Abstract



Coastal zones are dynamic geomorphological environments whose position and shape respond to marine, climatic, hydrodynamic, and sedimentary processes. In semi-enclosed and closed basins such as the Caspian Sea, small water-level fluctuations can cause large horizontal shoreline shifts because of very low coastal gradients. This study quantitatively analyzes spatiotemporal changes in the water–land boundary along the southeastern Caspian Sea coast, in Golestan Province, Iran, from the Turkmenistan border to Bandar Gaz. Four geomorphological units were compared: Gomishan Lagoon, Gorganrud Delta, the eastern coast of Gorgan Bay, and the southern coast of Gorgan Bay. The main objective was to determine the magnitude, rate, and spatial pattern of boundary displacement and to assess differences among these units over a 48-year period (1977–2025).



Landsat images from 1977 (MSS), 1985 and 1995 (TM), 2007 (ETM+), and 2017 and 2025 (OLI/TIRS) were selected based on minimal cloud cover and key Caspian water-level phases. Geometric and radiometric preprocessing was performed in ENVI 5.3 and ArcGIS 10.8. Image-to-image registration used the 2017 image as reference, with 20–30 ground control points and RMSE below 0.5 pixel. All images were projected to UTM Zone 39N, WGS84. Radiometric calibration and dark object subtraction were applied. The normalized difference water index (NDWI) was used to extract the water–land boundary, supported by false-color composites, high-pass filtering, and Google Earth visual checks. Water-level data were obtained from the Makhachkala coastal station (CASPCOM) for pre-1992 and from TOPEX/Poseidon and Jason-1, -2, and -3 satellite altimetry (Copernicus) for 1992–2025. Shoreline change was quantified using the Digital Shoreline Analysis System (DSAS v5.0). A 300-m landward baseline was defined relative to the highest 1995 water level; transects were generated every 100 m and extended 9 km. Net shoreline movement (NSM), endpoint rate (EPR), linear regression rate (LRR), and shoreline change envelope (SCE) were calculated. Positional uncertainty was set at half a pixel: ±40 m for 1977 and ±15 m for later images. NSM and SCE uncertainty was ±42.7 m, and EPR and LRR uncertainty was ±0.89 m/yr. All observed changes exceeded these thresholds and were considered statistically significant. Field observations recorded exposed shallow beds, shell remains, new sediment surfaces, vegetation colonization, new beach strips, and passable former seabed areas.



Results showed that seaward displacement of the water–land boundary was the dominant trend, but its magnitude and pattern varied strongly among units. Gomishan Lagoon exhibited the greatest and most uniform seaward shift. All transects had positive NSM, with a mean of 1223 m and a maximum of 2001 m. Mean LRR was 20.3 m/yr, EPR was 25.3 m/yr, and SCE was 6057.34 m. Gorganrud Delta showed a more heterogeneous pattern. Maximum seaward movement reached 4185 m, while maximum landward movement was 586 m. Mean NSM was 910 m, mean SCE exceeded 3100 m (maximum 4990 m), mean LRR was 16.4 m/yr, and mean EPR was 18.8 m/yr. The eastern coast of Gorgan Bay had a mean NSM of 855 m and a maximum seaward shift of 4467 m near the delta mouth. Mean LRR was 10.69 m/yr, EPR was 17.68 m/yr, and SCE was 1658.84 m. However, negative NSM up to 362 m occurred near the Ashuradeh channel. The southern coast of Gorgan Bay showed the smallest changes: mean NSM 311 m, maximum 655 m, no negative NSM, mean LRR 2.47 m/yr, EPR 6.44 m/yr, and SCE 1350.16 m.



Water-level analysis showed a historical minimum of about −29 m in 1977, a rise to about −26.5 m by 1995, and a sustained decline thereafter, with intensification after 2017. This decline is attributed mainly to climatic and anthropogenic factors, including reduced precipitation, increased evaporation, decreased Volga River inflow, and rising water consumption. The dominant seaward boundary shift is therefore consistent with water-level fall, especially on very low-gradient coasts. However, the results also reveal that water-level change alone cannot explain all spatial differences. The Gorganrud Delta reflects combined water-level and sediment-dynamic controls. The eastern Gorgan Bay coast is influenced by inlet morphology and water exchange through the Ashuradeh channel. The southern Gorgan Bay coast is relatively protected by the Miankaleh spit and exposed to lower wave energy. Comparison of the two low-water periods of 1977 and 2025 indicates that the present morphological condition of the Gorgan Bay inlet cannot be explained only by water level; cumulative sediment and morphological changes over recent decades are also important. Field evidence supports progressive drying and coastal change. These findings provide a quantitative basis for sustainable management of the sensitive southeastern Caspian coast and for maintaining the water exchange of Gorgan Bay. The main limitation is the lack of quantitative analysis of river discharge, sediment load, waves, and littoral sediment transport; therefore, causal interpretations remain partly hypothetical. Future research should integrate hydrological time series and wave and sediment-transport modeling to separate the relative contributions of water-level fall and sediment processes.



Keywords: Caspian Sea; shoreline change; DSAS; Landsat; NDWI; Gorgan Bay; Gorganrud Delta; Gomishan Lagoon; water-level fluctuation.