Lake Numayama-Oonuma is situated at the southern foot of Mt. Gassan. Since the first survey of this lake by Yoshimura, about forty-five years have passed. Regretably, it has been ignored by limnologists for the last thirty years. In order to examine the present status, a study was undertaken by the author between 1965 and 1966. In this paper, the new findings from the study are discussed with special reference to the geochemistry of the lake water.
1. The morphometrical data of the lake are summarized as:
(a) surface area: 9.94×10⁴m²
(b) volume: 1.13×10⁶m
(c) maximum depth: 31.7m
(d) shore development: 1.78
The bathymetric map reveals that this lake is a damned lake formed by a catastropic landslide.
2. The main solute in the lake water is sodium cholride derived from the atmospheric salt transported by monsoon and deposited in the snow cover. The quality of water has not changed in spite of the passage of time.
3. In midsummer, the development of thermal and chemical stratifications is observed and the layer from the depth of fifteen meters to the bottom is always in a stagnant condition.
4. Judging from the low photosynthetic productivity and the quality of water, this lake belongs to an oligotrophic lake.