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The Volga is the longest river in Europe and 16th longest in the world. The riverine landscape of the Volga is of exceptional scientific and economic importance to Russia; the basin contains approximately 40% of the Russian population and relates to 45% of the country's industrial and agricultural produce. The Volga River drains an area of 1.4 million km2, covering various biomes from taiga to semidesert. Anthropogenic impacts in the 20th century include pollution as well as hydropower production and navigation purposes, incurring a cost for its historically important migratory fish (e.g., sturgeons) and related fisheries. River basin management in Russia, since 2006, is based on the water code that determines federal competencies in water management. Extensive water quality monitoring programmes provide feedback to regional managers. Monitoring of biological parameters is spatially limited and should be extended in order to provide sufficient data for informed management. Some initiatives have been implemented in recent decades in order to restore the ecological health of the river and manage fisheries resources (e.g., restocking programmes and the definition of total allowable catches). As recreational fishing is popular but presently unregulated in Russia, we suggest additional monitoring. Finally, the headwaters and lower river floodplain of the Volga have remained as free‐flowing and relatively undisturbed systems. Because reference conditions with low levels of anthropogenic disturbance cannot be found in Central European lowland rivers, both the headwaters and lower Volga floodplains below Volgograd are of great importance on European level.

The impact of climate change on the development and disintegration of Maya civilisation has long been debated. The lack of agreement among existing palaeoclimatic records from the region has prevented a detailed understanding of regional-scale climatic variability, its climatic forcing mechanisms and its impact on the ancient Maya. We present two new palaeo-precipitation records for the central Maya lowlands, spanning the Pre-Classic period (1800 BCE–250 CE), a key epoch in the development of Maya civilisation. A beach ridge elevation record from world's largest late Holocene beach ridge plain provides a regional picture, while Lake Tuspan's diatom record is indicative of precipitation changes at a local scale. We identify centennial-scale variability in palaeo-precipitation that significantly correlates with the North Atlantic δ14C atmospheric record, with a comparable periodicity of approximately 500 years, indicating an important role of North Atlantic atmospheric–oceanic forcing on precipitation in the central Maya lowlands. Our results show that the Early Pre-Classic period was characterised by relatively dry conditions, shifting to wetter conditions during the Middle Pre-Classic period, around the well-known 850 BCE (2.8 ka) event. We propose that this wet period may have been unfavourable for agricultural intensification in the central Maya lowlands, explaining the relatively delayed development of Maya civilisation in this area. A return to relatively drier conditions during the Late Pre-Classic period coincides with rapid agricultural intensification in the region and the establishment of major cities.

The low-lying and populous Vietnamese Mekong delta is rapidly losing elevation due to accelerating subsidence rates, primarily caused by increasing groundwater extraction. This strongly increases the delta’s vulnerability to flooding, salinization, coastal erosion and, ultimately, threatens its nearly 18 million inhabitants with permanent inundation. We present projections of extraction-induced subsidence and consequent delta elevation loss for this century following six mitigation and non-mitigation extraction scenarios using a 3D hydrogeological model with a coupled geotechnical module. Our results reveal the long-term physically response of the aquifer system following different groundwater extraction pathways and show the potential of the hydrogeological system to recover. When groundwater extraction is allowed to increase continuously, as it did over the past decades, extraction-induced subsidence has the potential to drown the Mekong delta single-handedly before the end of the century. Our quantifications also disclose the mitigation potential to reduce subsidence by limiting groundwater exploitation and hereby limiting future elevation loss. However, the window to mitigate is rapidly closing as large parts of the lowly elevated delta plain may already fall below sea level in the coming decades. Failure to mitigate groundwater extraction-induced subsidence may result in mass displacement of millions of people and could severely affect regional food security as the food producing capacity of the delta may collapse.

