AUFEIS ACCUMULATIONS IN STREAM BOTTOMS IN ARCTIC AND SUBARCTIC ENVIRONMENTS AS AN INDICATOR OF GEOLOGIC STRUCTURE

Aufeis (German for "ice on top") accumulates during winter along stream and river valleys in arctic and subarctic environments. Aufeis forms by upwelling of river water behind ice dams, or by ground-water discharge. The latter mechanism prevails in high-gradient alpine streams as they freeze solid. Ground-water discharge is blocked by ice, perturbing the steady-state condition, and causing a small incremental rise in the local water table until discharge occurs along the bank at the top of the previously formed ice. During winter, successive freezing of the onlapping ice layers can lead to aufeis accumulations several meters thick. In two visits to the interior of Alaska during winter 2001 and spring 2002, extensive aufeis was observed. Seeps issue from the sides of the valleys leading to ice buildup and giving the ice surface a concave shape perpendicular to the stream direction. This concavity is evidence for ground-water discharge along the length of the aufeis, as opposed to discharge from a single upstream point. During spring thaw, stream flow is commonly observed out of the normal channel, having been displaced horizontally and/or vertically. This stream displacement is evidence for the occlusion of the channel (and probably the shallow sediments) by the aufeis. The location and extent of aufeis may be useful to relate local hydrology to geologic structure. In our studies in the Goodpaster River basin, the thickest (>3m) and most extensive aufeis (100's of m to km along valleys) coincided with locations of laterally extensive (>5 km) mapped high-angle brittle fault zones. This observation suggests that the faults are at least partially hydraulically conductive. These faults are aligned along the drainage valleys containing the aufeis. Minor or no aufeis was observed in many other small drainage valleys where no laterally extensive structures have been mapped, implying more than a topographical effect or surface-deposit discharge. Thus, the presence of thick, laterally extensive aufeis in high-gradient streams may be a useful aid to geologic structural mapping in arctic and subarctic climates.