Spatiotemporal Evolution of the Dongting Lake Beach in Recent 90 Years

Using historical topographic maps and aerospace remote sensing data since the 1930s, this study investigates the spatial and temporal evolution of Dongting Lake beach. The evolution characteristics of the beaches in different regions and the related formation mechanism were also analyzed. The results show that Dongting Lake beach expanded from 1 622.17 km2 in 1938 to 1 962.28 km2 in 2018. With the addition of 980.96 km2 of reclaimed high bay beach, the beach area increased by 1 321.07 km2. However, the change process fluctuated somewhat rather than continuously increased. Substantial expansion of the beach area occurred during 1938-1948 and 1958-1998, while slow contraction of the beach area occurred during 1948-1958 and 1998-2018. Dongting Lake beach was dominated by terrigenous debris, the sedimentary types included lacustrine deposits, river alluvial deposits, floodplain and main channel deposits, and river-lake interaction deposits. The rapid expansion occurred in the estuary delta of the east branch of the Ouchi River, which advanced 38.55 km from the estuary toward the lake over the past 90 years. The causes of the changes in the beach included beach reclamation, sediment changes, and lake sand mining. Seventy embankments (covering 2 057.77 km2) have been enclosed in the Dongting Lake area since 1930s, of which the high bay beach covered an area of 980.96 km2. The amount of sediment deposited in Dongting Lake has reached 230 857 x 104 m3 since 1950s, which is equivalent to an average deposition height of 0.85 m on the lake's bottom. The mining of lake sand caused the beach to shrink, and the proportion of the beach area decreased from 77.18% in 1998 to 72.60% in 2018. The results of this study provide objective data for protecting the lakeshore's ecosystem and biodiversity and supporting the ecological restoration and environmental protection of the Yangtze River Basin.