The upper part of the Krka River estuary and Prokljan Lake are a specific example of a well-stratified estuarine environment in a submerged river canyon. Here, we reconstructed the geomorphological evolution of the area and classified the data gathered in the study, integrating multibeam echosounder data, backscatter echosounder data, side-scan sonar morpho-bathymetric surveys, and acoustic sub-bottom profiling, with the addition of ground-truthing and sediment analyses. This led to the successful classification of the bottom sediments using the object-based image analysis method. Additional inputs to the multibeam echosounder data improved the segmentation of the seafloor classification, geology, and morphology of the surveyed area. This study uncovered and precisely defined distinct geomorphological features, specifically submerged tufa barriers and carbonate mounds active during the Holocene warm periods, analogous to recent tufa barriers that still exist and grow in the upstream part of the Krka River. Fine-grained sediments, classified as estuarine sediments, hold more organic carbon than coarse-grained sediments sampled on barriers. A good correlation of organic carbon with silt sediments allowed the construction of a prediction map for marine sedimentary carbon in this estuarine/lake environment using multibeam echosounder data. Our findings highlight the importance of additional inputs to multibeam echosounder data to achieve the most accurate results.

During the Last Glacial Maximum, a large part of the eastern Adriatic shelf was exposed, resulting in a distinctive landscape. With postglacial sea level rise, many coastal landforms were drowned and filled with Quaternary sediments. Here, we describe the geomorphological features and sedimentary processes shaping the submerged landscape of the Prokljan Lake, a part of the karst Krka River Estuary (KRE). KRE is a 24 km long microtidal salt-wedge estuary located in central Dalmatia. The estuary extends to the last active tufa waterfalls, Skradinski buk, which is a part of the National park Krka. High resolution multibeam bathymetry, sub-bottom profiling and side-scan sonar were used to determine the evolution of the seafloor morphology of the estuary shaped by postglacial sea level variability. The dataset covered approx. 6 km2 of a shallow seabed area with depths of up to 25 m. The observed paleolandscape is unique and well preserved, consisting of the former terrestrial landforms. The most prominent feature is a 3.5 km long complex tufa barrier system, with adjoining basins, at depths ranging from 5 to 20 m bsl. Other distinct features are numerous carbonate mounds and incised river valley. These features are covered or partially filled with fluvial/lacustrine and estuarine sediments. In addition, 36 grab samples and video observations provided ground truth for sediment determination and classification of the estuarine bottom, while 5 piston sediment cores enabled correlation with seismic data. Based on analyzed landform morphology and sediment stratigraphy, the flooding of the river canyon due to sea level rise in the Prokljan Lake started at approx. 10000–9500 y BP and the formation of the estuary at approx. 7500 y BP. The results of ongoing study provide a better understanding of past environments in this submerged system related to the postglacial sea level rise.This study was supported by the Croatian Science Foundation Project “Sediments between source and sink during a late Quaternary eustatic cycle: the Krka River and the Mid Adriatic Deep System” (QMAD) (HRZZ IP-04-2019-8505).