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Sediment is the most valuable natural resource for deltaic environments because it is required to build new land. For land building to occur, sediment must be retained in the delta instead of being transported offshore. Despite this, we do not know what controls sediment retention within a delta. Here we use a calibrated numerical model of Wax Lake Delta, Louisiana, USA to analyze sediment retention for different riverine flood magnitudes, tidal amplitudes, and vegetation extents. Our results show that as riverine flood magnitude increases, areally averaged vertical accretion increases from 0.33 to 2 cm per 60-day flood, but sediment retention decreases from 72% to 34%. For the uniform vegetation characteristics considered, the buffering effect, defined as the reduction of sediment flux onto the islands in the presence of vegetation, reduces the sediment flux onto the islands 14 to 22% on a fully vegetated delta. When sediment is transported onto the islands, vegetation enhances retention, which we refer to as the trapping effect, bỹ 10%. But, this does not offset the buffering effect, and vegetation decreases vertical accretion and retention in the delta up to 6% (or~0.5 cm per 60-day flood). We suggest that vegetation will increase sedimentation only when trapping compensates for buffering. Finally, greater tidal amplitude at higher discharges enhances vertical accretion by~0.5 cm per 60-day flood compared to smaller tidal amplitudes. These results provide insight on the mechanisms behind coastal systems growth, and suggest how sediment diversions might be operated more efficiently in deltas with reduced sediment supply.

River deltas sit at the interface of the terrestrial and ocean environments where relative sea-level rise causes flooding and possibly shoreline retreat. Since 1993, relative sea-level rise has averaged ∼3.1 mm yr −1 globally (Thompson et al., 2019), and this is predicted to cause more frequent flooding in low-lying, densely populated deltas due to storm surges (Edmonds et al., 2020;Hirabayashi et al., 2013;Muis et al., 2016). To counteract relative sea-level rise and potentially mitigate flooding or shoreline retreat, river deltas can adjust their surface elevation though deposition of organic and inorganic sediment in the interdistributary islands that host the existing wetlands (Paola et al., 2011). Sediment deposition on a deltaic surface is non-uniform at short time scales (e.g., Bevington et al., 2017;Nardin et al., 2016;Nienhuis et al., 2018), but at some point, the entire surface must aggrade by some fixed amount to counteract relative sea-level rise.

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Evidence suggests that most nitrate removal in deltas occurs in densely vegetated wetlands (DeLaune et al., 2005;Lane et al., 2003;Leopold, 1970), which are associated with shallowly submerged delta islands as opposed to deeper, open-water embayments and channels (Carle et al., 2014(Carle et al., , 2015Ma et al., 2018). For example, in Wax Lake Delta, 73% of the nitrate that is removed from surface water is associated with shallowly inundated island areas (Hiatt et al., 2018). Knights et al. (2020) showed that submerged island areas with denser vegetation have greater nitrate processing rates, and Henry and Twilley (2014) demonstrated a positive correlation between delta