Sediment delivery to deltaic land surfaces in the absence of artificial levees: Contrasting the Ayeyarwady and Mekong Deltas

Under natural conditions, deltaic land surfaces are maintained by sediment received during high-water periods (seasonal or tidal), when water levels exceed the elevation of banks and sediment is transported onto the subaerial delta floodplain. The Mekong Delta, like most other large Asian deltas (e.g., Ganges-Brahmaputra, Red) has been subjected to extensive human modification, including clearing of mangrove forests and the construction of levees that effectively isolate the delta floodplain from distributary channels. In contrast, the Ayeyarwady Delta maintains larger intact regions of mangrove forest and has experienced less levee construction, making it an ideal location to study natural processes that transfer sediment between distributary channels and deltaic land surfaces. Short-term instrument deployments in the Mekong and Ayeyarwady Deltas captured high-resolution hydrodynamics within mangrove forests during the SW monsoon (high discharge) and NE monsoon (low discharge). In the Ayeyarwady Delta, dendritic tidal channels incising the mangrove-colonized floodplain exhibit net-landward sediment fluxes over tidal cycles during the high-discharge season, and less exchange during the low-discharge season. In the Mekong Delta, where mangroves are limited to fringing forests seaward of artificial levees, magnitudes of exchange between mangroves and distributary channels are greatest during the low-discharge season due to enhanced sediment delivery from offshore. In both locations, sediment-accumulation rates within mangrove forests are on the order of 1 cm yr^-1, suggesting that these environments are effectively trapping sediment at a rate similar to local sea-level rise. In the Ayeyarwady Delta, intact tidal-channel networks are crucial conduits for supplying sediment to both mangrove forests and adjacent cleared fields, though remote sensing reveals the progressive degradation of natural channel networks following deforestation. Although mangrove forests in the Mekong Delta are effective sediment sinks, their extirpation from all but the land–ocean interface limits the extent to which they promote aggradation of the subaerial delta.­