Alluvial fans and alluvial terraces are well developed along the rivers around the Matsumoto Basin, Central Japan. Stratigraphy and morphological features of fluvial surfaces and mountain slopes were examined to clarify the landform development of the river basins through the Last Glacial Age and the Post-Glacial Age. The results are as follows;
1) Fluvial surfaces formed during the Last Glacial Age and the Post-Glacial Age are divided into four: the Hata surface (formed in the earlier half of the Last Glacial Age), the Moriguchi surface group (in the latter half of the Last Glacial Age), the Kamikaido surface group (in the Late Glacial and Post-Glacial Age) and the Oshide surface (at present). The Hata surface is a depositional one, while the Moriguchi surface is an erosional one. The Kamikaido and Oshide surfaces are generally depositional in the alluvial fan but erosional in the mountainous area.
2) Surveys on the deposits on mountain slopes indicate that periglacial slow mass movement on the slopes had been active in the latter half of the Last Glacial Age, yet it diminished in the Late Glacial and the Post-Glacial Age. Active transportation of materials by running water occurred along the tributaries and hollows on mountain slopes in the Late Glacial and the Post-Glacial Age, resulting in the dissection of mountain slopes. The changes in the processes on mountain slopes can be attributed to the climatic change from the cold climate in the Last Glacial Age to the wet and warm one in the Post-Glacial Age.
3) The increase in the effects of running water on mountain slopes in the Late Glacial and Post-Glacial Age suggests that river discharge increased in these ages. On the other hand, the changes in the activity of material movement on mountain slopes indicates that the debris supply from the slopes increased or was unchanged in these ages. Thus, the degradation of rivers in the mountainous areas during these ages is ascribable to the increase in river discharge. It is considered that the active transportation of gravels to the lower reach occurred during the Late Glacial and Post-Glacial Age due to the discharge large enough to remove the debris supplied from mountain slopes, resulting in aggradation in the alluvial fan areas.
4) The characteristics of terrace deposits and the previous studies of paleoclimate in Central Japan suggest that river discharge in the latter half of the Last Glacial Age was not larger than that in the earlier half. Thus, the degradation during the latter half of the Last Glacial Age is ascribable to the decrease in debris supply. The decrease is probably due to the diminution of material movement along tributaries and hollows on mountain slopes which was induced by the decrease in precipitation in the latter half of the Last Glacial Age.