A series of confirmed and suspected dammed palaeo‐lake sedimentary successions is scattered within the middle Yarlung Tsangpo valley in Tibet. However, the chronology, the genesis of the dam and its location, the water level of the dammed lake, the process of dam failure and the spatiotemporal relationships between the sedimentary successions remain controversial. Here, we focus on one sedimentary succession of the suspected dammed palaeo‐lake at Xigazê. We measured the grain‐size distribution, magnetic susceptibility, organic and inorganic carbon content, and δ
13C
org and δ
15N
total ratios of the sediments. In addition, we measured the δ
18O
shell and δ
13C
shell values of modern and fossil
Radix sp. shells, and the δ
18O
water and δ
13C
DIC values of the ambient water with different hydrological regimes. The results indicate that the δ
18O
shell values of modern
Radix sp. and the δ
18O
water of the ambient water body significantly depend on its hydrological status. In addition, a strong positive relationship was observed between δ
18O
shell values of modern
Radix sp. shells and the δ
18O
water of the ambient water on the Tibetan Plateau. According to this correlation, the δ
18O
water values of the palaeo‐water body are reconstructed using the δ
18O
shell values of
Radix sp. fossil shells in the Xigazê section. Further, based on the δ
18O
shell values of fossil
Radix sp., the reconstructed δ
18O
water of the palaeo‐water body and the specific habitats of
Radix sp., we infer that the sedimentary succession in the Xigazê broad valley was mainly formed within the backwater terminal zone of a dammed palaeo‐lake and that the elevation of the water level of the lake was approximately 3811 m a.s.l. AMS
14C dating indicates that the deposits of the dammed palaeo‐lake were formed at about 33–22 cal. ka BP. Finally, the presence of
Radix sp. fossil shells within the Xigazê section suggests that
Radix sp. survived the late Last Glacial Period on the Tibetan Plateau.
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