Technical description. As shown in a SGER-funded pilot study designed to promote Alaska?s Kennicott Glacier as a natural laboratory, glaciers slide whenever their hydrologic system is insufficiently developed to accommodate water inputs, either from melt or from a draining lake. The conceptual model suggested in a paper from the SGER study is being cited extensively to explain new observations in Greenland. The key to understanding the sliding of a glacier is that the hydrologic system beneath a glacier is always in a state of transience; water inputs vary widely, as do the hydraulic efficiencies of the channelized and unchannelized portions of the hydrologic system. The proposed research teams a geomorphologist with experience in both glacial fieldwork and analysis of the hydrology-sliding connection, with a hydrologist whose research includes the hydrology of faults and karst systems that are both pertinent to the glacial hydrology?sliding problem. The team will link fieldwork on Kennicott Glacier with numerical models to develop a rigorous understanding of the evolving hydrologic system and the basal sliding that it inspires. Task 1) Kennicott Glacier field experiment. This easily accessible 40-km long, 4-km wide glacier is ideal to study the connection between hydrology and glacial motion. Its hydrologic system is perturbed diurnally, by a major spring event each summer, and by an annual outburst flood. Several side-glacier and small supra-glacial lakes serve as natural manometers to document seasonal evolution of the drainage network, and passage of the outburst flood. The research will document the diurnal, seasonal, and flood-related inputs and outputs of water from the glacier, and the associated sliding response over two full years. GPS-derived ice speeds will document the horizontal and vertical displacement fields, which vary by several-fold on daily, seasonal and flood timescales. These data sets will serve as a rigorous test case for models of the hydrology-sliding link. Task 2) Numerical modeling of subglacial hydrology. Inspired by both old and new data on the Kennicott Glacier, and a working 1D hydro-sliding model, hydrologic models of differing complexity will be developed. Network and grid-based 2D distributed numerical models will have dynamic aperture geometry and hydraulic properties that allow transitions from distributed ?linked cavity? to ?conduit-like? behaviors resulting from temporally and spatially variable melt inputs. The models will predict spatial and temporal patterns of horizontal and vertical ice motion, and measurable features of the hydrologic system, including solute concentrations at the outlet. They will be immediately employed to diagnose the results from the 2006 pilot study, and will be refined as the full annual cycle is illuminated by the field efforts.
Broader significance. Glaciers transport ice down their valleys both by deformation of the ice and by sliding against their beds. Glacial research is motivated in part by the fact that the loss of ice from glaciers and ice sheets profoundly impacts both sea level and water resources in glaciated watersheds. While glaciologists understand the deformation of ice, sliding remains poorly understood. In addition, as only by sliding do glaciers erode the landscape over which they move, understanding of glacial landscapes that dominate many national parks requires knowledge of how sliding works. Variations in glacier sliding are clearly linked to the seasonal and daily delivery of meltwater into the glacier, but the linkage between the state of the water system in the glacier and this sliding remains unclear; no physically-based model of sliding exists. Recent acceleration of sliding of glacial outlets from the Greenland Ice Sheet has been shown to mimic the seasonal sliding history observed on alpine glaciers, making alpine glaciers an appropriate and less expensive laboratory to probe the hydrology-sliding connection that may drive sea level rise from ice sheets. The proposed research will make use of the spectacular natural laboratory of the Kennicott Glacier at the entrance to Alaska?s Wrangell St. Elias National Park. The research will generate a detailed data set of glacier sliding and of the evolving hydrologic state of the glacier that will guide the development of theoretical models of sliding. Numerical models in this research take advantage of the analogy between the ice-rock contact at the bed of a glacier and the rock-rock contact across a fault. In both cases, the interface is rough, with patchy contacts and gaps; and in both cases, sliding at this interface is promoted by high water pressures. Models constructed in this study will capture the full complexity of the evolving subglacial hydrologic system, including rapid inputs of water from the surface and from draining lakes that promote sliding, and feedbacks that serve to regulate the sliding.