Deltas are low-relief landforms that are extremely vulnerable to sea-level rise. Impact assessments of relative sea-level rise in deltas primarily depend on elevation data accuracy and how well the vertical datum matches local sea level. Unfortunately, many major deltas are located in data-sparse regions, forcing researchers and policy makers to use low-resolution, global elevation data obtained from satellite platforms. Using a new, high-accuracy elevation model of the Vietnamese Mekong delta, we show that quality of global elevation data is insufficient and underscore the cruciality to convert to local tidal datum, which is often neglected. The novel elevation model shows that the Mekong delta has an extremely low mean elevation of ~0.8 m above sea level, dramatically lower than the earlier assumed ~2.6 m. Our results imply major uncertainties in sea-level rise impact assessments for the Mekong delta and deltas worldwide, with errors potentially larger than a century of sea-level rise., Assessments of sea level rise risks depend on elevation data. Here, the authors present a new dataset on the Mekong Delta which shows it to have a much lower elevation (0.82 m above sea level) than previously thought – underlying principles may also imply major elevation uncertainties in other deltas.

Palaeoflood hydraulic modelling is essential for quantifying ‘millennial flood’ events not covered in the instrumental record. Palaeoflood modelling research has largely focused on one-dimensional analysis for geomorphologically stable fluvial settings because two-dimensional analysis for dynamic alluvial settings is time consuming and requires a detailed representation of the past landscape. In this study, we make the step to spatially continuous palaeoflood modelling for a large and dynamic lowland area. We applied advanced hydraulic model simulations (1D–2D coupled set-up in HEC-RAS with 950 channel sections and 108 × 10 3 floodplain grid cells) to quantify the extent and magnitude of past floods in the Lower Rhine river valley and upper delta. As input, we used a high-resolution terrain reconstruction (palaeo-DEM) of the area in early mediaeval times, complemented with hydraulic roughness values. After conducting a series of model runs with increasing discharge magnitudes at the upstream boundary, we compared the simulated flood water levels with an inventory of exceeded and non-exceeded elevations extracted from various geological, archaeological and historical sources. This comparison demonstrated a Lower Rhine millennial flood magnitude of approximately 14,000 m 3/s for the Late Holocene period before late mediaeval times. This value exceeds the largest measured discharges in the instrumental record, but not the design discharges currently accounted for in flood risk management.

Reconstruction of past topography in palaeo-DEMs serves various geomorphological analyses. Constructing a palaeo-DEM by stripping young elements from a LiDAR DEM can provide results for large study areas at high resolution. However, such a ‘top-down’ approach is more suited to recent periods and geomorphologically static parts of the landscape than to geomorphologically dynamic areas and periods farther back in time. Here, we explore this approach by reconstructing the early medieval (circa 800 CE) topography of the Lower Rhine river valley and upper delta in Germany and the Netherlands. The large (4500 km2) study area contains abundant anthropogenic terrain modification and stretches across geomorphologically active as well as inactive zones. We first removed all anthropogenic relief elements from the LiDAR DEM, using separate procedures for linear and non-linear elements. These steps were sufficient to obtain the palaeotopography of the inactive zone, characterized by inherited natural relief. Then, we reconstructed the topography and bathymetry in the fluvially-reworked active zone by incorporating geological and historical geographical information. We present and evaluate zonal averages of elevation differences between the modern and past valley floor topography in this densely populated area with complex land-use history, which allows us to approximate total anthropogenic volumetric change. Further comparisons with the modern LiDAR DEM elucidate changes in floodplain negative-relief connectivity, showing the potential importance of investing in palaeo-DEMs when assessing past river flooding. Our palaeo-DEM construction workflow is deployable at diverse spatial scales and widely applicable to other lowland areas, because of its top-down and generic nature. The relative importance of different workflow aspects depends on the time period that is targeted. Beyond a target age of 10–15 ka, valley floors that are too large in area are to be considered geomorphologically dynamic and the top-down approach to palaeo-DEM construction is no longer advisable.

