LONG-RUNOUT DEBRIS AVALANCHE IN RAINBOW CREEK AT MOUNT BAKER, WASHINGTON

Mount Baker, 50 km east of Bellingham, WA is one of the youngest stratovolcanoes in the Cascade Range. The modern cone began its activity ca. 50,000 years ago on the terranes of several older eruptive centers, one of which erupted the lava that now forms Lava Divide on the east flank of Mount Baker ca. 300,000 years ago.

The Mount Baker complex has a long history of slope failures from its flanks. In the late 19th century, approximately 15-20 million m³ of andesitic lava and poorly consolidated lava-flow breccia fell from the north side of Lava Divide into the valley of Rainbow Creek, running 140 m up the opposite valley wall. The deposit, known as the Rainbow Creek debris avalanche, extends 10.5 km through Avalanche Gorge to the confluence of Rainbow and Swift Creeks.

Our research focused on two main questions: 1) Why was the debris avalanche so mobile, flowing much farther than a normal alpine debris avalanche? 2) Was there more than one avalanche? To answer the first question, we collected matrix samples of the flow at 11 longitudinally dispersed sites and analyzed the sand, silt, and clay content. Additionally, we analyzed outcrop photos to determine the proportion of matrix and the distribution of coarse clasts up to boulder size. We infer that the mobility of the avalanche is due to a high percentage of fine material (8-21% silt and clay) that lubricated the flow. There are two unusual sources of fine particles: material cataclastically disaggregated during the near-vertical fall from Lava Divide, and clay-rich glacial till (from the Cordilleran Ice Sheet) entrained from the valley floor. To answer the second question, vegetation patterns in a 1928 aerial photo suggest a subsequent, smaller avalanche. Dendrochronology, field observations, and the accounts of J. Morovits, a pioneer geologist in the area from 1891-1918, suggest that the main avalanche occurred in 1890-1891. A second avalanche may have extended from the same source area to a point 4.7 km downstream. It likely occurred between 1918 and 1928, with the older limit based on Morovits' departure in that year (we believe he would have recorded the event) as well as differences in vegetation maturity apparent on the aerial photo. Further work is needed to define this possible second flow.