Abstract
<title>Abstract</title> <p>Suspended sediment sustains tidal wetlands, yet in shallow bays persistent sediment can also reflect erosion of the bay floor or marsh edge. Long-term sediment trajectories of coastal embayments, however, remain largely unquantified at broad scales. Here we show, using four decades of Landsat-derived suspended sediment concentration (SSC; 1985–2024) combined with bathymetric, tidal, wind and marsh-change datasets, that SSC has declined widely across U.S. East Coast bays and adjacent offshore waters. However, embayment sediment trajectories often diverge from the offshore background. Sheltered, marsh-rich bays with efficient sediment trapping tend to amplify offshore SSC decline, whereas wave-exposed or weakly marsh-buffered bays tend to attenuate it. Importantly, slower in-bay sediment loss coincides with greater marsh retreat, suggesting that sediment availability in the water column may be maintained by internal geomorphic reworking rather than indicating greater resilience. Embayment SSC trajectories therefore reveal when apparent sediment persistence masks erosion of the bay’s own geomorphic foundation, providing a framework to diagnose vulnerability of shallow coastal bays.</p>