Kura River plume patterns observed by satellite SAR in the coastal zone of the southwestern Caspian Sea

This paper examines the manifestation of river plumes in satellite data. It discusses the mechanisms that determine the radar signatures of coastal plumes formed by the outflow of the Kura River, which flows into the nontidal Caspian Sea and is a typical example of rivers on the Caspian's west coast. This research uses data collected over ten years, from January 2015 to December 2024, through satellite surveys conducted in the southwestern part of the Caspian Sea. Our primary data source is high-resolution radar imagery obtained by synthetic aperture radars (SAR) C-SAR onboard Sentinel-1A and Sentinel-1B satellites. To gain a more complete understanding of the formation and evolution of plumes generated by the Kura River outflow, we also utilized data in the visual (VIS) range of electromagnetic spectrum provided by multispectral sensors MSI (Multispectral Instrument) on Sentinel-2A and Sentinel-2B satellites, as well as scanning radiometers OLI (Operational Land Imager) and OLI-2 on Landsat-8 and Landsat-9 satellites. We analyzed 1,029 C-band SAR images and 880 VIS images. Initially, satellite data revealed the high variability and diverse morphology of the Kura River plumes. Based on these findings, we identified and described the main types of plumes formed by the Kura River outflow as seen in SAR images: plumes with a front at their boundary visualized through backscattered radar signal amplification; plumes with boundaries outlined by slicks; and plumes showing areas of increased or decreased radar backscatter without a distinct boundary front. We briefly discuss the physical mechanisms that govern each visualization type. Additionally, we provide quantitative estimates of changes in radar backscatter intensity within the plume and on its boundaries for these primary mechanisms. We also assess the relative contribution of each mechanism to the SAR signatures of river plumes in the study area. The extensive dataset allowed us to obtain statistically significant insights into the spatial and temporal variability of river plumes in the western Caspian Sea's shelf zone, demonstrating that satellite SAR data can expand the experimental base and facilitate a better understanding of river water distribution in seas with minimal tidal influence.