## 3 Case Study Results
### Historical Climate, Sea-Level Change, and Hydomorphology
#### 3.1.3 Hydomorphology
The channels build out to form the \"toes\" of the \"bird's foot\", between which there are large interdistributary bays (Figure 5c). These bays are relatively sheltered and are sites of fine-grained subaqueous sedimentation. In front of the channels, mouth bars have formed, localized to the areas in front of the individual delta lobes. Since the Ice Age, the MSL has risen about 10 mm/yr and constantly moved the mouth of the river deeper and deeper into the land, thus spreading the influence of seawater deeper into the land environment. This constantly reduced the river production zone (watershed), water, and sediment supply. The space on the delta plain generated by the MSL rise could not be filled, so the delta constantly transgressed. Therefore, throughout history, the total length of the river and river zone has constantly reduced (approximately 120 km) [22; 23; 28]. The rapid rise in sea level and the upstream movement of the estuary and delta averaged 4 cm/yr, ending at the beginning of antiquity. During the antiquity period, the MSL rise slowed down, the equilibrium changed, and a delta (bottom) regressive coast arose, moving the mouth downstream by about 12 km (at an average of 3 m/yr). This increased the influence of the river on delta hydomorphology and biodiversity. This process continued until the beginning of the 20 th century, when the equilibrium state changed again and the delta transgressed. The delta transgress process was caused by a rapid MSL rise and low sediment supply owing to the construction of large reservoirs in the river's upper zone and land reclamation in the lower zone. Today, sediment is transported directly to the coastal sea, outside the \"existing\" delta. The significant delta areas have less physical variety and connectivity to surrounding areas and do not flood as frequently. Sediment accumulation is significantly reduced, as is the transboundary flow of energy  
Figure 5: Historical sequence of the development of the Neretva Delta (CC BY 4.0). (**a**) The Neretva River in the Roman period [29]. (**b**) [PERSON]’s version of [PERSON] map of the region from 1701 [30]. (**c**) The Neretva Delta on the map of Dalmatia 1869 [31]. (**d**) River regulation project 1887 [32]. and mass. The river's hydomorphologies and biophysical environments are unstable and vulnerable and generate high entropy, thus reducing ecosystem services and increasing risks to nature, society, and the economy. Simultaneously, reclaimed land and polders are increasingly exposed to river and seawater energy. Free transboundary interactions between MSL change and seawater energy occurred only in the peripheral areas of the Neretva Valley. The sediment supply in these areas is extremely low because only local karst watershed areas generate sediment. The characteristics of the natural delta, which is currently a protected wetland area, have been partly preserved. Bottom transgression also occurs in the river itself owing to the influence of the sea and reduced sediment supply, as well as an increase in the level of surface freshwater owing to the intrusion of seawater, thereby raising the groundwater level. Reclaimed lands (polders) are functionally separated from the river and sea energy processes. However, indirect transgression affects polders due to saltwater intrusion into coastal aquifers. This is why the transgression effects will ultimately be the same (flooding) if water pumping does not reduce the levels of surface water and groundwater. Thus, global climate change and historic local human interventions create negative CIs on the delta's biophysical environment, society, and the biophysical economy. The described historic dynamics of change were confirmed by delta deposits at the town of Opuzen, which, in the distant past, was in the upper zone of the river, changing to the lower zone in the last 2,000 years (Figure 6) [4]. The structure of a deltaic deposit is the result of historical changes in the functional zone positions of streams, internal river characteristics, and delta formation processes.