Marsh Sedimentation Controls Delta Top Morphology, Slope, and Mass Balance

• Published in: Geophysical research letters 2022-06, Vol.49 (12), p.n/a
• Main Authors: Sanks, K. M., Zapp, S. M., Silvestre, J. R., Shaw, J. B., Dutt, R., Straub, K. M.
• Format: Article
• Language: English
• Subjects: Accumulation; Coastal plains; Coastal sediments; coastal sustainability; Coastal zone; Coastal zones; Deep water; delta; Deltas; Deposition; Fluvial sediments; Hypsometry; Mangrove swamps; Mangroves; Marshes; Mass balance; morphology; River sediments; Rivers; Sea level; Sea level changes; Sea level rise; Sediment; Sediment distribution; Sediment load; Sediments; slope; Slopes; Soil erosion; Sustainability; Vegetation; Wetlands
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2022GL098513

Rising sea levels, subsidence, and decreased fluvial sediment load threaten river deltas and their wetlands. However, the feedbacks between fluvial and non‐fluvial (marsh) deposition remain weakly constrained. We investigate how non‐riverine, elevation‐controlled deposition typified by marshes impacts sediment partitioning between a delta's topset, coastal zone, and foreset by comparing a delta experiment with proxy marsh accumulation to a control. Marsh accumulation alters fluvial sediment distribution by decreasing the slope in the marsh window by ∼50%, creating a 78% larger marsh zone. Fluvial incursions into the marsh window trap 1.3 times more clastic volume. The volume exported to deep water remains unchanged. Marsh deposition shifts elevation distributions toward sea level, which produces a hypsometry akin to field‐scale deltas. The elevation‐lowering effect of marshes on an equilibrium delta shown here constitutes an unexplored feedback and an important aspect of coastal sustainability. Plain Language Summary Low‐lying coastal zones, often with abundant vegetation (wetlands), are threatened worldwide because of rising sea level and decreased sediment supply. Coastal sediment accumulation is a fundamental process that helps these regions keep pace with rising sea level. This sediment may be delivered directly from rivers, or as mud and plant material in wetlands (e.g., marshes and mangrove forests) and shallow bays. Our study shows that sediment accumulated in the second manner alters the elevation distribution of coastal regions and the spatial deposition of the river sediment. These results provide important information for plans to help sustain and restore coastal land area. Key Points Marsh deposition decreases delta slope, creating feedbacks that alter the spatial deposition of clastic material A small addition of marsh material, almost doubled the area near sea level, which has significant implications for coastal restoration The interaction of marsh and clastic deposition creates a delta hypsometry more akin to global deltas than experiments without marsh