Changes in tropical cyclone activity present a risk to delta regions as these storms supply a large amount of sediment to such regions, helping to protect the deltas from rising sea. Analysis of 25 years of data from the Mekong River, presented in Nature this week, shows that a large proportion of sediment is supplied to its delta as a result of cyclone activity further upstream. The findings suggest that understanding the relationship between cyclone activity and sediment transport may help us to assess vulnerable coastal areas better.
Most large deltas are at risk of ‘drowning’ owing to rising sea levels, resulting in part from the removal of sediment from the system by human activity such as the building of reservoirs. Increased rainfall caused by tropical cyclones can increase the amount of sediment travelling towards the delta by triggering landslides in the river network, thereby compensating for losses elsewhere in the system.
Stephen Darby and colleagues show that, for the Mekong River, 32 per cent of the sediment delivery is associated with rainfall generated by tropical cyclones. Between 1981 and 2005 more than half of the decline in suspended sediment supply to the delta arose from shifts in tropical cyclone patterns, the authors report. Climate models predict changes in the routes followed by tropical cyclones that could affect the Mekong basin, which, together with the present results, indicates that future delta stability may be at risk.