The Hanjiang River, a major tributary of the Yangtze River in China, is noted for the current national South-to-North Water Diversion project. Palaeo-hydrological investigations were carried out along the upper reach gorges of the Hanjiang River that drains the Qinling and the Dabashan Mountains. A set of palaeoflood slackwater deposit beds (SWDs) was identified in Holocene pedo-stratigraphy of the riverbanks along the valley. These SWDs are interbedded in the eolian loess-soil profiles in the cliffy riverbanks and they thin out toward the upper slopes. The palaeoflood SWDs were differentiated from eolian loess and soil by the sedimentary criteria and analytical results. The minimum flood peak discharges were estimated to be 65,400−65,830m3s−1 by using palaeo-hydrological methods. They are about twice of the largest gauged flood (34,300m3s−1) that has ever been measured. They represent the largest flood events in the upper reaches of the Hanjiang River over the Holocene. These extraordinary flood events were dated to 1810−1710 a (AD 200−300) with the optically stimulated luminescence method and checked by archaeological dating of the human remains retrieved from the profiles. This indicates that the extraordinary floods occurred during the dynasties of Eastern Han to Western Jin (AD 25−316) in Chinese history, during which severe droughts and floods were recorded in documents. In the reconstructed 2000-year temperature time series based on high-resolution climatic proxies from tree-rings, stalagmites, ice-cores and lake sediments from over the world, these extraordinary palaeoflood events are correlated with an increased climatic variability characterized by cooling and drying during the period AD 150−350. This result is important for understanding the effects of global change on river system dynamics.

Archaeological excavations have exposed a shocking picture of the prehistorical catastrophes in the Lajia Ruins in the upper Yellow River. The grouped skeletons resting on the dwelling floors show a vivid scene of the mortal struggle of human being during catastrophes. Geological records of the major disasters in relation to the devastation of this prehistorical settlement were investigated by detailed field observations, sedimentological analysis and optically stimulated luminescence (OSL) and 14C dating. The results show that the prehistorical community of the Qijia Culture migrated into the Guanting Basin at c. 4200 yr BP. They chose the loess-blanketed riverbanks to build their settlement and cultivate millet crops. At c. 3950 yr BP, immediately followed by a major earthquake, the settlement was overtaken by immense mudflows coming along the tributary gullies from the hillsides behind. The enormous mudflows suddenly buried and destroyed the dwellings and killed the women and children at their homes. The source of the catastrophic mudflows was traced upslope to the gully heads behind the ruins, where the palaeoearthquake together with the rainstorms and flash floods induced enormous mass wasting on the hillsides. Referring to the causations of the catastrophic debris flows and landslides in the region, it is inferred that the soil erosion, mass wasting and accumulation of debris on the hillsides were intensified largely by human disturbance of the landscape by bush clearance from 6000 yr BP to 3950 yr BP. During the climate event of 4200–4000 yr BP, enhanced human activities and over-exploitation of natural resources increased the vulnerability of the communities to detrimental environmental change and catastrophe. This means that the prehistorical catastrophic mudflows were created partly by the early settlers themselves. These results are of important implications in understanding the prehistorical environmental change in the environmentally sensitive zones over the world.

Palaeoflood slackwater deposits (SWDs), are the natural record of overbank flooding and are often found within aeolian loess‐soil profiles along the river valleys of the middle Yellow River basin. These pedo‐stratigraphic sequences are studied using a multi‐disciplinarily approach to reconstruct Holocene hydro‐climatic variations. Our field investigations along the lower Jinghe River valley have identified palaeoflood SWDs at several sites along the riverbanks based on sedimentological criteria. Analytical results, including magnetic susceptibility, particle‐size distribution and concentrations of chemical elements, indicate that these well‐sorted palaeoflood SWD beds were deposited from the suspended sediment load in floodwaters. We identify two episodes of extraordinary palaeoflood events along the Jinghe River valley. These hydro‐climatic events were dated to 4200–4000 and 3200–2800 a BP, by using the optically stimulated luminescence method in combination with archaeological dating of retrieved anthropogenic remains, and with pedo‐stratigraphic correlations with the previously studied Holocene pedo‐stratigraphy in the Jinghe River drainage basin. The flooding events are therefore considered to be a regional expression of known climatic events in the northern hemisphere and demonstrate Holocene climate was far from stable. This study provides important data in understanding the interactions between regional hydro‐climatic systems and global change in semi‐arid and sub‐humid regions. Copyright © 2012 John Wiley & Sons, Ltd.

