Formation and infilling of oxbow lakes in the Ishikari lowland, northern Japan

Oxbow lakes are common landforms in meandering river floodplains. Despite the possible importance of oxbow lakes in floodplain evolution, little is known about sedimentary facies and sedimentation rates of oxbow fills, their formation ages and their persistence in the landscape. We analyzed infilling records by interpretation of borehole cores from four oxbow lakes in the Ishikari lowland, northern Japan. The borehole sediments were interpreted as basal coarse-grained streambed sediments (Unit 1) succeeded by channel fills before complete disconnection (Unit 2) and overbank deposits after complete disconnection (Unit 3). Unit 2 is thick in one core but very thin or absent in the other cores. This difference may result from the diversion angles of the incipient oxbow at the time of cutoff rather than the cutoff mechanism (meander cutoff or local avulsion). Detailed chronology based on the ¹⁴C ages, tephra analysis, and ¹³⁷Cs suggests that the oxbow lakes formed during the last centuries and the sedimentation rates are approximately 45-90 mm/y in Unit 2 and 3.9-22.0 mm/y in Unit 3. Oxbow lakes in the Ishikari lowland may persist in the landscape during 600-1300 years since the initiation of cutoff. The relationship between the production rate of oxbow lakes and their persistence suggests that the production rate of oxbow lakes was high during 1899-1959. The number of oxbow lakes in the landscape may be variable in time scales of several decades or a century depending on the production rate of oxbow lakes.