A CIRCA A.D. 1781 ERUPTION AND LAHAR'S AT MOUNT HOOD, OREGON—EVIDENCE FROM TREE-RING DATING AND FROM OBSERVATIONS OF LEWIS AND CLARK IN 1805-6

Cross dating of a western redcedar stump buried by a lahar in the Sandy River and of a buried Douglas fir in the tributary Zigzag River shows the trees died near the end of the AD 1781 growing season or by the earliest part of the 1782 growing season. A damaged survivor Douglas fir in the upper Sandy River valley showed extreme suppression of rings, starting with the 1782 ring, probably owing to abrasion trauma, while some undamaged survivor trees along the valley margins show an abrupt growth release beginning in 1782 due probably to the increased availability of light caused by removal of competitors by the flow. Thus, this lahar (the lahar which defines the Old Maid eruptive period) likely occurred before 1782, and perhaps before the end of the growing season in 1781. Cross dating of a scarred cedar along the Muddy Fork of the Sandy River shows a dramatic decrease in growth after 1786. It appears to have been damaged when a lake impounded by the Old Maid lahar in a tributary drainage breached catastrophically. The buried and scarred trees were cross dated using a local chronology developed from low-density "light rings" and other pointer rings.

The Sandy River lahar was triggered by a block-and-ash flow and pyroclastic surge originating at Crater Rock dome near the summit of Mount Hood. The coarse, debris-flow phase of the lahar buried forests up to 10 m along the Zigzag and Sandy Rivers for at least 35 km, and it likely continued more than 45 km farther to the Columbia River as a hyperconcentrated runout. The lahar was followed by probably decades of above-normal sediment yields, which caused more than 15 m of channel aggradation. Previous work bracketed the laharís age to the late 1700s using radiocarbon and lichenometric dating, minimum-age of trees growing on the lahar surface, and historical evidence from Lewis and Clark, who graphically described the shallow channel and quicksand conditions typical of such a volcanic disturbance and named it the Quicksand River (Moulton, 1988; Cameron and Pringle, 1986).