Introduction Recent investigations in the Tanana Flats of Interior Alaska (Figure 1), have identified large areas of unfrozen fen peatlands maintained by the discharge of groundwater through unfrozen gravels and silts (Racine and Walters, 1994). Where these unfrozen fens border slightly higher birch forest, underlain by per- mafrost, there is widespread evidence of bank collapse, forest drowning, and moat formation, suggesting the degradation of the permafrost underlying these forests (Figure 2). In contrast, where the fens border black spruce forest and shrub bog, there is little evidence of thermokarst. This study is a documentation of the evi- dence of preferential thawing of permafrost associated with birch forests and possible causes in relation to per- mafrost characteristics. Because of the long and complex history of the Tanana Flats in terms of both the surface and subsurface move- ment of glacial meltwater from the Alaska Range and the present mean annual temperature of about -3.3¡C in this area, we believe that permafrost conditions here are extremely dynamic. This is especially important in light of projected climatic warming scenarios. The present study is part of a larger project to investigate per- mafrost dynamics in the Tanana Flats. Our approach includes an attempt to understand, classify, and map the landscape-scale pattern of permafrost distribution in relation to vegetation and near-surface sediment type. Study area The Tanana Flats is situated on the distal slopes of a large alluvial fan complex built out of the Alaska Range on the south (Figure 1). Climatic fluctuations during the Quaternary caused glacial expansion and recession in the Alaska Range, which in turn built a broad slope of coalesced alluvial fans, pushing the Tanana River north- ward against the Yukon-Tanana Upland. The thick unconsolidated deposits of the fan complex document a long and complicated record of alternating cycles of silt and gravel deposition and erosion along with the for- mation and destruction of permafrost (Pw and Reger, 1983). The northwest portion of the Tanana Flats, where we conducted our studies, is dominated by thick (3 to 4 m) abandoned floodplain cover deposits and organic floa- ting-mat fens over gravel (Jorgenson et al. , 1996). Common vegetation includes birch (Betula papyrifera) forests, alder (Alnus tenuifolia) shrub swamps, floating- mat fens dominated by buckbean (Menyanthes trifoliata), Abstract The Tanana Flats is a wetland region located on the distal slopes of an extensive alluvial fan complex built out of the Alaska Range. Vegetation in the Flats consists of a mosaic of fen, birch forest, black spruce forest, shrub, and bog. Permafrost is not present in the fen and bog areas, but it exists on the bordering forested or shrub areas 0.5 to 2 m above water level. Our studies show that permafrost in the Flats is relatively warm at -0.2 to -0.7¡C, and that the distribution and characteristics of permafrost are related to the geobotanical conditions at a specific site. In general, permafrost is more ice rich and shows higher secondary porosity where finer-grained sediments (silts) are abundant. These are environments characterized by birch forest vegetation. Permafrost in areas of birch forest appears more susceptible to thaw and is currently showing signs of extensive degradation. James C. Walters, et al. 1109 CHARACTERISTICS OF PERMAFROST IN THE TANANA FLATS, INTERIOR ALASKA James C. Walters 1 , Charles H. Racine 2 , M. Torre Jorgenson 3 1. Department of Earth Science, University of Northern Iowa, Cedar Falls, Iowa 50614-0335, USA e-mail: james.walters@uni.edu 2. U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire 03755-1290, USA e-mail: cracine@crrel41.crrel.usace.army.mil 3. Alaska Biological Research, Inc., P.O. Box 80410, Fairbanks, Alaska 99708, USA e-mail: tjorgenson@abrinc.com PERMAFROST - Seventh International Conference (Proceedings), Yellowknife (Canada), Collection Nordicana No 55, 1998 ?