Predicting the future stability of vulnerable and valuable deltas requires accurate constraints on sediment retention and export at the river-ocean interface. However, sediment discharge for most rivers is measured above the influence of tides, even though the lower, tide-influenced reach of a river can retain 40%-70% of the total sediment load (Goodbred & Kuehl, 1998; Milliman et al., 1985; Nittrouer et al., 1995). Previous in situ studies of tidal-to-estuarine sediment dynamics have revealed multiple pathways and mechanisms for sediment retention along the tidally influenced reach of a river. Starting in the fresh, tidal river, sediment export tends to dominate within channels, while regular overbank flows deposit sediment on floodplain surfaces (

Mangroves are often considered integral engineers of morphologic evolution, but mangroves can also opportunistically respond to morphologic change created by abiotic sedimentary processes. Consequently, predicting the response of individual estuarine environments to changes in mangrove extent is challenging. Here, the impact of mangrove extent was explored using in‐situ observations and numerical modeling of the quiescent Waikaraka Estuary in Tauranga Harbor, Aotearoa, New Zealand. Mangrove coverage expanded in the estuary from 1940 until a removal program began in 2005 with the goal of reducing fine sediment and restoring bivalve habitat. In June 2019 water velocity, turbidity, wave height, and bed‐sediment grain size were measured at multiple sites to identify how sediment dynamics in the estuary were responding to removal. Flow in the predominantly sandy lower estuary was ebb dominant, resulting in net sediment export, while flow in the muddy upper estuary was weakly flood dominant, resulting in sediment import and retention. Therefore, fine sediment is unlikely to be flushed out of quiescent estuaries after mangrove removal. A Delft3D numerical model calibrated with in‐situ data showed that tidal asymmetry, velocity skew, and peak ebb‐tide shear stress were not significantly altered by varying the mangrove extent between the 2005 maximum coverage and complete removal. Mangroves did not significantly impact flow in this system; instead, the hydrodynamics and net sediment transport were controlled by tidal interactions with bathymetry. In model runs with mangroves covering all tidal flats, the water velocity on the intertidal flats decreased while the peak ebb‐tide velocity in the main channel increased, indicating that the potential for fine‐sediment export may actually decrease with mangrove removal. These results emphasize the role of mangroves as opportunistic colonizers, not engineers, and highlight the importance of considering site specific parameters when planning mangrove removals.

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Vegetated mid-channel islands play an important though poorly understood role in the sediment dynamics and morphology of tide-dominated deltas. Meinmahla Island is a mangrove-forest preserve at the mouth of the Bogale distributary channel, in the Ayeyarwady Delta, Myanmar. In this relatively unaltered mid-channel island, sediment dynamics can be directly connected to morphology. Field measurements from 2017 to 2019 provide insight into the pathways for sediment transport and resulting morphological evolution. Water depth, salinity and turbidity were monitored semi-continuously, and velocity profilers with turbidity and salinity sensors were deployed seasonally in single-entrance (dead-end/blind) and multi-entrance tidal channels of the island. The morphological evolution was evaluated using grain size, 210 Pb geochronology, remote sensing and channel surveys. The data show that ebb-dominant, single-entrance channels along the island exterior import sediment year-round to the land surface. However, these exterior channels do not deliver enough sediment to maintain the observed ca 0.8 cm/yr accretion rate, and most of the sediment import occurs via interior, multientrance channels. Interior channels retain water masses that are physically distinct from the water in the Bogale distributary, and estuarine processes at the tidal-channel mouths import sediment into the island. Sediment is sourced to the island from upriver in the wet season and from the Gulf of Mottoma in the dry season, as the location of the estuary shifts seasonally within the Bogale distributary. The salinity and biogeochemistry of the distributary water are affected by interactions with sediment and groundwater in the island interior. The largest interior channels have remained remarkably stable while the island has aggraded and prograded over decadal timescales. However, the studied multi-entrance channel is responding to a drainage-network change by narrowing and shoaling. Overall, mid-channel islands trap sediment and associated nutrients at the river-ocean interface, and these resilient landscape features evolve in response to changes in drainage-network connectivity.