Glacier National Park Montana

Summary:

The signatures of glacial erosion are ubiquitous in alpine landscapes. Cirques – bedrock basins near mountain crests - are particularly intriguing features in that they are often used as proxies for past climate, e.g., cirque elevations are assumed to represent elevation of the zero-degree isotherm during the Last Glacial Maximum (~10,000 years ago). The detailed physical processes acting to form cirques, however, are not well understood. I have been measuring glaciological and geomorphic parameters related to cirque development at Grinnell Glacier in Glacier National Park in an effort to examine how these bedrock basins are formed. This includes measuring air temperature, snowpack thickness, water discharge, and ice motion using GPS over the summer months, as well as mapping landslide deposits to determine rates of non-glacial erosion. In summer 2005 we conducted lake coring in Swiftcurrent Lake and Lake Josphine, downvalley of Grinnell glacier, in an effort to better constrain rates of erosion and examine environmental change in the basin. About 20 meters of lake sediment were collected, spanning ~10,000 years of time. See Grinnell Lake Cores for more information.

In summer 2004 and 2005, I conducted research with a team of 3-4 students staying in Many Glacier and making daily trips to the glacier. This required hiking ~13 miles round-trip each day, sometimes with heavy packs! See photos for more information.

Scientific Motivation:

Much research by glaciologists focuses only on the dynamics of ice motion and climatic controls on glaciers, whereas geomorphologists tend to ignore the role of glaciers when looking at the forces responsible for landscape evolution over geologic time. My research examines the role of glaciers in shaping alpine landscapes; namely, how do glaciers erode and deposit sediment, why do they produce mountain landscapes that look so different from those dominated by fluvial systems, and how do the physical ‘signatures’ of glaciation (like cirques and moraines) relate to the climate history they have been subjected to? The purpose of my proposed summer research is to initiate measurements of glaciologic and geomorphic processes that we believe are most important for the evolution of cirque basins. My long-term goal is to provide a physical explanation, based on field evidence and numerical modeling, for cirque development. In addition, I hope to provide evidence to illuminate the relationship between erosional and depositional glacial landforms and climate history. Understanding the relationship between climate (forcing) and landforms (result of physical processes that move water, ice and sediment) is an important way to decipher the puzzle of our planet’s history, and continues to be at the heart of surface processes research.

The development of the glaciated valley profile by temperate mountain glaciers is a classic problem in alpine geomorphology. Twentieth century geologists noted unique glacial features such as stepped long valley profiles, hanging tributary valleys, cirques, fjords, paternoster lakes, and the U-shaped cross-sections of glacial valleys (Gilbert 1904; Tarr 1907). These landforms are intriguing because they are ubiquitous in glaciated landscapes and rare in fluvial settings, suggesting that valley glacier mechanics operate on the landscape in a distinctly different way than do rivers. In places such as Yosemite Valley and Glacier National Park, this signature of glaciation is evident; however, no quantitative explanation of the evolution of these features exists. Although many mass balance and glaciological numerical models exist, only three (Oerlemans 1984; Harbor 1992; MacGregor et al. 2000) have been extended to consider landform development through erosion. Cirques, or bedrock overdeepenings at/near the headwalls of glaciated valleys, are particularly intriguing features in that they are extremely common in glaciated mountain ranges. Importantly, the elevations of cirque floors are often used as proxies for Last Glacial Maximum temperature and/or snowline patterns, or as past equilibrium line altitudes (ELA’s). However, their use as a paleoclimate indicator is not supported by process-based studies, and possible mechanisms for their formation, including increased sliding and therefore erosion at the ELA, are only hinted at in the literature (e.g., Trenhaile 1977, p. 446).

Field Measurables: Grinnell Glacier

Temperature and Climate: Using Hobo temperature sensors, we will monitor air temperature at several locations on the glacier over the course of the summer. This will be important for modeling of melt rates across the ice surface and within the contributing drainage basin. In addition, we will use regional climate records to examine longer-term trends in temperature and precipitation.
Mass Balance: Mass balance is an important term in ice dynamics calculations. Glacier mass balance is the sum of winter precipitation (positive snow contribution) and summer melt (loss of snow and ice). We will make measurements of snowpack across the glacier using both snow probes and mapping of the snow surface with GPS. Melt (loss of snow) will be made using snow stake measurements.
Ice Dynamics: Using GPS, we can characterize cirque glacier motion over the summer, including horizontal and vertical components of ice velocity. By looking at ice motion at several locations on the glacier we can calculate subglacial sliding (a critical parameter for subglacial erosional processes). Our data suggests Grinnell Glacier is moving at 5 cm/day, of which ~4 cm/day results from basal sliding. This value is greater than that measured by Anderson and others (1982) in the 1970’s, suggesting that the glacier is speeding up, despite dramatic shrinkage.