Karl Lillquist

Hydro‐isostatic deflection and tectonic tilting in the central Andes: Initial results of a GPS survey of Lake Minchin shorelines

Sufficiently large lake loads provide a means of probing rheological stratification of the crust and upper mantle. Lake Minchin was the largest of the late Pleistocene pluvial lakes in the central Andes. Prominent shorelines, which formed during temporary still-stands in the climatically driven lake level history, preserve records of lateral variations in subsequent net vertical motions. At its maximum extent the lake was 140 m deep and spanned 400 km N-S and 200 km E-W. The load of surficial water contained in Lake Minchin was sufficient to depress the crust and underlying mantle by 20–40 m, depending on the subjacent rheology. Any other differential vertical motions will also be recorded as departures from horizontality of the shorelines. We recently conducted a survey of shoreline elevations of Lake Minchin with the express intent of monitoring the hydro-isostatic deflection and tectonic tilting. Using real-time differential GPS, we measured topographic profiles across suites of shorelines at 15 widely separated locations throughout the basin. Horizontal and vertical accuracies attained are roughly 30 and 70 cm, respectively. Geomorphic evidence suggests that the highest shoreline was occupied only briefly (probably less than 200 years) and radiocarbon dates on gastropod shells found in association with the shore deposits constrain the age to roughly 17 kyr. The basin-wide pattern of elevations of the highest shoreline is composed of two distinct signals: (27±1) m of hydro-isostatic deflection due to the lake load, and a planar tilt with east and north components of (6.8±0.4) 10−5 and (−5.3±0.3) 10−5. This rate of tilting is too high to be plausibly attributed to steady tectonism, and presumably reflects some unresolved combination of tectonism plus the effects of oceanic and lacustrine loads on a laterally heterogeneous substrate. The history of lake level fluctuations is still inadequately known to allow detailed inferences of crust and mantle rheology. However, it is already clear that the effective elastic plate thickness is closer to 40 km than the 60–70 km crustal thickness in the central Andes and the effective viscosity is less than 5 1020 Pa s.

Historical Glacier and Climate Fluctuations at Mount Hood, Oregon

Abstract Terminus fluctuations of five glaciers and the correspondence of these fluctuations to temperature and precipitation patterns were assessed at Oregons Mount Hood over the period 1901–2001. Historical photographs, descriptions, and climate data, combined with contemporary GPS measurements and GIS analysis, revealed that each glacier experienced overall retreat, ranging from −62 m at the Newton Clark Glacier to −1102 m at the Ladd Glacier. Within this overall trend, Mount Hoods glaciers experienced two periods each of retreat and advance. Glaciers retreated between 1901 and 1946 in response to rising temperatures and declining precipitation. A mid-century cool, wet period led to glacier advances. Glaciers retreated from the late 1970s to the mid-1990s as a result of rising temperatures and generally declining precipitation. High precipitation in the late 1990s caused slight advances in 2000 and 2001. The general correspondence of Mount Hoods glacier terminus fluctuations with glaciers in Washington and Oregon suggests that regional, decadal-scale weather and climate events, driven by the Pacific Decadal Oscillation, play a key role in shaping atmosphere-cryosphere interactions in Pacific Northwest mountains. Deviations from the general glacier fluctuation pattern may arise from local differences in glacier aspect, altitude, size, and steepness as well as volcanic and geothermal activity, topography, and debris cover.

Stream Tables and Watershed Geomorphology Education.

Watersheds are basic landscape units that are fundamental to understanding resource and environ- mental issues. Stream tables may be an effective way to learn about watersheds and the dynamic processes, factors, and landforms within. We review the copious stream table literature, present new ideas for assembling stream tables, and provide a watershed approach to stream table exercises. Our stream tables compact size and low cost permits the purchase and use of multiple units to maximize active learning. The included stream table modules allow introductory students to experiment and observe the effects of factors–i.e., climate (Module A–Precipitation, Overland Flow, and Channel Initiation and Module B–Stream Discharge and Channel Formation), topography (Module C–Watershed Topography and Channel Formation), land cover (Module D–Watershed Cover Types and Channel Formation), and base level (Module E–Local Base Level Changes via Dams and Reservoirs) –on fluvial processes and landforms in a watershed. Course evaluations and exams show that students enjoy the stream table exercise more, and learn the concepts of fluvial geomorphology better, than via traditional topographic map and aerial photograph interpretation exercises.

The Post-World War II Origin and Evolution of Mountain Snowshoes and Mountain Snowshoeing in North America

The origins of mountain snowshoes can be traced back to the 1940s and 1950s, when small, maneuverable wooden frames were developed in the Adirondacks and Green Mountains. Fixed traction devices and forward hinge points originated in the Cascades in the late 1950s, enabling better access to steep terrain. The 1960s brought strong, lightweight aluminum-framed snowshoes from the Sierra Nevada. During this time, neoprene-coated nylon began to replace rawhide lacing and bindings in the Adirondacks, while neoprene-coated nylon decking was first used in the Cascades. Integrated plastic frame/decking snowshoes were a 1960s Rockies weight- and labor-saving innovation. From the Cascades in the early 1970s came hinged pivot bindings, with attached claws, that provided better traction and tracking. Integrated aluminum frame/decking snowshoes came from the Sierra Nevada in the 1980s. Fixed-pivot bindings and rear traction cleats for added maneuverability and stability originated in the Rockies and Sierra Nevada in the late 1980s and early 1990s. More substantial and user-friendly bindings also developed in the Green Mountains during this time. Further refined, integrated plastic frame/decking snowshoes from the Rockies came along in the 1990s. In the last decade, innovations from the Cascades and Rockies included integrated aluminum frame/traction snowshoes that further enhanced stability on steep slopes. Seventy years of research and development across North America has resulted in smaller, lighter, stronger, and more maneuverable mountain snowshoes. These innovations, and aggressive marketing initiated in the late 1980s, have helped snowshoeing become the fastest-growing winter sport in the U.S., with approximately five million participants.

Mass Wasting in the Swauk Watershed, Washington

Mass wasting evidence is common along the margins of the Columbia River Basalts. I identified, mapped, dated, and assessed the environment of nearly 160 discrete slope failures (excluding rockfall) along the margins of the Columbia River Basalts in the Swauk watershed of central Washington. Rotational slides, translational slides, flows, and complex slide-flows were identified via topographic map, airphoto, and field analysis. Geographic information systems analysis revealed that these features cover 38% of the watershed. Translational slides are the most numerous of the slope failures, whereas complex slide-flows cover the most area. I placed each slope failure into a relative age category (active, inactive-young, inactive mature, and inactive-old) based on the characteristics of the main scarp, lateral flanks, internal morphology, vegetation cover, and toe relationships. Most Swauk watershed slope failures are inactive-mature. Organic sediments from an inactive-mature sag pond formed ∼6880 14C yr BP, whereas inactive-young sediments dated at ∼5930 14C yr BP. Inactive slope failures are often associated with steep slopes, inclined beds, incompetent geologic units, or streamcuts. Streamcuts, roadcuts, or clearcuts typically accompany active slope failures. Rain-on-snow events and associated mass wasting in winter 1996 provide a plausible trigger analog for inactive mass wasting. Rockfall deposits cover ∼29% of the watershed, range from inactive to active in age, and occur atop pre-existing slope failures in well-jointed Columbia River Basalts. Mass wasting has played a key role in shaping the topographic and hydrologic patterns of the watershed. [Key words: mass wasting, watershed, Washington state, Columbia River Basalts, rain-onsnow.]