Abstract
Grain size of lake sediments is often measured in paleolimnological studies, especially investigations of past paleoclimatic and paleohydrologic changes. The implications of such measures, however, remain unclear, since watershed hydrology and the related transfer of materials to the lake are affected by local climate variables, hydrological shifts, and vegetation cover variables. Sediment from Wanghu Lake in the middle reaches of the Yangtze River have apparently been affected by land cover changes and lake-river system transitions caused by a sluice gate built at Fuchi in 1967. These changes influenced the watershed hydrology, thereby confounding paleoclimatic and paleohydrologic interpretations by proxy records in sediment cores. We collected sediment cores from the center of Wanghu Lake and analyzed trends in pollen and physical properties through sedimentary records to investigate land cover changes and hydrological transitions during the past 90 years. The grain size of the sediment core increased with precipitation and the significant relationships between pollen and grain size after 1967 indicated that sediment transfer to the lake was controlled by rainfall and land cover changes due to human-induced deforestation and farming in the lake catchment. Interestingly before 1967, there was no significant relationship between the pollen and grain size or between the precipitation and grain size, indicating that the sediment of WanghuLake was not simply from the lake catchment. The different relationships patterns before and after 1967 indicated that the sediments in the lake were not only transported following precipitation and discharge from the lake catchment but also came from the Yangtze River draining back into the lake during the flood seasons before 1967. These results highlight matters needing attention and the potential application of grain size of sediments for reconstructions of past hydro-environmental changes.
Highlights
The lake-river systems associated with the Yangtze River are experiencing dramatic hydrological changes
Before the 1960s, the watershed was in a natural state with negative population growth, and its economy was backward, according to Yangxin Country records [47]
The sediment core was characterized by high clay percentages and fine grain size, indicating that the watershed remained relatively stable and less erodible (Figure 6)
Summary
The lake-river systems associated with the Yangtze River are experiencing dramatic hydrological changes. This is partially due to human activities within the catchment, such as reservoir construction and land cover changes. Such transitions in hydrology impact river and floodplain ecology within the catchment [1]. The limited availability of pre-instrumental observations in the catchment undermines our ability to identify site-specific causes of the hydrological shifts [2]. Alternative methods are required to identify the trends in and the main causes of historical change in the hydrology of lake-river systems.
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