Fluvial lowlands have become attractive human settling areas all around the world over the last few millennia. Because rivers kept changing their course and networks due to avulsion, the sedimentary sequences in these areas are archives of both fluvial geomorphological and archaeological development. We integrated geological and archaeological datasets to demonstrate the concurrence of the gradual abandonment of a major Rhine channel (Utrecht, The Netherlands), the development of human habitation in the area, and the interactions between them. The Utrecht case study highlights the stage-wise abandonment of a natural river channel, due to avulsion, coincident with intensifying human occupation in Roman and Early Medieval times (1st millennium AD). The analyses make maximum use of very rich data sets available for the study area and the tight age control that the geo-archaeological dataset facilitates, offering extra means of time-control to document the pacing of the abandonment process. This allows us to quantify change in river dimensions and meander style and to provide discharge estimates for successive stages of the abandonment phase over a 1000-year period of abandonment succession, from mature river to eventual Late Medieval overbuilt canal when the Rhine branch had lost even more discharge. Continued geomorphic development during this period - which includes the 'Dark Ages' (450-1000 AD) - appears to have been crucial in the development of Utrecht from Roman army fortress to Medieval ecclesial centre. The settlement dynamics in and around the city of Utrecht changed during the various phases of abandonment. In the bifurcating network of river branches forming the Rhine-Meuse delta, the main Rhine branch hosted the Roman limes military border and transport route. The Rhine- Vecht bifurcation at Utrecht provided an excellent location to raise a Roman fort. Continued geomorphic activity during abandonment in Early Medieval times was characterised by enhanced overbank sedimentation and shifts in the position of bifurcations. River flooding became more incidental in this stage, and alluvial-ridge occupancy became sensitive to flooding events for several centuries. We conclude by demonstrating that similar human-river interactions during Roman times occurred in several other deltas within the former Roman empire, with differences depending on the position of a settlement within the delta, the overall hydrological situation, and the ability of societies to control the changing environment.

This paper is a result of the open IGBP PAGES Focus 4 workshop “Sediment and carbon fluxes under human impact and climate change” which was held in July 2011 in Bern Switzerland and is a contribution to PAGES' Soil and Sediment theme. ABSTRACT: Is anthropogenic soil erosion a sink or source of atmospheric carbon? The answer depends on factors beyond hillslope erosion alone because the probable fate of mobilized soil carbon evolves as it traverses the fluvial system. The transit path residence times and the resulting mechanisms of C loss or gain change significantly down basin and are currently difficult to predict as soils erode and floodplains evolve – this should be a key focus of future research.

To evaluate floodplain functioning, monitoring of its vegetation is essential. Although airborne imagery is widely applied for this purpose, classification accuracy (CA) remains low for grassland (< 88%) and herbaceous vegetation (

The reach-scale effects of human-induced disturbances on the channel network in large braided rivers are a challenge to understand and to predict. In this study, we simulated different types of disturbances in a large braided river to get insight into the propagation of disturbances through a braided channel network. The results showed that the disturbances initiate an instability that propagates in the downstream direction by means of alteration of water and sediment division at bifurcations. These adjustments of the bifurcations change the migration and shape of bars, with a feedback to the upstream bifurcation and alteration of the approaching flow to the downstream bifurcation. This way, the morphological effect of a disturbance amplifies in the downstream direction. Thus, the interplay of bifurcation instability and asymmetrical reshaping of bars was found to be essential for propagation of the effects of a disturbance. The study also demonstrated that the large-scale bar statistics are hardly affected.

Land cover maps provide essential input data for various hydromorphological and ecological models, but the effect of land cover classification errors on these models has not been quantified systematically. This paper presents the uncertainty in hydromorphological and ecological model output for a large lowland river depending on the classification accuracy (CA) of a land cover map. Using four different models, we quantified the uncertainty for the three distributaries of the Rhine River in The Netherlands with respect to: (1) hydrodynamics (WAQUA model), (2) annual average suspended sediment deposition (SEDIFLUX model), (3) ecotoxicological hazards of contaminated sediment for a bird of prey, and (4) floodplain importance for desired habitat types and species (BIO-SAFE model). We carried out two Monte Carlo (n = 15) analyses: one at a 69% land cover CA, the other at 95% CA. Subsequently we ran all four models with the 30 realizations as input. The error in the current land cover map gave an uncertainty in design water levels of up to 19 cm. Overbank sediment deposition varied up to 100% in the area bordering the main channel, but when aggregated to the whole study area, the variation in sediment trapping efficiency was negligible. The ecotoxicological hazards, represented by the fraction of Little Owl habitat with potential cadmium exposure levels exceeding a corresponding toxicity threshold of 148 @mg d^-^1, varied between 54 and 60%, aggregated over the distributaries. The 68% confidence interval of floodplain importance for protected and endangered species varied between 10 and 15%. Increasing the classification accuracy to 95% significantly lowered the uncertainty of all models applied. Compared to landscaping measures, the effects due to the uncertainty in the land cover map are of the same order of magnitude. Given high financial costs of these landscaping measures, increasing the classification accuracy of land cover maps is a prerequisite for improving the assessment of the efficiency of landscaping measures.

