Terrace Topography in the Tsugawa Basin and its Vicinity

Abstract

In the Tsugawa Basin, Niigata Prefecture, there are several levels of river terraces along the course of the Agano and the Tokonami Rivers.
The terraces along the Tokonami River are classified into three groups; the higher, the middle and the lower terraces. The terrace surfaces are generally covered with the veneers of thin gravel beds. There is a remarkable pattern of the river course, and an asymmetrical distribution is seen as to the terraces. These characteristics are the results of the influence of the geological structure.
In the course of the Agano River the author classifies the terraces into three groups; the higher, the middle and the lower terraces. The surfaces of the lower terraces are subdivided into two groups; the upper and the lower surfaces. The upper surfaces are covered with pumiceous materials ejected from the Numazawa Volcano and transported by the Tadami and the Agano Rivers. The pumiceous materials are accumulated on the old river floor with the narrow gorges as its temporary base-levels as seen in the Tadami River and the upper course of the Agano River. A part of the upper surfaces was dissected soon after the accumulation, and by this dissection the lower surface was formed.