Fluvial landscapes of the Harappan civilization.

The collapse of the Bronze Age Harappan, one of the earliest urban civilizations, remains an enigma. Urbanism flourished in the western region of the Indo-Gangetic Plain for approximately 600 y, but since approximately 3,900 y ago, the total settled area and settlement sizes declined, many sites were abandoned, and a significant shift in site numbers and density towards the east is recorded. We report morphologic and chronologic evidence indicating that fluvial landscapes in Harappan territory became remarkably stable during the late Holocene as aridification intensified in the region after approximately 5,000 BP. Upstream on the alluvial plain, the large Himalayan rivers in Punjab stopped incising, while downstream, sedimentation slowed on the distinctive mega-fluvial ridge, which the Indus built in Sindh. This fluvial quiescence suggests a gradual decrease in flood intensity that probably stimulated intensive agriculture initially and encouraged urbanization around 4,500 BP. However, further decline in monsoon precipitation led to conditions adverse to both inundation- and rain-based farming. Contrary to earlier assumptions that a large glacier-fed Himalayan river, identified by some with the mythical Sarasvati, watered the Harappan heartland on the interfluve between the Indus and Ganges basins, we show that only monsoonal-fed rivers were active there during the Holocene. As the monsoon weakened, monsoonal rivers gradually dried or became seasonal, affecting habitability along their courses. Hydroclimatic stress increased the vulnerability of agricultural production supporting Harappan urbanism, leading to settlement downsizing, diversification of crops, and a drastic increase in settlements in the moister monsoon regions of the upper Punjab, Haryana, and Uttar Pradesh.

Socio-economic impacts on flooding: a 4000-year history of the Yellow River, China.

We analyze 4000-year flood history of the lower Yellow River and the history of agricultural development in the middle river by investigating historical writings and quantitative time series data of environmental changes in the river basin. Flood dynamics are characterized by positive feedback loops, critical thresholds of natural processes, and abrupt transitions caused by socio-economic factors. Technological and organizational innovations were dominant driving forces of the flood history. The popularization of iron plows and embarkment of the lower river in the 4th century BC initiated a positive feedback loop on levee breaches. The strength of the feedback loop was enhanced by farming of coarse-sediment producing areas, steep hillslope cultivation, and a new river management paradigm, and finally pushed the flood frequency to its climax in the seventeenth century. The co-evolution of river dynamics and Chinese society is remarkable, especially farming and soil erosion in the middle river, and central authority and river management in the lower river.

Anthropogenic impacts on the decreasing sediment loads of nine major rivers in China, 1954-2015.

Over the past 60 years, because of the combined impacts of human activities and climate change, the sediment load of the nine major rivers (the Yellow, Yangtze, Pearl, Songhuajiang, Liaohe, Haihe, Huaihe, Qiantangjiang, and Minjiang rivers) in China has dropped by 85%, which had caused serious environmental problems such as reservoir siltation and estuary erosion. However, quantitatively evaluating the impact of different human activities on this decline is still an unsolved and complex problem. Based on a big new data set from 27 gauge stations and 469 meteorological stations, we established five methods to assess sediment loss of China's nine major rivers. During 1954-2015, the sediment load into the marginal seas via these nine rivers was characterized by a marked decline, from 1.95 Gt/yr (1954-1968) to 1.40 Gt/yr (1969-1985), 890 Mt/yr (1986-1998), 450 Mt/yr (1999-2003), and 310 Mt/yr (2004-2015), reflecting an 85% decrease between 1954-1968 and 2004-2015. The cumulative sediment load into the marginal seas was ~71.0 Gt, constituting ~7% of the global sediment load. The Yellow River, Yangtze River, Pearl River, and other six major rivers contributed 40.9 (58%), 22.9 (32%), 4.1 (6%), and 2.96 Gt (4%), respectively. We estimate that ~53.0 Gt of terrestrial sediment has been retained on the mainland China because of human activities, with reservoir trapping, water resource utilization, and water-soil conservation measures accounting for 45.5%, 29%, and 25.5% of the total, respectively. The contribution of climatic factors was assessed to be secondary. This drastic reduction in river sediment load could lead to a series of negative effects on deltas: decreased sediment delivery, coastal erosion, aggravated reaction to storm disasters, and most importantly, loss of new land for human use. In addition, the large amounts of sediment trapping by reservoirs over long periods will cause siltation that could reduce reservoir water storage capacity.

Assessing the effect of natural controls and land use change on sediment yield in a major Andean River: the Magdalena drainage basin, Colombia.

The Magdalena River, a major fluvial system draining most of the Colombian Andes, is a world-class river, in the top 10 in terms of sediment load (approximately 150 MT/yr). In this study, we explore the major natural factors and anthropogenic influences behind the patterns in sediment yield on the Magdalena basin and reconstruct the spatial and temporal pattern of deforestation and agricultural intensification across the basin to test the relationships between land use change and trends in sediment yield. Our results show that sediment yield for the whole Magdalena catchment can be explained by natural variables, including runoff and maximum water discharge. These two estimators explain 58% of variance in sediment yield. Temporal analyses of sediment discharges and land use show that the extent of erosion within the catchment has increased over the last 10 to 20 years. Many anthropogenic influences, including a forest decrease by 40% in a 20-year period, an agriculture and pasture increase by 65%, poor soil conservation and mining practices, and increasing rates of urbanization, may have accounted for the overall increasing trends in sediment yield on a regional scale.

Sediment flux and the Anthropocene.

Data and computer simulations are reviewed to help better define the timing and magnitude of human influence on sediment flux--the Anthropocene epoch. Impacts on the Earth surface processes are not spatially or temporally homogeneous. Human influences on this sediment flux have a secondary effect on floodplain and delta-plain functions and sediment dispersal into the coastal ocean. Human impact on sediment production began 3000 years ago but accelerated more widely 1000 years ago. By the sixteenth century, societies were already engineering their environment. Early twentieth century mechanization has led to global signals of increased sediment flux in most large rivers. By the 1950s, this sediment disturbance signal reversed for many rivers owing to the proliferation of dams, and sediment load reduction below pristine conditions is the dominant signal today. A delta subsidence signal began in the 1930s and is now a dominant signal in terms of sea level for many coastal environments, overwhelming even the global warming imprint on sea level. Humans have engineered how most water and sediment are discharged into the coastal ocean. Hyperpycnal flow events have become more common for some rivers, and less common for other rivers. Bottom trawling is now widespread, suggesting that even continental shelves have received a significant but as yet quantified Anthropocene impact. The Anthropocene attains the level of a geological climate event, such as that seen in the transition between the Pleistocene and the Holocene.

Impact of humans on the flux of terrestrial sediment to the global coastal ocean.

Here we provide global estimates of the seasonal flux of sediment, on a river-by-river basis, under modern and prehuman conditions. Humans have simultaneously increased the sediment transport by global rivers through soil erosion (by 2.3 +/- 0.6 billion metric tons per year), yet reduced the flux of sediment reaching the world's coasts (by 1.4 +/- 0.3 billion metric tons per year) because of retention within reservoirs. Over 100 billion metric tons of sediment and 1 to 3 billion metric tons of carbon are now sequestered in reservoirs constructed largely within the past 50 years. African and Asian rivers carry a greatly reduced sediment load; Indonesian rivers deliver much more sediment to coastal areas.