Sediment accumulation rate assessments for a large reservoir via ²¹⁰Pb and bulk density profiles

Accumulation of sediment in reservoirs behind dams limits their long-term viability for storing water. Thus, estimates of sediment accumulation rates (SARs) are important for evaluating a reservoir's future sustainability. Since many large dams worldwide were built in the 20th Century, the approximately 100-year range of radiometric dating via the natural isotope ²¹⁰Pb (T_1/2 = 22.3 yr) seems appropriate. Unfortunately, prior studies have shown that some ²¹⁰Pb depth profiles in reservoirs are often complex and difficult to interpret. Investigation of sediment cores from a large reservoir in central Thailand shows that good estimates of SARs can be derived from measured sequences of sediment above a recognized bulk density (D_B) interface between the old soil horizon and the overlying relatively recent reservoir sediment. Estimated SARs can then be based on corrected ²¹⁰Pb inventories via a modified Constant Rate of Supply (CRS) model. Sedimentation rates were also obtained via matching of documented periods of low water level anomalies for this reservoir to sedimentary evidence of subaerial exposure. Several cores showed transient occurrences of elevated D_B that can be matched to dates of one of three known extreme low water periods. Nearly 80 % of the 18 cores examined in this investigation yielded useful SAR assessments via D_B profiles or water level anomalies without reliance on radiometric dating.