Abstract
In the alluvial lowlands along the seacoast in Japan, there are many maritime coastal lakes (Fig. 1). It is important that we have precise knowledge about these lakes in order to understand the development of the alluvial lowlands. The purpose of this paper is to clarify the geomorphological features of the maritime coastal lakes in Japan and to consider their geomorphological development.
The number of maritime coastal lakes in Japan is 31, a majority of the 51 lakes whose are as aregreater than 4km2. The significant feature of these lakes is that the maximum depth of each one is very small compared with its area (Fig. 2). Along the shoreline of a maritime coastal lake, we can generally find lacustrine lowlands, which develop at an altitude of 0-5m and at widths of 200-700m, and also a littoral shelf whose depth is only a few meters.
The lakeside lowlands are composed of three different parts: that is, lacustrine terrace I (at an altitude of 2-5m, width of 100-200m), lacustrine terraae II (at an altitude of 1-2m, width of 100-500m), and present sand beach or littoral marsh (at an altitude of 0-lm). Both the lacustrine terraces may have been formed at two different high lake water levels in the past, and have been connected with high sea levels in the.Holocene. It is assumed that lacustrine terrace I was formed in the period of the “Jomon transgression” (about 6, 000 y. B. P.), and lacustrine terrace II was formed in the period of the “Jomon re-transgression” (about 4, 000-3, 000 y. B. P.) or in the Heian era about 1, 000 years ago.
On the other hand, the littoral shelf can be divided into two other terraces, based on the differences in depth, sediment, and shape (Fig. 4 and Fig. 5). Littoral shelf I develops widely and continuously at a depth of 0.5-2.0m, width of 200-300m. It is certain that littoral shelf I is now under construction at the present lake water level. But littoral shelf II is supposed to have been formed as a topset surface of a river delta, a tidal delta, a sand bar, a sand spit, or a wave cut bench, at a low lake water level in the past. This low lake water level is assumed to be correlated with the low sea level whichappeared three times after the maximum transgression in the Holocene; that is, about 4, 500 y. B. P., 3, 000-2, 000 y. B, P., and in the 16-17th centuries. The present sand beach and littoral shelf I tend to develop around the widespread lacustrine terrace II, where lacustrine terrace I often remains conspicuous. Fhis fact suggests that the present landforms develop under the strong influence of past ones.
Finally, the author would like to discuss the issue from the applied geomorphological point of view. Recently practical use of the maritime coastal lakes has come to be emphasized. But many kinds of human activity-for example, construction of a littoral bank, a large-scale reclamation, or artificial control of the lake water level-have affected the natural ecosystem and traditional land utilization by an area's inhabitants. We should first pay attention to the geomorphological features of maritime coastal lakes and should estimate accurately the influence of human activities, in order to make the best use of such lakes.
