Lacustrine sediments provide an excellent opportunity to study climate change beyond the instrumental period and further to elucidate its intrinsic interactions with ecosystem. However, the radiocarbon (¹⁴C)-based chronological framework of such natural archives, on which robust interpretation of geochemical proxies primarily depends, is commonly challenged due to the difficulty in properly correcting carbon reservoir effect. Here we combine new and existing ¹⁴C dates on total organic carbon (TOC) in two sediment cores from Lake Qinghai and Manas, northwest China, to evaluate the reservoir effect (expressed by reservoir age, RA) over the past ~1500 years. Generally, these two cores characterize RA values ranging from ~2100 to 4200 and from ~350 to 2100 years respectively. Straightforward comparison with paleorecords generated from the same cores indicates a close correlation between RA changes and lake level fluctuations on multi-centennial timescales. Our results suggest that lake level changes (or hydrological variations), which are expected to control the relative input of terrigenous old carbon, merit careful consideration to reduce dating uncertainty. It is essential to select TOC samples across discrete depths, under the control of similar lake status, to allow comparable RAs and, as a result, to establish reliable chronology.