Long-term (1953-2020) changes in morphology of Chikugo River, Japan in response to natural and anthropogenic forces

The sediments transported by the rivers are the primary source of materials to the downstream estuaries and tidal flats. Hence, human activities and increased natural forcing (driven by climate change) can strongly influence the sediment supply by the rivers to the downstream. This can directly impact the existence of tidal flat areas, the rate of bank erosion, and the health of the aquatic ecosystem. A quantitative analysis of the changes in river morphology on a long-term basis is necessary to understand the current situation and develop management strategies. Therefore, this study aims to analyze the long-term (1953-2020) changes in the riverbed elevation of the Chikugo River, Japan. This 143 km long river has a macrotidal estuary downstream (0-23 km). The bathymetric data measured at 200 m intervals from the river mouth (0 km) to the upstream (64 km) for 68 years (1953-2020) was collected from the Japanese government. Further, topographic data in the upstream estuary (10.2-17 km from the river mouth) at 1 km intervals were surveyed for 17 years (2005-2021).

Based on the time of human activities and disasters occurrence, the study period was divided into three periods: (1) the period of human activities (1953-1998), (2) the period of no human activities and no disasters (1998-2003), and (3) period of disasters (2003-2020). During period-1, the riverbed of the whole river was lowered, with maximum degradation occurring between 20-30 km. This was caused by extensive human activities such as dredging for flood control and land reclamation and sand mining for commercial use. During period-2, the riverbed (23-64 km) became stable because the dredging was stopped. However, bed elevation in the estuary (0-23 km) increased by nearly 1 m due to tide-induced landward sediment transport. During period-3, extreme floods and landslide disasters supplied massive sediments into the Chikugo River, which was deposited between 30 to 64 km and increased the riverbed elevation. However, the riverbed between 23 to 30 km was almost stable and the bed elevation in the estuary was (6-23 km) decreased. It seemed that sediments supplied by disasters were trapped by a bed sill located at 28.7 km and were not enough to reach the estuary. Further, the extreme flood discharge was strong enough to erode the sediment deposited upstream of the estuary by the tidal forcing and transport back downstream. The erosion of existing deposits and lack of sediment supply from the upstream caused the decreased bed elevation of estuarine areas. According to the results, the river is redistributing and restoring the sediments supplied by disasters at the place of extracted sediments in the past, which has reached mid-stream currently and is expected to arrive at the estuary and downstream soon.