Albert J. Kettner

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 worlds 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.

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.

Changes in water and sediment exchange between the Changjiang River and Poyang Lake under natural and anthropogenic conditions, China

To study the fluvial interaction between Changjiang River and Poyang Lake, we analyze the observed changes of riverine flux of the mid-upstream of Changjiang River catchment, the five river systems of Poyang Lake and Poyang Lake basin. Inter-annual and seasonal variations of the water discharge and sediment exchange processes between Changjiang River and Poyang Lake are systematically explored to determine the influence of climate change as well as human impact (especially the Three Gorges Dam (TGD)). Results indicate that climate variation for the Changjiang catchment and Poyang Lake watershed is the main factor determining the changes of water exchanges between Changjiang River and Poyang Lake. However, human activities (including the emplacement of the TGD) accelerated this rate of change. Relative to previous years (1956-1989), the water discharge outflow from Poyang Lake during the dry season towards the Changjiang catchment increased by 8.98 km(3)y(-1) during 2003-2010. Evidently, the water discharge flowing into Poyang Lake during late April-late May decreased. As a consequence, water storage of Poyang Lake significantly reduced during late April-late May, resulting in frequent spring droughts after 2003. The freshwater flux of Changjiang River towards Poyang Lake is less during the flood season as well, significantly lowering the magnitude and frequency of the backflow of the Changjiang River during 2003-2010. Human activities, especially the emplacement and operation of the TGD and sand mining at Poyang Lake impose a major impact on the variation of sediment exchange between Changjiang main river and Poyang Lake. On average, sediments from Changjiang River deposited in Poyang Lake before 2000. After 2000, Changjiang River no longer supplied sediment to Poyang Lake. As a consequence, the sediment load of Changjiang River entering the sea increasingly exists of sediments from Lake Poyang during 2003-2010. As a result, Poyang Lake converted from a depositional to an erosional system, with a gross sediment loss of 120.19 Mty(-1) during 2001-2010, including sand mining.

Modeling suspended sediment discharge from the Waipaoa River system, New Zealand: The last 3000 years

HydroTrend, a hydrologic-transport model, is used to simulate the water and suspended sediment discharge of the Waipaoa River system over the last 3 Kyr, a time period in which a well-documented sequence of natural events and anthropogenic activities that profoundly impacted drainage basin processes occurred. Comparisons between observed and simulated data show that the model output replicates the frequency and distribution of flow events and the suspended sediment concentration-discharge relationship, and the long-term trends in suspended sediment discharge are consistent with the sediment record preserved on the middle shelf. Water discharge tracks precipitation, and average annual discharge may have been up to 20% higher and 6% lower at different times in the past. Suspended sediment discharge changed from 2.3 ± 4.5 to 14.9 ± 8.7 Mt yr?1 during the Anthropocene, increasing by 140% after Polynesian arrival, by 350% after European colonization, and by 660% after the catchment headwaters were deforested.

Early Anthropogenic Transformation of the Danube-Black Sea System

Over the last century humans have altered the export of fluvial materials leading to significant changes in morphology, chemistry, and biology of the coastal ocean. Here we present sedimentary, paleoenvironmental and paleogenetic evidence to show that the Black Sea, a nearly enclosed marine basin, was affected by land use long before the changes of the Industrial Era. Although watershed hydroclimate was spatially and temporally variable over the last ~3000 years, surface salinity dropped systematically in the Black Sea. Sediment loads delivered by Danube River, the main tributary of the Black Sea, significantly increased as land use intensified in the last two millennia, which led to a rapid expansion of its delta. Lastly, proliferation of diatoms and dinoflagellates over the last five to six centuries, when intensive deforestation occurred in Eastern Europe, points to an anthropogenic pulse of river-borne nutrients that radically transformed the food web structure in the Black Sea.

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 embankment 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.

HydroTrend v.3.0: A climate-driven hydrological transport model that simulates discharge and sediment load leaving a river system

HydroTrend v.3.0 is a climate-driven hydrological water balance and transport model that simulates water discharge and sediment load at a river outlet, by incorporating drainage basin properties (river networks, hypsometry, relief, reservoirs) together with biophysical parameters (temperature, precipitation, evapo-transpiration, and glacier characteristics). HydroTrend generates daily discharge values through: snow accumulation and melt, glacier growth and ablation, surface runoff, and groundwater evaporation, retention and recharge. The long-term sediment load is predicted either by the ART-QRT module based on drainage area, discharge, relief, and temperature, or the BQART module that also incorporates basin-average lithology and anthropogenic influences on soil erosion. Sediment trapping efficiency of reservoirs is based on reservoir location in the river network and its volume that determines the residence time of water within the reservoir. Glacial influence is based on the extent of ice cover, equilibrium altitude, and freezing line mobility. HydroTrend v.3.0 captures the inter- and intra-annual variability of sediment flux by using either high-resolution climate observations or a stochastic climate generator for simulations over longer geological intervals. A distributary channel module simulates the flow conditions and transport capacity across a multiple deltaic channel system. Simulations of the Metauro and the Po rivers, in Italy, are used as case studies to demonstrate the applicability of the new model.

