Bathymetry and Bottom Sediments of Oxbow Lakes along the Lower Reaches of the Ishikari River

Abstract

Many small-scale oxbow lakes formed by meandering cutoffs are distributed along the lower reaches of the Ishikari River, northern Japan. We conducted soundings in five oxbow lakes (Pira, Toi, Tsuki, Hishi, and Ito) using a rubber dinghy equipped with a hand-controlled trolling motor and a GPS-equipped fish finder, which are easily handled by researchers, and produced bathymetric maps. We also collected bottom sediments from the lakes and measured wet and dry bulk densities, particle size, and loss on ignition (LOI). In all lakes, the maximum water depth occurred at a site with a large curvature remaining from when it was previously a channel of the Ishikari River. In particular, the maximum water depth of Lake Hishi is greater than that of the other four lakes, and it is thought to have been deeply scoured. The bending sections in Lakes Pira, Hishi, and Ito are deeper along the outer than the inner shores, which is a typical morphological feature of meandering channels. In contrast, the deepest parts of Lake Tsuki and the northern part of Lake Toi are not noticeably close to the outer shore, with shallow water and little irregularity of the lake bed. These characteristics may reflect the difference in the initiation of the oxbow fills. The particle size of the bottom sediments shows that fine silt and clay have accumulated in all the lakes. In particular, very fine-grained sediments rich in organic matter have been deposited on the surface in areas of greater water depths in Lake Pira and Lake Hishi.