| Based on comprehensive field survey, the ZJC profile, a typical loess-palaeosol profile, was selected and sampled in high-resolution at the sensitive climatic zone in the upper reaches of the Jinghe River. And two typical slackwater deposits profiles including CJC profile in the middle reaches from Binxian to Yongshou and LCQ profile in the reaches from Liquan to Chunhua, were sampled and studied. The proxy indicators of environment were determined and analysised by experiments with magnetic susceptibility, loss on ignition, percentage composition of calcium carbonate, particle-size distribution and chemical element in laboratory. This research deals with the study of palaeofloods with respect to climate changes over the last few millennia years.Through the analysis of macroscopical sedimentologic characteristics and physicochemical property of the ZJC profile, characteristics of the Holocene climatic variability and environmental change in the Jinghe River Bansin were reconstructed, providing the background of climatic variability for the Holocene palaeofloods occurrence. Palaeofloods slackwater deposits of the Jinghe River were identified in the CJC and LCQ profiles, which providing information on hydrologic variability and extreme floods over long-term intervals (100-10 000 yr). Hereby the water levels and peak discharges of the extreme palaeofloods with ten-millennia-year return period were reconstructed by hydrological methods, extending the time scale of floods from decades to millennia. Through the modern flood and the relation of peak discharge and drainage area, the palaeoflood discharges were tested reliable. Palaeofloods were used in combination with historical flood data (last 1 000 yr) and the gauged record (last 30-50 yr), we drew the rating curves of the peak discharges and frequency since ten millennia years. Palaeofloods data can also provide physical evidence for the upper limits of flood events in a basin without relying on instrumental records, improving the conventional procedure involves statistical extrapolations from short gauged hydrological records documenting 30-40 year records of observed floods to the estimation of the discharges of very large, rare floods, and the discharges precision was also improved. Thus it is significant to the engineering construction of flood mitigation, water conservancy and hydropower in the Jinghe River Basin.The main results are as following: 1. Through the analysis of proxy environmental indicators of the ZJC profile, this paper revealed the Holocene loess-palaeosol material origin and established the process of the Holocene climatic and environmental changes in the Jinghe River Basin. All the indexes including magnetic susceptibility, loss on ignition, percentage composition of calcium carbonate, particle-size distribution and chemical element, revealed as many as four periods of climatic change in Holocene, they are the late glacial period (14 700-11 500 a B.P.), the early Holocene (11 500-8 500 a B.P.), mid-Holocene (8 500-3 100 a B.P.) and the late Holocene (3 100 a B.P. to present). The characteristics of the Holocene climatic and environmental change in the Jinghe River Basin were reavealed from the ZJC profile, providing a climatic context of palaeofloods occurrence.2. Through stratigraphical and sedimentologic analysis, slackwater flood deposits in two profiles were identified accurately, providing geological evidence for reconstruction water level and peak discharge of the palaeofloods in the Jinghe River. Based on identification of field macroscopical sedimentary characteristics and laboratory analysis on the proxy indexes of magnetic susceptibility, loss on ignition, percentage composition of calcium carbonate, particle-size distribution, we identified five extreme floods event recorded in the CJC profile. Cultural relics in the late period of the Longshan Culture were discovered in the stratigraphy between 420 cm and 450 cm, compared with thermoluminescence dating of other profiles in the Jinghe Basin, we obtained that the extreme palaeoloods occured during the late period of the Longshan Culture (4 200-4 000 a B.P.). The LCQ profile recorded two extreme floods, and their chronologies were similar to the CJC profile. Over the last millennium, the palaeoflood chronologies of the Jinghe River are in accord with the paleaofloods period of the Qishui River in the Weihe River Basin, so it is common that extreme floods occurred during 4 200-4 000 a B.P. in the middle reaches of the Yellow River.3. Reconstruction of water level and shape of cross-section, the peak discharge of palaeoflood were worked out by hydrological method. Through reconstruction of modern floods and the relation of peak discharge and catchment area, the results were verified. From sedimentological evidence of floodwater elevations reached by past floods, water level of palaeoflood were reconstructed by riverload during the late period of the Longshan Culture. Baesd on the methodology of Stratigraphy, Archaeology, Hydrology and referred to 1/10000 terrain map, shape of cross-sections was restored and regulated, other parameters including channel roughness and slope of river bed were determined, adopting slope-area method and hydraulic calculations, peak discharges of extreme palaeofloods during the Holocene were received. The modem extraordinary floods were reconstructed from floodmarks by slope-area method. The error of the calculated results was less than 5%, and rationality of application the slope-area method was verified. Relation between peak discharge and catchment area tested that the calculated peak discharges of palaeofloods were reasonable and credible.4. Taking advantage of palaeoflood discharge data, together with gauged data and historical flood infromation, applying procedure of the Pearson curves in Hydrology, peak discharge-frequency curves of the Jinghe River have been established, which prolonging flood data timeseries of the Jinghe River. The method of design flood calculation altered extrapolation into interpalation, improving the precision of flood frequency, reducing unnecessary funds waste. This research fills the domestic gap of palaeoflood in the Jinghe River, and it is very valuable to the engineering construction of flood mitigation, water conservancy and hydropower, development and utilization of water resources, eco-environmental construction between urban and rural areas, water pollution control in the Jinghe River Basin.5. The research discovered that floods mainly resulting from rainstorm of the Jinghe River Basin were highly sensitive to climate change, and Holocene extreme palaeofloods occurred in the late period of Longshan Culture (4 200-4 000 a B.P.). Extreme floods were unequivocally related to anomalous weather (rainfall) conditions in the late Megathermal of the Holocene. Climatic instability, uneven distribution of annual precipitation within a year, change of intensity of summer monsoon, monsoonal rain-belt moving to the north and activating frequently in the middle reaches of the Yellow River during 4 200-4 000 a B.P., which are the main reasons to the extreme floods in the period.Through field survey, the palaeoflood deposits were found and analyzed on T1 terrace of the Jinghe river. Using stratigraphy, physiognomy and archaeology, it is certain that the extreme palaeoflood events happened in the period of 4 200-4 000 a B.P. since 11 500 a B.P., respectively. Then the peak discharges of palaeofloods were calculated by the hydrological model, which greatly prolonged the flood data series of the Jinghe River. And the flood rating curves were established to improving the reliability to the engineering construction of flood mitigation, water conservancy and hydropower in this basin. This research results are not only practical to the flood mitigation, water resources exploitation and eco-environmental construction, but also fill in the blank of the palaeoflood hydrological study in the Jinghe River Basin, which are of great scientific significance to the comprehensive improvement of the eco-environment of the basin.
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