CLIMATE AND KINEMATICS OF NORTH CASCADES ROCK GLACIERS

Rock glaciers are common in many mountains, particularly those with cold, relatively dry polar or continental climates (e.g., Brookes Range, Rockies) where discontinuous permafrost also occurs. As a result, rock glaciers have been used to infer distribution of permafrost and permafrost climates (i.e., mean annual temperatures <-2˚C). However, rock glaciers also occur in some maritime mountains such as the Southern Alps and the North Cascades where large zones of permafrost seem unlikely. This apparent incompatibility begs the question: do rock glaciers in the North Cascades represent previously unrecognized zones of permafrost conditions? Or conversely, are rock glaciers not dependable indicators of such climates? To test these questions, our study has three goals: 1) to catalogue and map the extent of active North Cascade rock glaciers, 2) to investigate the thermal regimes and climate at two rock glaciers, and 3) to test the value of Terrestrial Laser Scanning (TLS) to constrain movement of rock glaciers.

Based on aerial and satellite imagery, we identified 10 rock glaciers in the North Cascades that appear to be actively flowing based on established morphologic characteristics (e.g., over-steepened termini, convex surfaces). These rock glaciers primarily occur east of the range crest on north to northeast facing slopes and cirques. On two of these, we assessed climatic conditions and detailed movement over the course of 15 months. We placed temperature loggers at several places on each rock glacier, at the surface and to depths of up to 3 m in the coarse rubble. The loggers recorded temperatures six times a day for ~15 months. Summer surface temperatures of these sensors correlate well to nearby SNOTEL measurements. Average summer temperatures at depth were generally cooler, more muted, and lagged surface temperatures by 2-7 hrs. After the first major snowfall, temperatures at depth generally stabilized at –1 °C to –4 °C, although at some sites temperatures increased with depth. TLS scans with an ILRIS 3D reveal movement rates up to 12-cm/yr on one rock glacier; TLS data for the other rock glacier are still being processed. Our preliminary results remain inconclusive regarding our original question, but suggest that rock glaciers may not be unequivocal indicators of alpine permafrost.