Palaeo-hydrological study was carried out in the Qishuihe River valley in the middle reaches of the Yellow River. Several bedsets of flood slackwater deposit were identified in the Holocene loess-soil sequences on the riverbanks. They were differentiated from aeolian loess and soils by the parallel and waving beddings and the distinctive stratigraphic breaks separating individual palaeoflood events. Chronology of the flood events was established by OSL dating, checked by archaeological identification of the anthropogenic remains retrieved from the sequences. The results show that successive floods occurred between 4300 and 4000 a BP in association with the abrupt climatic event of 4200 a BP. These overbank floods had the riverbank settlement inundated repeatedly. Another series of extraordinary floods occurred between 3200 and 3000 a BP when monsoonal climate shifted from the mid-Holocene Climatic Optimum toward late Holocene dry conditions. The climatic event of 4200 a BP and the climatic decline at 3100 a BP were believed to be characterized by droughts previously. This work provides solid evidence that both severe droughts and extreme floods were parts of the climatic variability during abrupt climatic event and climatic decline in the semi-arid to sub-humid zones over the world.

The surface landscape of the Chinese Loess Plateau features densely developed gully systems. Gully incision and extension are extremely active over the elevated regions of loess hills, ridges and tablelands. This paper presents the results of investigations on major gullying phases over the Loess Plateau during the Quaternary. The objective is to establish a chronology of gullying events and to examine their relations with monsoonal climatic changes and neo-tectonic uplifts. Results from field observations, geomorphologic and pedo-stratigraphic analysis indicate that pre-historical gullying events are recorded by buried palaeogullies and pedo-stratigraphic unconformities preserved in loess sections. There were four major gullying phases identified at the loess to palaeosol transitions of L6/S5, L3/S2, L2/S1, L1/S0 in the stratigraphy over the Loess Plateau. The age of a gullying event is closely related with the bottom unit of the palaeosol series in the overlying loess blanket. The Gullying phases were therefore dated to ca. 600 ka, 240 ka, 125 ka and 11.5 ka, respectively, by loess chronology. This means that gully incision occurred when large-scale monsoonal climatic shift met with coincided neo-tectonic uplift of the land mass. Neo-tectonic uplift provided the gullying with potential for deep down-cutting. Excessive runoff resulted from precipitation brought on by intensified maritime monsoon was a dynamic forcing of the gullying at the transitions of MIS 16/15, 8/7, 6/5 and 2/1. In response to each wave of gully incision and cross-section development occurred the outspreading of the tributary gullies, and a new order of gullies formed. Gully density is therefore increased and the loess land became more dissected hills and ridges. These results provide new insights into history of gully erosion and long-term landscape development in the semi-arid loess regions of the world. (C) 2011 Elsevier Ltd and INQUA. All rights reserved.

