At the mouth of the Mataquito River, a coastal bar has formed due to the high availability of sediments from the ocean–basin interaction. The evolution of this bar has been marked by disruptive events such as the earthquake followed by the tsunami of February 27th, 2010, frontal systems, and strong ENSO Niño Niña phenomena. The bar erodes and builds up naturally, and in the last decade has been affected by increased storm surges and sediment scarcity due to the mega drought in central Chile since 2010, as well as intensive forestry use in the upper sections of the basin, which reduce the flow of sediment to the ocean and put this landform at risk. The objective of this work is to quantify the multidecadal evolution and spatio-temporal changes of the bar from 1985 to 2024 using Landsat satellite images (5, 7, 8 and 9 and Sentinel 2), ERA5 wave data, and in situ data. The analysis includes the effect of waves and sediment characteristics, thus taking into account the inherent complexity of the coastal environment. The dynamics of the mouth are reconstructed, showing the point at which the bar was turned to the left by the 8.8Mw 2010 earthquake, during which the bar disappeared, showing the force of the tsunami versus the inertia of the river and favouring the growth of an elongated channel with variable width towards the north. In this bar, fluvial avulsion is evident, which promotes coastal erosion since sediments are temporarily retained in the floodplain. The capacity of the existing riverbed to transport all the water and sediments it receives is noteworthy. On the other hand, the subsidence caused by the 2010 earthquake caused ebb and deposition, changing the morphology of the bar.