The Hongqiaocun Site: The earliest evidence of ancient flood sedimentation of the water conservancy facilities in the Chengdu Plain, China

•The construction of water conservancy facilities is related to ancient floods.•Frequent flooding may be related to the climate background of about 4000a BP.•The floods are synchronized with those in the Yangtze and Yellow River basins. The earliest water conservancy facilities in the Chengdu Plain were found at the Hongqiaocun Site of about 4.0 ka before present, nearly 2,000 years earlier than the world-famous Dujiangyan Irrigation Project. In order to understand the background of the construction of the water conservancy project, we sampled and analyzed the ancient river profile TS20W13 near the water conservancy facilities. Sedimentology demonstrated that the 4th to 6th layers of the TS20W13 profile were the strongest stage of hydrodynamic force in the river. The Zr/Rb and Ba/Nb values indicated that the 4th to 7th layers might have been those layers of a time of frequent flooding in this ancient river. The results of zircon psephicity analysis also showed that the psephicity of the zircon at the 4th to 6th layers were the best, indicating that along this stage of the river, the average transporting force was the strongest. According to the OSL dating results and the above analysis, at around 4.0 ka BP, the Hongqiaocun Site was in the midst of a flood-prone period, which was synchronous with the ancient flood records of the Yangtze River and Yellow River Basin. The increase in flooding during this period might be related to the aggravation of climatic fluctuation by about 4.0 ka BP. In the context of the above environmental background, the ancient people at the Hongqiaocun Site built water conservancy facilities to withstand floods.