A century of stream burial due to urbanization in the Tokyo Metropolitan Area

The alteration of hydrologic systems due to urbanization causes many problems including urban flood inundation, water quality degradation, and stream ecosystem damage related to river channel modifications such as stream burial. The purpose of this study was, therefore, to clarify the factors affecting the process of stream burial by performing a time series investigation of the changes in six river systems flowing through the Tokyo Metropolitan Area from 1909 to 2020 using historical topographic maps. The observed changes revealed that the stream burial process consists of: (1) river improvements such as meander cutoff and integration of flow channels by branch abolishment, (2) stream burial of relatively small tributaries, and (3) stream burial of large tributaries and part of the mainstream. Stream burial was remarkable in the center of the Tokyo Metropolitan Area, where the river channels and condition of stream burial were nearly the same in 2020 as they were in 1945. However, meandering channels partly remained in the tributaries of relatively large stream systems. Straight channels constituted about 10% of all channels in 1909, but constituted greater than 40% in 2020, suggesting that the remaining open channels were considerably influenced by river improvements such as meander cutoff. Furthermore, the degree of stream burial was observed to be strongly related to the proportion of urbanization and the scale of the target river system. Stream burial was found to be advanced not only by changes in land use, but also by social factors including earthquake and war damage reconstruction projects as well as measures to counter flooding and improve water quality. The findings of this study advance research into evaluating measures and goals for urban river restoration projects.