Many north-facing slopes in the Yukon-Tanana Uplands of Alaska show signs of downhill movements in the form of hummocky surface and leaning trees. Measurements of movements and pore pressures were made at several sites in the Caribou-Poker Creek Research Watershed. It was found that, on slopes with inclinations near 30º, most of the movements occurred immediately after thaw when pore pressures were high.

To evaluate the slope stability, the soil strength was measured by direct shear tests. The strength of the moss layer and tree roots was evaluated by performing tensions tests on the roots and the moss-root complex. Results of stability analyses show that local failures involving individual wedges are likely. The displacements associated with a wedge slide would result in a hummock or step. The strength of the moss-root complex was found to be a significant factor in the stability of the wedge and of the step.

Key words: moss, roots, permafrost, pore pressure, shear strength, slope stability, thaw.