Riverbank freeze‐thaw erosion along a small mountain stream, Nikko volcanic area, central Japan

Processes of riverbank erosion involve fluvial entrainment, mass failure and subaerial erosion. The role of freeze‐thaw action in these erosion processes was investigated along a small mountain stream in a region with seasonal frost. Riverbank profiles, horizontal erosion, soil temperature, frost depth and soil water content were monitored over 20 months including two winters. Diurnal and annual freeze‐thaw action directly triggers subaerial erosion and indirectly contributes to mass failure and fluvial entrainment. The rate of subaerial erosion reaches a maximum during the thawing period, reinforced by increasing water content and decreasing soil hardness. The subaerial erosion, which occurs uniformly along the riverbank and recurs every year, contributes to progressive deepening of a notch developed above the water level as both preparatory and erosive processes. The overdeepening of the notch decreases bank stability, followed by occasional mass failure immediately after the thawing period and periodic fluvial entrainment during the summer flood period. The annual amount of subaerial erosion is comparable to that of fluvial entrainment. These observations demonstrate that freeze‐thaw erosion plays a fundamental role in erosion of riverbanks subject to deep seasonal frost. Copyright © 2006 John Wiley & Sons, Ltd.