GEOMORPHIC AND STRATIGRAPHIC EVIDENCE OF OUTBURST FLOODS FROM A PERSISTENT MELTWATER CONDUIT, BERING GLACIER, ALASKA

Outburst floods from a persistent ice front conduit during the 1965-67 and 1993-95 surges were responsible for large-scale foreland modification. A knowledge of pre-flood topography serves as an essential frame of reference for assessing landforms gained and landforms lost during surge-related floods. An annual monitoring of post-flood erosion and deposition along an active peripheral drainage system provides a basis for distinguishing landforms produced by flooding from those formed by normal post-flood alteration.

Of the four islands constituting the eastern sector of the 30 km wide Bering piedmont lobe, only Weeping Peat Island was in the path of outburst floods. Thus, all flood-related foreland modifications are restricted to this island and adjacent Tsivat and Tsiu lake basins. The drumlinized and fluted terrain of other foreland islands (Peat Falls, Bentwood and Arrowhead) remains essentially as it was prior to flooding, although all were overridden during the surge.

Flood generated sandar, expressed as pitted outwash fans, represent the dominant depositional landform, whereas kilometer-scale scour basins uncovered by retreat are situated immediately up-glacier from ice front outburst sites. Within a few decades geomorphic evidence is progressively masked by a developing mantle of vegetation, whereas diagnostic sedimentary structures remain as indicators of paleo-flooding. These include sub-kilometer-scale scour and fill channels with meter-scale lag boulders, massive and repetitive graded bedding, and a conduit fill esker.