Palaeolimnological Study on Coastal Closed Lake Inferred from Frequency of Overflow

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A purpose of this study is to clarify how a hydrological regime has changed during a drastic, climatic change, that is, from the last glacial age to the postglacial age. In this connection, Lake Horokayanto at the Tokachi district in Hokkaido was subjected to a co-operative research on the subject of" Palaeolimnological Study on Coastal Closed Lake Inferred from Frequency of Overflow" by the members of the research headed by Professor Kinshiro Nakao; the research was conducted for three years of 1983 (preliminary study) and 1985 - 1986 (principal studies). This lake is located on the seashore and is a closed lake dammed up by a sand bar in the estuary. The interaction between the climatic change and hydrological regime could be shown by recognizing the frequency with which the lake will overflow into the sea. The overflow of the lake can raise a breakdown of the sand bar, followed by an inflow of sea water. In the beginning, a survey on a water budget of the lake was performed in 1983. … More As a result, the lake was seen to have the overflow at the rate of one time per four or five years in the present climatic condition. The frequency of the overflow can be also traced as the number of coarse-grained laminae in the sedimentary facies below the lake bottom. These results gave preparatory recognition to trace the change in the hydrological regime of Lake Horokayan-to.A long sediment core was sampled in Lake Horokayan-to in August 1985 to reconstruct a history of the paleoenvironment. Its coring was performed on a raft fixed at the center of the sub-basin. The core sampler reached a fluvial gravel bed at a depth of 12 m below the bottom. The core sample obtained was 10.8 m long. The gravel bed consists of bed materials of a river which would have discharged into the sea through the valley of the present lake basin during a period of low sea-level. The following conclusion through inspection of the core can be led: The river was dammed up in the estuary about 8,400 years ago, when a postglacial transgression caused the sea level to rise. In the Jomon transgression, the higher sand bar by 2 m than today had dammed the mouth of the river valley at the same site as that in the present day. Various analyses for the sediment core show the fact that the hydrological regime of Lake Horokayan-to has been varied under the interaction of the changes in sea-level and climate. Less