佐渡島加茂湖の自然環境とその歴史  [in Japanese] Natural Environment and History of Lake Kamo, Sado Island  [in Japanese]

潟湖である加茂湖は新潟県佐渡島の国中平野の北東部に位置し, 両津市街をのせる砂州によって両津湾からへたてられている。潟の北東部が人為的に設けられた水路によって連絡しているため, 海水ないし汽水で満たされている。最終氷期の海面低下期には現在の両津湾へつづく谷の再奥部付近にあたり, 小佐渡・大佐渡山地からの河川が集中していた。晩氷期以降の海水準上昇期には海進の進行とともに現在の両津湾につづく入り江が最奥部に出現し, 海湾の一部となった。その後入り江での埋積が進行し, また住吉野城段丘の延長部に砂州が成長をはじめ, 入り江の口が狭められ潟湖に変わった。とくに弥生海退期の頃から海水の流入が減退し, 潟湖は汽水から淡水化した。明治時代に洪水対策から, 水路が切り開かれ, 再び海水の流入する潟湖となり, 現在にいたっている。なお, 本論では加茂湖およびその周辺の地形・地質, 湖底下の地質, 湖水, その底質などの資料を概説している。

The lagoonal Kamo Lake is located in the north-eastern part of Kuninaka Plain, Sado Island, Central Honshu, Japan. The lagoon is separated from Ryotsu Bay by a sand bar where Ryotsu Town is on it. The lagoon is filled with marine water, because there is an arrow waterway which was made artificially between the lagoon and sea in the northern site. At the fall of sea-level of the last glacial age a valley was formed from the Lake Kamo area toward Ryotsu Bay and several rivers from the surrounding mountains flowed in to that valley. Rapid sea-level rise during the postglacial age brought a embayment as far as the innermost part of the valley. After that, the inlet was gradually filled with crastic sediments from the surrounding mountains. Moreover, a sand bar grew from the cape of Sumiyoshi-nojyo Terrace northernwards and the marine inlet changed into a lagoonal condition because the exit of the inlet narrowed. Especially, the regression of the Yayoi period affected the inflow of sea water from Ryotsu Bay. The lagoon was filled with brackish or fresh water after that period until the Meiji period. At that period, a artificial waterway was made for prevention of a over flow. In this report, moreover, the general geomorphology and geology, underground geology, lakewater, lake deposits and natural environment of Lake Kamo and its surrounding area are summarized and discussed.