Sedimentary records of the Holocene extraordinary floods were investigated in the upper reaches of the Weihe River, a major tributary in the middle Yellow River basin. Palaeoflood slackwater deposits (SWD) were identified at several sites on the riverbanks. These clayey silt beds are inserted into the Holocene aeolian loess-soil profiles and slope clastic deposits. They have recorded the extraordinary palaeoflood events which occurred between 3200 and 3000 yr BP as dated by OSL method and checked by the archaeological remains of the Neolithic and Bronze Age retrieved from the profile. The minimum flood peak discharges were estimated at between 22 560 and 25 960 m3/s by using palaeohydrological methods. It is 4.5–5.0 times the largest gauged flood (5030 m3/s) that has ever occurred since 1934. The palaeoflood slackwater deposits were found inserted into the pedostratigraphic boundary between the late-Holocene loess (L0) and the mid-Holocene Luvisol (S0) in the riverbank profile. This indicates that the extraordinary flood events were synchronous with the pedogenic regression at the end of the mid-Holocene climatic optimum. The climatic proxies from the studied profile and the correlative profiles in the river valley and the Loess Plateau show that the pedogenic regression was forced by climatic aridity and intensified dust storms and dust falls in connection with monsoonal shift over the Yellow River basin at about 3100 yr BP. The extraordinary flood events were documented not only on the Weihe River, but also on the mainstream and other tributaries of the Yellow River. These suggest that both extraordinary floods and droughts were parts of the climatic variability during the monsoonal shift. These findings are of great importance in understanding the interactions between hydrological system and climatic change in the semi-arid and subhumid regions of the world.

Holocene palaeohydrological investigations were carried out in the middle reach of the Jinghe River that drains the central part of the Loess Plateau. A set of palaeoflood slackwater deposits was found interbedded in the Holocene loess-soil sequence within the cliff riverbanks. Both the sedimentary criteria and the analytical results show that this sediment was sourced from a suspended sediment load of floodwater. The major cultural layer of a late Neolithic settlement (4300−4000a BP) occurring at the same site is blanketed by this flood deposit directly. Slopewash with pottery shards, charcoal and burnt earth occurs in between the palaeoflood slackwater deposit beds. It indicates that each of the slackwater deposit beds has recorded one individual flood event, and that these extraordinary floods occurred when the late Neolithic people occupied the riverbank terrace land. This pre-historical settlement was abandoned after the floods eventually due presumably to repeated inundation by overbank floodwater. The flood events were OSL dated to between 4100 and 4000a BP and checked by archaeological dating of the anthropogenic remains retrieved from the sequences. Peak discharges of the floods were estimated to between 19,500 and 22,000m3s−1, which is several times gauged maximum floods. During the 4200–4000a BP climatic event, severe droughts were documented in China's monsoonal regions and great floods were known from an ancient legend. This study provides a well-dated sediment record of pre-historical floods which occurred at the transition from the Neolithic to the Bronze Age in the Yellow River basin. It shows that extraordinary floods were part of the abrupt climatic variability during the 4200−4000a BP climatic event. The highly variable, unstable and catastrophic climate typified by both droughts and floods resulted in settlement abandonment, and possibly the decline of the highly developed late Neolithic civilizations in China's monsoonal regions.

Chinese loess deposits are generally considered to be the product of dust storms and dust falls from the central Asia arid zones that were transported across China by the northwesterly continental monsoon. In contrast, the Zhengzhou Loess found southeast of the Loess Plateau, adjacent to the floodplain of the Yellow River, records a different eolian regime and dust source. The Zhengzhou Loess was investigated by field observations, measurements of magnetic susceptibility, particle-size distribution, loss-on-ignition, CaCO3 and chemical contents. Both field observations and the laboratory results indicate that, during the last glacial, the Zhengzhou Loess was supplied by two different eolian regimes and dust sources, one was from the fresh flood deposits of the Yellow River driven by the northeast winds from the low-lying floodplain, and the other was from the dust storms and dust falls that traveled across the Loess Plateau driven by the northwesterly continental monsoon from the central Asian arid lands. The early Holocene, 11,500–8500aBP, was a transition during the change in eolian regime and dust source because of the weakened northwesterly monsoon along with the global climatic amelioration. Following the retreat of the northwesterly monsoon from the onset of the mid-Holocene Climatic Optimum at 8500aBP, dust supply from the drifting sand zone on the Yellow River floodplain became dominant because of the intensified strength of the northeast winds from the Bohai Sea. From 3100aBP onwards, climatic aridity and extensive human disturbance have resulted in intensive eolian processes causing the incursion of the drifting sand into the Zhengzhou Loess zone. These results show that loess accumulation is more complex than traditionally assumed. The origin of loess deposits elsewhere outside the Loess Plateau may be related to dust sources derived from alluvial sediments of major river systems.

