LATE QUATERNARY GLACIAL HISTORY OF THE DESCHUTES RIVER BASIN III: THE TUMALO CREEK AND UPPER DESCHUTES RIVER WATERSHEDS; CENTRAL CASCADE RANGE, OREGON

The Three Sisters volcanic platform in the Oregon Cascade Range is a youthful, dynamic landscape comprised of four major Pleistocene stratovolcanoes forming an “L” shaped highland. The platform is anchored at its ends by the North Sister and Broken Top Volcanoes, primarily constructed during the late middle Pleistocene, and likely extinct by about 100,000 years ago. Middle and South Sister Volcanoes lie near the bend in the “L” and were formed by concurrent late Pleistocene eruptions beginning roughly 50,000 years ago. Tumalo Creek and the upper Deschutes River drain east and south from this highland and both watersheds exhibit an extensive, well-preserved cover of glacial drift. Relative dating of surficial deposits indicates only one period of glacial activity that we correlate with the late Pleistocene Cabot Creek glaciation of Scott (1977) and oxygen-isotope stage 2 (the LGM). Deposits of Scott’s older Jack Creek and Abbott Butte glaciations are presumably buried beneath a blanket of Cabot Creek material in these drainages, a pattern of ice cover matching that of Whychus Creek (Bevis et al., 2011). A more extensive Cabot Creek glaciation can likely be attributed to growth of a considerably larger volcanic highland source-area during the interglacial period between Jack Creek and Cabot Creek glaciations. The surface of this former alpine glacial system was reconstructed by dividing it into three lobes, establishing ice surface contours for each lobe by assuming convergent and divergent flow in accumulation and ablation areas, and modifying ice surface contours until basal shear stresses computed from ice thickness and surface slope were within 50 to 150 kPa. Using an accumulation-area ratio of 0.70+/-0.05 for the Tumalo-Upper Deschutes ice cap, we determined the weighted-mean equilibrium-line altitude (ELA) during Cabot Creek glaciation to be about 5951 ft (1814 m), similar to its counterpart in the Whychus Creek watershed (Bevis et al., 2011). However, the ELA for each individual sublobe decreases southward mirroring the reduction in height and massiveness of the Cascade Crest away from the Three Sisters area. Modern precipitation patterns exhibit a distinctive rainshadow effect from the Three Sisters and a higher paleo-ELA in the area is expected assuming that former precipitation patterns were similar to today.