THE SANDY RIVER DELTA, OREGON: A RECORD OF LATE HOLOCENE VOLCANISM AT MT. HOOD AND SEDIMENT INTO THE COLUMBIA RIVER

The Sandy River has formed a 10-km² delta at its confluence with the Columbia River, approximately 35 km east of Portland, Oregon. This delta is composed largely of sediment that traveled 75 km down the Sandy River valley and into the Columbia River during two late Holocene eruptive episodes of Mt. Hood. Stratigraphy exposed along two cutbanks of the Sandy River and in six drill-cores in the Sandy River Delta reveals at least two lahars from the Timberline eruptive episode. Radiocarbon dating of material within and below the lahars indicates the lahars were emplaced between A.D. 300 and A.D. 600. These lahars are overlain by up to 3.5 m of cross-bedded, lithic-rich sand deposited by post-Timberline aggradation stream flows and hyperconcentrated flood-flows. Radiocarbon dating of material within overlying Columbia River sediment indicates the post-Timberline aggradation phase persisted less than 300-400 years. Total aggradation of the delta was as much as 7.5 m above present low-flow elevation of the Sandy River. Timberline-age deposits are overlain locally by deposits of lithic sands associated with the Old Maid Flats eruptive episode of the early 1780s. Filled paleochannels indicate approximately 3.5-4 m of post-Old-Maid-Flats aggradation in the decades following that eruption and up to 8 m of Columbia River and Sandy River overbank deposits.