Response of interconnected estuarine lakes to sea-level rise and large hydrological structures

The Hii River basin, and Lakes Shinji and Nakaumi in Japan. This study examines the impacts of sea-level rise (SLR) and hydrological structures on the salinity dynamics of Lakes Shinji and Nakaumi (SHINJI and NAKAUMI), two interconnected estuarine lakes in Japan. Over the past two decades, a significant upward trend in sea level has been observed downstream of these lakes. Furthermore, the construction of a dam and a diversion channel upstream over the past decade has introduced additional hydrological modifications. To quantitatively assess these impacts, we employed the HSPF runoff analysis model and the AEM3D lake water quality prediction model. The results reveal that SLR exerts a more substantial influence on the salinity of SHINJI (upstream) than on NAKAUMI (downstream). This discrepancy arises from the smaller increase in water level in SHINJI compared to NAKAUMI and the adjacent sea, rendering SHINJI more vulnerable to saltwater intrusion. Numerical simulations further indicate that the impact of hydrological structures on lake salinity remains minor, even during extreme flooding events. These findings underscore the need for further investigation into the effects of extreme climate conditions and the development of adaptive management strategies to ensure the sustainable conservation of these lake ecosystems. • Sea-level rise (SLR) notably raises salinity in estuarine lakes, especially in the upstream lake, Shinji. • The Ohara Dam and Hii River diversion channel show minor effects on lake salinity, even during an extreme flood. • Sea-level rise boosts water exchange between Lakes Shinji and Nakaumi, increasing salinity, particularly in shallower areas. • Shallower regions experience a stronger impact from seawater intrusion due to SLR compared to deeper areas. • Findings highlight the necessity for adaptive strategies to mitigate SLR impacts on estuarine ecosystems and local fisheries.