Sources of fine-sediment reservoir deposits from contrasting lithological zones in a medium-sized catchment over the past 60 years

Abstract

Reservoirs influence the processes associated with the downstream transportation and deposition of sediments in various sizes of catchments. Patterns of sediment yield from different lithological zones in a catchment can differ greatly. Assessing the historical sources of fine sediment deposited in a reservoir derived from contrasting lithological zones aids our understanding of the characteristic pattern of sediment yields from different lithological environments and improves our knowledge of sediment sources in reservoir systems. However, few previous studies have addressed this issue. In this study, geochemical fingerprinting was used to identify the sources of the fine sediments derived from contrasting lithological zones and deposited in a reservoir in a medium-sized catchment in a rocky mountainous region of northern China over the past 60 years, using the chronology determined by 137Cs and 210Pbex tracers. The influence of grain-size sorting on the source of the fine sediments deposited in the reservoir was examined. Soil samples were collected from three types of lithological zones within the catchment, namely, the granite (205 samples), limestone (120 samples), and shale (86 samples) zones, and the elemental composition and grain size of these samples was determined. Six sediment cores (Cores 1–6) were collected from the perennial backwater area of the reservoir to determine the elemental composition, grain size, chronologies, and the source of fine sediments derived from the three types of lithological zones. Two further cores (Cores 7 and 8) were collected from the reservoir’s upstream floodplain, and the grain size distributions and chronologies of these deposits were also determined. Overall, the sources of fine sediment identified in Cores 1–6 reflected the changing trends in sediment contribution from the three types of lithological zones over the past 60 years, which was linked to the differing sediment yields from the contrasting lithological zones. The granite zone occupies half of the catchment, has thicker soils, is relatively susceptible to soil erosion, and has been subjected to greater disturbance by human activity than the limestone and shale zones, but contributed only 25% of the fine sediment in Cores 1–6. In contrast, the limestone and shale zones occupy 23% and 15%, respectively, of the catchment but contributed 42% and 33% of the fine sediment in Cores 1–6. This result is related to grain-size sorting during downstream sediment transportation and deposition in the reservoir system, as demonstrated by a comparison of the grain size distributions of the soil samples from the three types of lithological zones, Cores 7 and 8, and Cores 1–6. These findings suggest the sediment source has been strongly influenced by grain-size sorting. This study provides new insights into the source of sediments deposited in a reservoir system from contrasting lithological zones at the scale of a medium-sized catchment over multi-decadal timescales.