The Ustic Isohumisol (Chernozem), widely distributed over the Chinese Loess Plateau, is classified as a modem soil by agricultural pedologists and as a palaeosol by geoscientists. High-resolution investigations into this soil, including field observations, element analysis, optically simulated luminescence dating and measurements of magnetic susceptibility and particle-size distribution, were carried out over the plateau uplands. The results show that the formation of physico-chemical properties and development of the soil profiles has kept pace with the land surface growth because of incessant dust accumulation during the Holocene. The continuously intensified weathering of accumulated dust from 11,500 a B.P. had resulted in an increased chemical differentiation through time during the early Holocene. The development of the Ustic Isohumisol occurred over the uplands between 8500 and 3100 a B.P., when the chemical differentiation reached its maximum because of increased precipitation and ample soil moisture in response to the mid-Holocene Climatic Optimum. The soil formation had sustained for ca. 5400 years and ceased at 3100 a B.P. A major pedogenic regression occurred from 3100 a B.P. because of climatic aridity and intensified dust accumulation over the Loess Plateau. The newly accumulated loess and topsoil has the Ustic Isohumisol buried at a depth of ca. 50-80 cm in general. The pedogenic regression has been intensified by incorporation of the reworked components by increased runoff and erosion-redeposition. Large-scale land reclamation extended to the uplands at ca. 2170 a B.P. An agricultural landscape of dry farming has formed since ca. 1500 a B.P. Thereafter, the present topsoil has accumulated and has been affected by the intensified cultivation and addition of barnyard manure in history. The solubles are retained in the topsoil because of dry climate and scant soil moisture. (C) 2009 Elsevier B.V. All rights reserved.

In Chinese history, the development of the Zhou tribes and the dynasties that succeeded them between 3550 and 2200 BP in the middle reaches of the Yellow River is known as the period of 'Five Relocations'. Most of these relocations appear to have been forced by the pressure of nomadic invasions and occupations from the northern steppe. Historians simply attributed these relocations to political and military causes. However, palaeoclimatic studies show that the Zhou tribes and their successor dynasties developed at the demise of the Holocene Climatic Optimum when regional climate became highly variable and unstable. An integrated analysis of environmental change over the Loess Plateau and the Mongolian steppe facilitates a credible understanding of the linkage between climatic events and these relocations. It indicates that the relocations caused an expansion or contraction of the settled regions over the drought-prone semi-arid lands. During climatic amelioration, dry farming societies pushed upward to the Loess Plateau where increased precipitation and soil moisture allowed cereals to be cultivated. When hit by droughts and the associated disasters, both the dry farming societies and the nomadic tribes had to move southward to find an environment suitable for their food production. Migration and relocation were, therefore, basic strategies to secure the resources necessary to sustain an agricultural economy. The settled frontier was pulled back as dry farming societies from the upland plateau retreated to the lowland riverbanks of the Guanzhong Basin. Even though there were political and military intentions, climatic events played an essential role in the relocations of the Zhou tribes and the successive dynasties. (C) 2008 Elsevier Ltd. All rights reserved.

