Long-term (68 years) morphological changes of the alluvial river and macro-tidal estuary affected by river works and extreme rainfall disasters

• Riverbed dredging caused 3–4 m decline in riverbeds of mid-stream and estuary. • Mid-stream riverbed became coarser, and estuary became finer with cohesive sediment. • After weakening dredging, riverbed was almost stable until the disasters occurred. • Sediments supplied by disasters are currently deposited in mid-stream (1.13 Mm 3 ) • High shear stress (3–20 N m −2 by disasters eroded 1.92 Mm 3 of estuarine riverbed. The effects of riverbed dredging and recent disasters on the long-term morphological changes of the Chikugo River were determined utilizing the 68-year (1953–2021) monitoring bathymetry data from the river mouth to upstream. During the human activities dominated period, P1 (1953–1998), a significant decline (3–4 m) in the riverbeds of the mid-stream and the estuary was observed due to severe riverbed dredging. This intensive degradation coarsened the riverbed of mid-stream while that of the estuary became finer with cohesive sediments transported by the tide. During lesser human activities and no severe disaster period, P2 (1998–2009), the riverbeds of upstream and mid-stream were almost stable from 1998 to 2003, however, deposition was found from 2003 to 2009 as the river flow increased. Conversely, bed elevation of the estuary increased by about 0.36 m from 1998 to 2003 and erosion was initiated between 10–23 km from 2003 to 2009. During disasters dominated period, P3 (2009–2021), morphological responses differ spatially. Although disasters supplied huge sediment, they deposited in the upstream first and gradually moved to mid-stream. Currently, 1.13 Mm 3 were deposited in mid-stream where the largest sediment volume was extracted during P1. Thus, sediments supplied by disasters have not reached the estuary yet. Furthermore, the bed shear stress of the estuary during P3 was high (3–20 N m -2 ) due to the high river flow caused by disasters. That high river flow eroded the surface mud and bottom sand layers, resulting in the parallel decline of both layers. As a result, 1.92 Mm 3 of sediments were eroded from the estuary from 2009 to 2019. These results indicate that both high river flow caused by disasters during P3 and insufficient sediment supply caused by riverbed degradation during P1 are responsible for the erosion of the estuary during P3.