Terrestrialization and vegetation succession in cut-off channels were studied in the Nishibetsu River, Hokkaido, Japan, to clarify causal factors that governed the spatiotemporal change in vegetation over the last 100 years. According to aerial photo interpretation of 128 cut-off channels, terrestrialization and vegetation succession began on the river banks of main channels in natural river sections without artificial modification. In channelized sections with artificial levees, terrestrialization of the channels depended on the time when they cut off. Channels that cut off from the river before channelization underwent gradual terrestrialization and succession because the water table level dropped after river incision in the straightened channels. In contrast, little terrestrialization occurred in the cut-off channels that were formed by channelization where artificial levees decreased flood frequency to reduce sedimentation in the cut-off channels.<br>Species composition of the terrestrial vegetation changed in cut-off channels: vegetation dominated by the pioneer species Salix pet-susu changed to swamp-forest species such as Fraxinus mandshurica var. japonica, Ulmus davidiana var. japonica, and Alnus hirsuta. This change occurred on alluvial plugs 40–60 years after being cut, whereas it occurred in interior areas ca. 25 years after being cut. The vegetation succession was probably caused by changes in the frequency and duration of flooding; flooding was reduced by the raised land surface of alluvial plugs. The succession in interior areas may have been influenced by changes in groundwater level fluctuations; the water table was stabilized by the sedimentation of silt and clay.<br>Microlandforms with various surface elevations are one of the important factors controlling the spatial arrangement of terrestrial vegetation. Trees were arranged in the order of surface elevation as follows: Ulums davidiana var. japonica and Syringa reticulata; Fraxinus mandshurica var. japonica, Alnus hirsuta, and Sambucus racemosa subsp. kamschatica; and herbs distributed were Sasa kurilensis, Equisetum hyemale, Urtica platyphylla, Carex caespitosa, and Carex rhynchophysa.<br>There were differences in the terrestrialization processes between upper and lower alluvial plugs. Terrestrialization and therefore vegetation succession on the lower plugs were delayed by erosion that occurred when flooded water was discharged through the plugs. The ground surface of lower alluvial plugs consisted of coarser sediment (sand) than that of upper plugs, resulting in differences in vegetation between upper and lower plugs. In the interior areas of cut-off channels, old landforms that had formed before being cut, such as terraces, provided diverse environments for various types of vegetation. In addition to such old landforms, debris flows from valley side slopes modified the vegetation in the interior areas.