The volcanic mudflow occures as an accident in the process of the landform formation. It builts up such unusual landforms as the mudflow hills and river terraces caused by the volcanic activity. In Japan there are many active volcanoes having the mudflow deposits on their mountain-foots. Some of the deposits can be transported to the alluvial land near the volcanoes. But there are few studies adout the geomorphological changes in the alluvial land due to the volcanic mudflow. The paper deals with the case that the volcanic mudfiow directly gave effect to the valley plain alone. It aims at clarifing how the volcanic mudflow has influences on the formation of the alluvial land-forms directly and indirectly. It is well-known that the Bandai Volcano broke away by one-third due to the steam explosion on the 15th of July, 1888 and that the huge dry mudflow depo~fits filled up the valleys in the upper reaches of the Nagase River. As the result of it, many mudflow hills and lakes were formed. The area studied can be divided into two parts: the valley pla, in and the alluvial plain of the Nagase Rivers. The Nagase River originates in the north foot of the Bandai Volcano and flows into Inawashiro Lake. When the mudflow deposits filled up the valley, many mudflow hills were formed on the valley plain. The average height of the mudflow hill reliefs is about 30 metres at the neck of the valley near Tsuchiyu-Dani. At the lower reach of the neck the height of the mudflow hills abruptly become small. It seems that the mudfiow deposits have a thickness of more than 40 metres in the valley. The maximum diameter of the boulders found in the mudflow are 500 to 700 centi metres at the lower reach of the neck. Since the time when the valley of the Nagase River was filled up by the mudfiow deposits, fioods had occured very frequently until the construction of the dams in 1916 in the alluvial plain of the Nagase River. The small natural levees were formed along the river course every flood. The location of the Nagase River mouth advanced about 500 metres offshore and it made the delta temporarily large. As the result of the construc-tion of the dams, the river mouth has today gone back to its original location. Ultimately the Ura-Bandai dry mudflow caused a great direct geomorphological influ-ence, with the formation of the mudfiow hills, upon the valley plain of the Nagase River. The indirect effects of the mudflow for the alluvial plain of the lower reach of the Nagase River were a slight expansion of the delta and the formation of the small natural levees. But the effects were not so great as estimated. The reasons why the landforms of the alluvial plain were little changed may safely be put as follows : l. The m,udflow was dammed up by the neck of the valley. 2. The foreset slope of the Nagase delta was so steep that the mudflow deposits did not advance the delta offshore. 3. The water volume of the Nagase river has been controled by the dams and waterways since 1916.