The Optically Stimulated Luminescence (OSL) signal from fluvial sediment often contains a remnant from the previous deposition cycle, leading to a partially bleached equivalent-dose distribution. Although identification of the burial dose is of primary concern, the degree of bleaching could potentially provide insights into geomorphic processes. However, comparison of bleaching between samples is complicated by sample-to-sample variation in aliquot size and luminescence sensitivity. Here we develop an age model to account for these effects. With measurement data from multi-grain aliquots, we use Bayesian computational statistics to estimate the burial dose and bleaching parameters of the single-grain dose distribution. We apply the model to 46 samples taken from fluvial sediment of Rhine branches in the Netherlands, and compare the results with environmental predictor variables (depositional energy and environment, sample depth, depth relative to mean water level, dose rate). We find no significant correlations between any predictor variable and the bleaching parameters, although large uncertainties may be obscuring relationships. However, the best bleached samples are found close to the mean water level. Based on these results, we hypothesize that bleaching occurs mainly during fluvial transport rather than upon deposition, with extra bleaching possible for sediments near the transition of channel to overbank deposits due to local reworking after deposition either by wind or water.

This thesis examines the largest floods of the Lower Rhine river over the past centuries to millennia (Late Holocene to historic time frame). Its central objective is to quantify these events. First, the thesis determines maximum water levels in the past based on sedimentological interpretation of archaeological stratigraphy and geographical survey of epigraphic marks. Next, it provides detailed reconstructions of the alluvial terrain of the Lower Rhine valley and delta, covering the early modern channel bathymetry restored from historic maps and the medieval floodplain topography restored from geomorphological analysis and interpolation. The past landscape context then serves to set up a palaeoflood hydraulic model that resolves the discharges of the largest floods by linking the simulation results to reconstructed water levels. The insights gained in this thesis may ultimately contribute to the design of future flood protection measures. The overarching findings demonstrate that reliable quantification of past floods in lowland river settings necessitates a two-dimensional approach, when it comes to the required flood level and landscape reconstructions and the discharge calculations. Furthermore, the findings show that palaeoflood research benefits from a multidisciplinary approach, with the primary methodological focus depending on the selected casus and targeted time period. The results include successfully reconstructed pre-instrumental flood levels across the Lower Rhine valley and delta by integrating archaeological, geological, and historical sources (Chapters 2, 3, and 6). Additionally, the results encompass successful reconstructions of the landscape for two key periods: before the onset of river normalisation in the mid-nineteenth century and before the onset of embankment in medieval times (Chapters 4 and 5). Finally, the thesis provides an estimate for the ‘millennial’ discharge in the Late Holocene by numerical flood simulations in the pre-embanked landscape context, resulting in a value of 14,000 m3/s (Chapter 6). Further output of the Floods of the Past, Design for the Future project shows that the landscape context prior to normalisation is crucial to constraining discharge magnitudes of individual historic flood events.

Water management should preferably bring solutions that sustain even if conditions change. In anticipating change, a sustainable plan should not only achieve economic, environmental, and social targets, but it should also be robust to uncertainty and able to be adapted over time to (unforeseen) future conditions. The objective of this Ph.D. research was to develop and test a method for exploring adaptation pathways for sustainable water management in river deltas into an uncertain future. The research resulted in two main products: 1) A stepwise policy analysis framework for the development of a sustainable plan that can cope with changing conditions. The key principles of this framework are: the use of transient scenarios representing a variety of relevant uncertain changing conditions over time; the exploration of adaptation pathways after an adaptation tipping point; and an adaptation map showing the set of most promising adaptation pathways and options for transferring from one pathway to another in the format of a metro-map, and 2) A fast, Integrated Assessment MetaModel (IAMM) that allows for exploring many policy pathways under a multiplicity of transient scenarios, and helps to assess when a policy’s tipping point might occur at earliest and at latest (time-span). The approach proved to be valuable for informed decision making on a sustainable water management plan, and has been adopted in the concept of adaptive delta management of the Delta Programme.