Significance of fluvial landform evolution and river reorganization in settlement development over the past 50 ka: A case study of Baodun site on Chengdu Plain, SW China

The Baodun archaeological site, which is representative of Baodun Culture (4.5–3.6 cal. ka BP), is a well-known walled site covering large areas of the Chengdu Plain (CDP), which is located in the upper reaches of the Yangtze River. The CDP is a low-lying sedimentary basin that is one of the cradles of Chinese civilization and was formed by many rivers originating from the northeastern part of the Qinghai–Tibetan Plateau. Geological and hydrological characteristics have strongly influenced the development of ancient cultures on the CDP. However, their relationship has not been validated. Based on field investigations, ground surveys, microgeomorphological discrimination, grain-size measurements, tree species identification, and the analysis of fluvial deposits, including age dating, height measurement, and observations of gravel layers, we investigated the effects of geomorphology and river channel migration on the evolution of the Baodun site. The results showed that three grades of terraces developed at the Baodun site region during the past ~50 ka BP, which had important impacts on the regional evolution of geomorphology and river systems. The chronology constrains the ages of terraces T1, T2, and T3 to ~7, ~22, and ~48 ka, respectively; T2 and T3 were closely related to tectonic activity, whereas T1 was related to climate variation. Similar to the modern river system, ancient waterflow primarily originated from the northwest; however, it mixed with water from other directions in the past. We attribute the construction of the Baodun site to the persistently outcropped terraces and the attraction of more stable rivers, which provided space and resources for the Baodun people. The evolution of the river system influenced the safety of the Baodun site, and flooding was the main factor leading to its decline and abandonment. The stability of the terrace and sedimentary environments and the decreased precipitation were important factors influencing the large-scale habitation of archaeological sites on the CDP during the middle and late Holocene.