Geoderma (Nov 2023)
Application of 137Cs tracer technique in floodplain deposition research in mesoscale river basins
Abstract
Floodplain deposition can provide important historical information about river systems. 137Cs tracer measurement is the most widely used technique for studying floodplain deposition. One-time sampling is normally used to date or estimate erosion and deposition rates, while time-interval sampling has rarely been applied. Northeast China is an important grain production area characterized by intense erosion and a low sediment transport ratio. Most studies of the region have focused on slope sedimentation, while floodplain sedimentation has received little attention. In this study, two-time steps of cross-sectional sediment core sample collection were performed to examine the floodplain sedimentation in the river basin of the Laolai River, a tributary of the Songhua River. The potential for using 137Cs to estimate the sediment deposition rate for a given time-interval was also investigated. A total of 18 cross-sections were acquired, and three repeated cross-sections were selected for double sampling. Significant differences were observed between the results obtained from the samples collected in separate rounds, suggesting the presence of internal heterogeneity during floodplain sedimentation and a significant influence of sampling positions on the analysis results. In the basin examined, sedimentation often occurs midstream, upstream, and at the outlet, and was mainly affected by topography, reservoir construction, and land use. Sedimentation occurs easily at wide, flat outlets or river intersections in braided water systems. It was observed that downstream floodplain sedimentation was increased by sediment interception by upstream reservoirs, but reduced by forests near floodplains. Sedimentation rates were significantly different between the 1954 and 1963 time markers, and we found that using rates from 1954 alone would lead to underestimation. When 137Cs sedimentation and floodplain distribution are used to determine flooding occurrence, variations in 137Cs content and particle size distribution should be considered. River bank erosion in this region should be taken into account, and riparian protection is required to reduce erosion and floodplain deposition. Since 137Cs is absent in fine particles when the sediment is predominantly sand, 137Cs measurement may underestimate the sediment deposition rate.
