教材としての河床堆積物 : その取扱いの視点

The river has been treated as traditional and important teaching materials in Japanese school since the Meiji era. But,it cannot have been necesarily the conspicuous teaching materials to observing. What is the reason of it ？
The conception has lied in the back of heads in Japanese people that gravels and sands of river sediments are more rounded to the downstreams. But,gravels and sands that a learner observes really on the river bed do not necessarily exist in such shapes. The features of Japanese rivers in comparison with continental rivers are that the river distance is short, the speed of a running fluid is rapid and specific discharge is very large. From such view points, the characteristics is discussed on the river sediments