关键词: 古洪水, 滞流沉积物, 全新世, 丹江

Abstract: The Danjiang River，a major tributary of the Hanjiang River，is noted for the current national South- to-North Water Diversion project. Recent flood disasters and flood damages have attracted media and public attentions from the world. The data generated from palaeoflood hydrology will facilitate an appropriate assessment of these flood events and the regional response of the hydrological system to global climatic change at a long time scale. Palaeoflood slackwater deposit （SWD） is a major evidence used for inference about hydrological parameters of the past flood events. This fine-grained sediment is deposited at high flood stage from suspended load in overbank floodwater. It could be buried and preserved by various surface processes in the Holocene pedo-stratigraphy in the riverbanks. Field investigations were carried out along the upper reaches gorge of the Danjiang River that drains from the south piedmont of the Qinling Mountains. A set of 4 palaeoflood slackwater deposit beds （SWDs） of the Holocene age were found in the depth range of 10-40 cm by using the sedimentological criteria at the LZT sites，which is situated at the riverbank terrace land. The front of the terrace land is ca. 16 m above the normal river water level. It formed an outstanding threshold （censoring level） for registration of extraordinary overbank floods through deposition of the suspended sediment load. It was further checked by comparison with the modern flood events and their SWDs deposited on the channel sides in the same reaches. They were identified by sedimentary criteria during fieldwork initially，sampled and invested in detail. Particle-size distribution of the samples was determined using a Mastersizer-S laser analyzer. Analysis of the grain-size distribution indicates that these SWD consist of sandy silt，sourced from the suspended sediment load of the floodwater. Therefore they were well sorted during the deposition. The shape of particle-size distribution curves of the SWD is high and thin. The curves of the palaeoflood SWDs at the LZT site peak between 5-8 μm （SWD2 and SWD4） and 30-40 μm （SWD1 and SWD3）. They were corresponded to the types of Ⅶ（SWD1 and SWD3） and Ⅷ（SWD2 and SWD4） in CM patterns. By an integration of the field observations and analytical results，the sediment in the depth range of 10-40 cm in the LZT profile is therefore defined as palaeoflood slackwater deposits. These palaeoflood SWD beds distribute at the highest elevation and were seen at many sites in the riverbanks along the upper reach gorges of the Danjiang River and the Hanjiang River. They have recorded the largest palaeoflood events occurred in the Danjiang River basin during the last 11 500 years. This study is of great significance to establish a long time scale hydrological data for flood control and water-soil conservation in the upper reaches of the Danjiang River.

Key words: palaeoflood, slackwater deposits, Holocene, the Danjiang River