Global mapping of storm surges and the assessment of coastal vulnerability

The measurement and recording of the height and spatial extent reached by coastal storm surges is fundamental to scientific progress in understanding these phenomena. Such information is required for better prediction and for risk assessment. Model-based evaluation of increasing delta vulnerability, for example, cannot be tested without long-term, consistent, and sustained observation of actual events. Also, storm surges occur within the temporal context of tidal variation, which must first be characterized through observation. Present standard approaches for measuring storm surges are not optimum. Thus, tidal gauges provide information at one point, whereas the heights reached by surges vary spatially. Also, post-surge ground surveys are expensive, laborious, and commonly lack comparison to similar data obtained for previous surges or for high tides. The advent of moderate spatial resolution, high temporal resolution remote sensing initiated by the launch of the two NASA MODIS sensors greatly reduces these constraints. For over a decade, daily coverage of most coastal land areas, though restricted by cloud cover, has systematically captured the maximum extents reached by both high tides and by storm surges. Automated water classification algorithms are now transforming the incoming image data into GIS water boundary files, again at daily or near-daily time steps. This paper provides a retrospective view of sample storm surges as mapped via these sensors and describes: (a) the present, MODIS-based surface water surveillance system, (b) the mapping enhancement to be provided by frequent-repeat, wide-swath satellite radar imaging, and (c) the emerging prospects for routine global surveillance of storm surge events. Such will be necessary if long-term trends are to be recognized, characterized, and understood, along coastal zones now being affected by both increasing subsidence and rising sea level.

Predicting the Flux of Sediment to the Coastal Zone: Application to the Lanyang Watershed, Northern Taiwan

Abstract A global scale approach involving data assimilation schemes (e.g. Distributed Oceanographic Data System) is designed to simulate the discharge of sediment to the coastal ocean at the dynamic level (daily). The result is either a real-time, hindcast or forecast picture of coastal hydrology optimized to estimate sediment loads of rivers. The approach links a compendium of global and regional web-based databases into a GIS system. Relational and spatial methods (i.e. RiverToolst, HYDRO1k, ArcInfot) facilitate the process of data acquisition useful to sediment discharge models (i.e. HydroTrend). As a climate-driven hydrological model, HydroTrend incorporates drainage basin properties (river networks, hypsometry, relief, lakes or reservoirs, distributary channels) through high-resolution digital elevation models, along with other biophysical parameters (basin-average temperature, precipitation, evapo-transpiration, canopy, soil depth, hydraulic conductivity, ice fields). The schema is designed to provide important boundary conditions for marine sediment-dispersal models, concomitant with ocean data (wind, wave, currents). Considering that <4% of world-rivers are monitored for their sediment loads, the approach provides a unique means to predict the sediment flux across an entire coastline at a high-resolution temporal scale. Model comparison to long-term (1950–1994) observations from the Lanyang River (Hsi), Taiwan is shown to capture average conditions and inter- and intra-annual variability of water discharge, sediment concentration and loads.

A Spatial Simulation Experiment to Replicate Fluvial Sediment Fluxes within the Magdalena River Basin, Colombia

Spatial variation of natural conditions (geomorphology, geography, and geology) and human perturbations (deforestation and reservoir retention) in river basins determine the variability of within-basin riverine sediment fluxes. Originally developed to determine a 30-yr normal of sediment load at a river mouth or for a “whole basin,” the scaling model BQART is validated to predict the sediment discharge from “within-basin” tributaries. Subbasins of the monsoonal-influenced Magdalena River are used here as case studies. By taking into account spatially variable geological, climatological, and human influences of the Magdalena tributaries, and by incorporating observational data to adjust for floodplain trapping due to a tectonic depression, the BQART model overestimates the cumulative sediment flux of 21 tributaries by 25% compared with observations ( ). For a given subbasin, BQART estimates are well within a factor of 3. The relatively short durations of the observations of each of the tributaries (6–25 yr), in combination with rapidly changing subbasin environments (like deforestation and mining intensification), likely affect the goodnesses of fit of the comparisons.