海岸湖沼の海への開口頻度の水収支評価と沼環境の変遷

概要

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Lakes Oikamanai-numa and Horokayanto-numa are coastal lakes facing the Pacific Ocean on the east, in Tokachi Province, Hokkaido. Their morphometric features are: 3.48 km^2 in area, 5 m maximum depth (Lake Oikamanai-numa); 0.65 km^2 in area, 3.6 m maximum depth with the lower lake level (Lake Horokayanto-numa). The lakes were formed by a sand bar which dammed a discharging river on the Pacific coastal line; Lake Oikamanai-numa in about 6,000 yr B.P. and Lake Horokayanto-numa in about 9,000 yr B.P. The latter differs greatly from the former in breaking frequency, namely the sand bar is broken once every four or five years in the latter while in the former it is broken several times a year. A hydrological explanation for this is that the amount of river in flow into these lakes fundamentally effects the breaking frequency. In addition the groundwater discharge from the lakes into the sea is characteristic of artesian groundwater which flows through a coarse stratum sedimented under the sand bar during the period in which the sand bar is broken. Cored samples obtained were 9.5 m in length in lake Oikamanai-numa and 10.8 m in lake Horokayanto-numa. Upon examintion of a total core, it was clear from vertical changes in its granulometric character that the coarsest strata of about 1,200 yr B.P. was deposited under the most wet climate and the tendency of the climate to become wet progressed increasingly from 5,000 to 1,200 yr B.P.
1990-11-30