Reconstruction of paleoenvironments of drainage basin by using oxbow lake sediments

通过对日本北方石狩川牛轭湖和河漫滩的柱状沉积物分析，研究了牛轭湖的形成和沉积历史，并探讨了石狩川下游的环境变化。本研究的主要结果如下所示。(1)提出了利用探鱼声纳和GPS制作牛轭湖水深图的常规方法。(2)湖泊底部沉积物主要由细粒泥组成，菱角在浅水中特别是在夏季繁荣。(3)这些湖泊是在过去几百年中形成的，并迅速被填满。(4)沉积物粒度的变化可能与夏季降水的变化有关。(5)河漫滩的演变受到冰后期海平面变化速率的强烈影响。

This study investigated the formation and depositional history of oxbow lakes based on the analysis of core sediments obtained from the oxbow lakes and the floodplain of the Ishikari River, northern Japan, and discussed the environmental change of the lower reaches of the river. The main results of this study are shown below. (1) We proposed the conventional method to make a bathymetric map of oxbow lakes by using fish finding sonar and GPS. (2) Bottom sediments of the lakes are mainly composed of fine-grained mud and water chestnuts prosper in shallow water especially during summer season. (3) The lakes formed during the last several hundred years and have been filled rapidly. (4) The change of the sediment grain size may be correlated with the change of summer precipitation. (5) Evolution of the floodplain has been influenced strongly by the rate of postglacial sea-level change.