The relative contributions of baseline hydroperiod, event-driven exceedances, and tropical-cyclone extremes to marsh accretion in microtidal regimes

Document Type

Article

Publication Date

8-15-2026

Abstract

The relative influence of baseline hydroperiod (monthly tidal datums), event-driven high-water exceedances (predominantly associated with frontal weather systems) and tropical-cyclone extremes, on marsh vertical accretion in Louisiana (USA) microtidal systems remains uncertain. We examined their contributions at stations spanning freshwater–brackish–saline marshes in the Atchafalaya and Terrebonne basins during water year (WY) 2020–2021, which aligned hourly CRMS water-level records with contemporaneous vegetation and accretion observations and includes multiple tropical events for contrast. Our approach was a reduced-form monthly partitioning framework: hourly water levels were used only to derive monthly hydroperiod metrics and within-month exceedance tiers. By reducing hourly water levels to monthly means and within-month exceedance tiers (minor, moderate, major), we modeled distinct pathways of mineral and organic accumulation. Suspended sediment concentration (SSC) is temporally controlled by river discharge seasonality and varies spatially across the deltaic landscape, while aboveground biomass regulates vegetation-mediated trapping. The distal saline site is mineral-dominated due to its low elevation, high SSC, and elevated mean sea level, whereas both freshwater sites are organic-dominated. We found that event-driven exceedances contribute roughly 40–45% of the annual mineral accretion, primarily from frequent minor exceedances, highlighting that accretion depends more on event frequency than individual magnitude. Named tropical cyclones are treated as discrete extremes when storm peaks exceed the statistical major tier; their incremental contribution was ∼2–3% of the annual total, reinforcing outer-bay marshes during the cool-season wind–wave regime. Vertical accretion profiles showed mineral loading tapering with height as flood depths contract, while organic productivity peaks at intermediate elevations. Independent comparisons with NOAA tide gauges and river–coastal data confirmed that these patterns reflect genuine environmental controls. Results indicate that marsh resilience depends less on tropical-cyclone extreme storms and more on the cumulative effects of frequent, shallow, sediment-bearing floods acting on a rising baseline hydroperiod.

Publication Source (Journal or Book title)

Estuarine Coastal and Shelf Science

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