Geospatial Assessment of Three Decades of Shoreline Shifts and Two Decades of Vegetation Change in the Grand Saloum Transboundary Wetland Complex, Senegal-The Gambia

Abstract

Coastal wetlands at the land–sea interface are on the frontline of climate change, yet integrated evidence on geomorphic and ecological responses remains limited in West Africa. We quantified shoreline trajectories (1990–2020) and landcover dynamics (2000–2020) across the transboundary Grand Saloum complex (Senegal–The Gambia) using Landsat surfacereflectance time series, spectral indices (NDVI, NDWI, NDBI), and the Digital Shoreline Analysis System (DSAS). Shorelines were extracted from NDWI-based water masks, filtered and vectorized, then analyzed in DSAS with End Point Rate statistics. Vegetation was mapped in Google Earth Engine with a Random Forest classifier (mangrove, other vegetation, built/bare, water). The coastline is dominated by erosion (mean −2.44 m·yr⁻¹) interspersed with localized accretion (mean +1.84 m·yr⁻¹). Erosion hotspots concentrate in central sectors, whereas mixed erosion–accretion patterns occur near the northern and southern mouths. Concurrently, mangrove cover expanded from 57,867.61 ha in 2000 to 66,840.17 ha in 2020 (~+15.5%), while other vegetation declined from 23,483.18 ha to 16,146.11 ha (~−31.3%). Within a 1-km coastal buffer, mangroves remained broadly stable to slightly increasing (16.43%→16.81%). These findings depict a dynamic yet resilient system where mangrove gains coexist with heterogeneous shoreline retreat and conversion of non-mangrove covers to bare substrates and water. Management should safeguard landward migration corridors, target erosion-prone reaches with nature-based measures, and institutionalize a transboundary monitoring, reporting, and verification framework that updates DSAS and satellite products at 2–3-year intervals while integrating in-situ elevation, salinity, and sediment data. Our workflow provides transferable, decision-relevant evidence for coastal adaptation and blue-carbon planning in data-limited deltas and policy design.