudes and frequencies of past floods from the sedimentary deposits with the modern record of floods will help develop a long-term flood-frequency record for the Yakima River.