This is a guest post by Erin McEwan.
River systems are shaped by both gradual and sudden geological processes, and the influence of active tectonics on river behaviour is a fundamental concept in tectonic and fluvial geomorphology. Despite this, much is still unknown about how earthquake surface deformation can alter flood hazard. This is concerning as human populations are increasingly expanding onto floodplains in seismically active regions. A recent review by McEwan et al (2025) in Earth-Science Reviews addresses this knowledge gap by analysing data from 52 sites where fault deformation is known to have induced an immediate change in river behavior; otherwise referred to as a Coseismic River Response (CRR).
How Do Rivers Respond to Fault Deformation?

Case study data enabled the identification of four main classes of near-fault CRR. These range from in-channel streamflow diversion or ponding, through to overbank flooding or avulsion, wherein a channel partially or fully shifts into an enduring new course within the floodplain (Fig. 1). CRR events are most often triggered by dip-slip fault ruptures, where the elevated side of the fault partially or fully dams the river channel. More rarely, oblique motion can behead channels, or multi-meter vertical displacements can tilt riverbeds backward, reversing flow direction and promoting lake formation or avulsion upstream of the fault-river intersection (Fig. 2).
Why Do Some Rivers Respond Differently?
The study highlights key differences in how river types respond to seismic events. Braided rivers, characterized by their wide, shallow channels and mobile sediments, can distribute flow across multiple pathways, often mitigating the obstruction effect produced by fault ruptures, particularly in confined settings. In contrast, erodible substrates in unconfined environments can also facilitate rapid avulsion, as occurred during New Zealand’s 2016 Papatea fault rupture and Waiau Toa / Clarence River avulsion (Fig.1). In contrast, single thread meandering rivers are more vulnerable to rapid bank overtopping and prolonged flooding. In semi-confined settings such as intermontane basins, this can lead to the formation of a long-lived coseismic lake, as documented in Algeria’s 1980 El Asnam rupture (Fig.3). Meandering floodplains, composed of saturated fine-grained sediments, are also more susceptible to liquefaction, which can lower the ground surface and exacerbate flooding by reducing drainage capacity. In any river environment, paleochannels can enhance the potential for, and spatial extent of coseismic flooding and avulsion, highlighting the role that floodplain features play in CRR.

Why Does CRR Matter?
Fault river intersections are prevalent at plate boundaries and CRR has profound implications for flood risk, especially in regions where human modification has altered natural floodplain dynamics. Globally, flooding is one of the most damaging and costly natural hazards, and models estimate that around 1.81 billion people are directly exposed to 1-in-100-year floods. Seismic activity can compound this risk by undermining flood defenses through surface rupture, liquefaction, and ground deformation. Currently, many of these impacts are overlooked when assessing flood hazard and implementing river control and flood mitigation measures.

Lessons from Past Earthquakes
New Zealand’s 2010–2011 Canterbury Earthquake Sequence (CES) provides a clear example of how seismic events can fundamentally reshape flood hazards. The CES triggered a coseismic avulsion in the Hororata River and widespread liquefaction-induced subsidence in Christchurch, permanently altering flood thresholds for thousands of residential properties. These effects rendered entire suburbs uninhabitable, leading to large-scale property buyouts and long-term shifts in urban planning. This example underscores the far-reaching consequences of earthquake-induced changes to river and floodplain systems. The pre-earthquake population of Christchurch numbered ~400,000 and the societal and economic risk posed by a large earthquake in a comparable geographic, but more densely populated and flood-prone setting, could be much higher.
Historical earthquakes in the Indian subcontinent have triggered rapid and sustained changes in river behavior, altering drainage networks and flood regimes across vast areas. However, the population density and infrastructure exposure in the region are now far greater than in past seismic events. Given the scale of settlement and dependence on floodplains for agriculture and urban development, a large earthquake today could produce severe and long-lasting disruptions to both human and natural systems. Bangladesh hosts the world’s largest lowland delta, the Ganges-Brahmaputra-Meghna (GBM) Delta, which spans ~760,000 km², half of which lies below 5 m elevation. Over 100 million people live on this delta, where seasonal flooding already affects 30–35% of the land annually, increasing to over 60% during severe monsoons. Climate change is expected to exacerbate flood hazards, while human modification of floodplains, such as stopbanks and land reclamation have reduced natural floodplain capacity and increased liquefaction susceptibility. These changes may prime river channels for coseismic flooding or avulsion following major earthquakes. Without proactive risk assessments and planning that account for coseismic river response, the consequences of such an event could be devastating for millions living near Bangladesh’s river channels.
Preparing for Future CRR Events
Understanding CRR is critical for seismic hazard planning. By integrating active fault databases with river centerline data, researchers can identify locations where future earthquakes may induce severe river responses. Probabilistic models can then assess the likelihood of overbank flooding due to fault rupture, while site-specific hydrodynamic modeling can refine hazard assessments. As floodplain settlement continues in earthquake-prone regions, accounting for CRR in disaster risk management is essential for mitigating future losses.
Reference
McEwan, E., Stahl, T., Langridge, R., Davies, T., Howell, A., & Wilson, M. (2025). Seismic hazard and shifting channels: Exploring coseismic river response. Earth-Science Reviews, 105042.
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