In the semiarid loess regions, slackwater deposition of overbank flooding over the piedmont alluvial plains was episodic and alternated with dust accumulation and soil formation throughout the Holocene. The records of past hydrological events are therefore preserved within the architecture of loess and soils and are protected from subsequent erosion and destruction. Several Holocene loess–soil sequences with the deposits of overbank flooding over the semiarid piedmont alluvial plains in the southeast part of the middle reaches of the Yellow River drainage basin were investigated by field observation, OSL and C14 dating, measurement of magnetic susceptibility, particle-size distribution and chemical elements. This enables the reconstruction of a complete catalog of Holocene overbank flooding events at a watershed scale and an investigation of hydrological response to monsoonal climatic change as well. During the Holocene, there are six episodes of overbank flooding recorded over the alluvial plain. The first occurred at 11,500–11,000aBP, i.e. the onset of the Holocene. The second took place at 9500–8500aBP, immediately before the mid-Holocene Climatic Optimum. After an extended geomorphic stability and soil formation, the third overbank flooding episode came at about 3620–3520aBP, i.e. the late stage of the mid-Holocene Climatic Optimum, and the floodwater inundated and devastated a Bronze-age town of the Xia Culture built on the alluvial plain, and therefore the town was abandoned for a period of ca 100 years. During the late Holocene, the alluvial plain experienced three episodes of overbank flooding at 2420–2170, 1860–1700 and 680–100aBP, respectively. The occurrence of these overbank flooding episodes corresponds to the anomalous change in monsoonal climate in the middle reaches of the Yellow River drainage basin when rapid climate change or climatic decline occurs. During at least the last four episodes, both extreme floods and droughts occurred and climate departed from its normal condition, which was defined as a balanced change between the northwestern continental monsoon and southeastern maritime monsoon over time. Great floods occurred as a result of extreme rainstorms in summers caused by rare intensive meridianal airflows involving northwestward moving tropical cyclone systems from the Pacific. These results could be applied to improve our understanding of high-resolution climatic change, and of hydrological response to climatic change in the semiarid zones.

Colluvial components incorporated in loess-soil sequences have preserved evidence of soil erosion and redeposition in the middle reaches of the Yellow River drainage basin. Holocene loess-soil sequences on the gentle sloped lands were studied by measuring: magnetic susceptibility, particle-size distribution, CaCO3 and extraction and identification of the non-dust materials. The analytical results indicate that in the early Holocene soil erosion and redeposition by overland flow occurred, even on the loess lands with small ground gradient, in response to climatic amelioration linked to global climatic change. No obvious soil erosion and redeposition events were found to correspond with the climatic fluctuations recorded in the loess-soil sequence during the Holocene Climatic Optimum. The primary Neolithic farming did not cause apparent erosion and redeposition because the loess landscape was mantled by well-developed Ustic Luvisols and probably by forest vegetation, both provided adequate resistance to sheet erosion by overland flow. Enhanced soil erosion and redeposition are traceable back to ca. 4000 a B.R It is attributed to intensified land use of arable farming and human settlement from the early Bronze Age onward. A major event of sheet erosion and redeposition by overland flow occurred in the loess landscape during 4000-3600 a B.R in the Xia dynasty. Soil degradation and reduced resistance caused by climatic aridity and intensified dust accumulation may have played a secondary role in promoting soil erosion since 3100 a B.P. These results provide new insights into the history of soil erosion in the semi-arid and semi-humid loess regions and into the evaluation of human-induced accelerated soil erosion against the background of the Holocene natural erosion forced by monsoonal climatic fluctuation. (c) 2006 Elsevier B.V. All rights reserved.

Charcoal preserved in accretionary loess–soil profiles within the southern part of the Loess Plateau has recorded fire history and landscape evolution connected with climatic variations and human activities. Local wildfires occurred frequently during the late last glacial period and the early Holocene before 8500years BP when steppe vegetation expanded from the Inner Mongolian Plateau southerly over the Loess Plateau. During the Holocene climatic optimum between 8500years BP and 3100years BP, natural wildfires were largely reduced. Pedogenesis of the Luvisols and Isohumisols was intensified because of the increased monsoonal precipitation and soil moisture. Localized fires in connection with human activities occurred in different patterns at the study sites. Fire seems to have been applied to vegetation clearance for land reclamation for millet cultivation at many places during the Neolithic and early Bronze Age. Levels of biomass burning were very high during the late Holocene, when the climate became drier and historical land-use became more intensive. At some sites, the intensity of human disturbance by fire and cultivation increased continuously during the last 3100years. At the other sites, local fires occurred most frequently between 3100years BP and 1500years BP during the major period of land reclamation for cereal cultivation. Human burning of the landscape has been reduced since then.