E. Stouthamer

Factors Controlling the Holocene Avulsion History of the Rhine-Meuse Delta (The Netherlands)

ABSTRACT The avulsion history of the Holocene Rhine-Meuse delta (The Netherlands) was reconstructed, on a timescale of millenia, using detailed paleogeographic maps based on approximately 200,000 lithological borehole descriptions, over 1150 14C ages, and 36,000 archaeological artifacts. Gradient lines were constructed for all channel belts. These allowed determination of gradients, paleo-flow direction, relative age of channel belts, and time correlation of undated channel-belt fragments. Avulsion sites were inferred from the paleogeographic reconstruction. At least 91 avulsions occurred over the past 10,000 years, of which 82 could be dated with an accuracy of ± 200 14C years. The location and shifting of Holocene avulsion sites in the Rhine-Meuse delta in space and time are related to: (1) Relative sealevel rise. In the Early Holocene, avulsions could not take place, because rivers were still incised. Around 7500 yr BP, avulsions occurred in the western part of the present delta as a result of backfilling of the Late Weichselian valley. Between 7500 and 3700 yr BP, the zone where avulsions occurred shifted inland as a result of relative sealevel rise. (2) Neotectonics. Between 4900 and 1700 yr BP, the location of avulsion sites seems to have been influenced by neotectonic movements of the upthrown Peel Horst. Four out of six avulsion nodes in the Rhine-Meuse delta were located in the Peel Horst fault zones. (3) Increased discharge and/or within-channel sedimentation. From 2800 until about 1500 yr BP, avulsion sites were located all over the delta. During this time, the number of channels was high, and avulsion frequency reached a maximum, at a time when aggradation rate decreased with a reduction in the rate of sealevel rise. After 2000 yr BP meander wavelength of alluvial channels increased considerably. The increased meander wavelength and the high avulsion frequency are attributed to increased bankfull discharge or within-channel sedimentation (leading to channel widening), or both. (4) Human influence. Between 1000 and 650 yr BP, all the rivers were embanked, and avulsions could no longer take place. The few that occurred were induced by humans.

Avulsion Frequency, Avulsion Duration, and Interavulsion Period of Holocene Channel Belts in the Rhine-Meuse Delta, the Netherlands

ABSTRACT Avulsion parameters greatly influence alluvial architecture, because they determine channel density and interconnectedness. Reliable data on avulsion parameters is still scarce. From a detailed reconstruction of the Holocene avulsion history of the whole Rhine-Meuse delta we determined quantitative values for the avulsion parameters: avulsion frequency, avulsion duration, and interavulsion period. In the Rhine-Meuse delta the number of coeval channels is related to the avulsion frequency. Calibrated 14C intensity histograms are used to analyze 14C dates of beginning and ending river activity. The histograms show that instantaneous and gradual avulsions were almost equally important in the Rhine-Meuse delta. A high avulsion frequency occurred from 8000 to 7300 cal yr BP (a total of 17 avulsions, i.e., 2.43 avulsions/100 years). During this period the avulsion frequency was related to the high rate of sea-level rise, which induced fluvial sedimentation in the present near-coastal area. After 7300 cal yr BP avulsion frequency decreased as a result of a continuously decreasing rate of sea-level rise. From 7300 to 3200 cal yr BP avulsion frequency was low; 35 avulsions took place within 4100 years (0.85 avulsions/100 years). Approximately 5000 cal yr BP the coastal barriers became closed and large-scale peat formation occurred. This resulted in fixation of the river channels and low cross-valley gradients, reducing the chances for avulsion. Between 3200 and 1400 cal yr BP avulsion frequency was high again (a total of 34 avulsions, i.e., 1.89 avulsions/100 years) as a result of increased discharge and/or within-channel sedimentation, which enhanced chances for avulsion. The period of activity of channel belts shows no significant trend on the time scale of the Holocene. It is highly variable and averages 1280 ± 820 cal yr. The avulsion duration fluctuates between less than 200 and 1250 cal years and averages 335 cal years. The avulsion duration shows no significant trend through time and remained constant until at least 1900 cal yr BP. Avulsion duration seems not to be influenced by aggradation rate. Average interavulsion period is shorter than the period of activity of channel belts, and is approximately 945 cal years. In the Rhine-Meuse delta, on the time scale of the Holocene, interavulsion period and avulsion duration are on average constant. Therefore, the number of coeval channels is directly related to avulsion frequency. Available evidence from other rivers and deltas around the world, however, suggests that the relationship between these avulsion parameters is not everywhere the same.

From river valley to estuary: the evolution of the Rhine mouth in the early to middle Holocene (western Netherlands,Rhine-Meuse delta)

The aim of this paper is to reconstruct the evolution of the early to middle Holocene Rhine-Meuse river mouths in the western Netherlands and to understand the observed spatial and temporal changes in facies. This is achieved by constructing three delta wide cross-sections using a newly accumulated database with thousands of core descriptions and cone penetration test results, together with a large set of pollen/diatom analyses and OSL/14C-dates. Most of the studied deposits accumulated in the fluvial-to-marine transition zone, a highly complex area due to the interaction of terrestrial and marine processes. Understanding how the facies change within this zone, is necessary to make correct palaeogeographic interpretations. We find a well preserved early to middle Holocene coastal prism resting on lowstand valley floors. Aggradation started after 9 ka cal BP as a result of rapid sea-level rise. Around 8 ka most parts of the study area were permanently flooded and under tidal influence. After 8 ka a bay-head delta was formed near Delft, meaning that little sand could reach the North Sea. Several subsequent avulsions resulted in a shift from the constantly retreating Rhine river mouth to the north. When after 6.5 ka the most northerly river course was formed (Oude Rijn), the central part of the palaeovalley was quickly transgressed and transformed into a large tidal basin. Shortly before 6 ka retrogradation of the coastline halted and tidal inlets began to close, marking the end of the early-middle Holocene transgression. This paper describes the transition from a fluvial valley to an estuary in unprecedented detail and enables more precise palaeo-reconstructions, evaluation of relative importance of fluvial and coastal processes in rapid transgressed river mouths, and more accurate sediment-budget calculations. The described and well illustrated (changes in) facies are coupled to lithogenetic units. This will aid detailed palaeogeographic interpretations from sedimentary successions, not only in the Netherlands, but also in other estuarine and deltaic regions.

Sedimentary products of avulsions in the Holocene Rhine–Meuse Delta, The Netherlands

Abstract The main objective of this study was to describe the characteristics of deposits related to avulsion, to determine if avulsion locations can be recognized by characteristic sediment architecture. In the Rhine–Meuse Delta, the majority of avulsion sites is accompanied by crevasse–splay deposits with a relatively large areal extent. These crevasse–splay complexes occur near the base of the overbank deposits of the newly formed channel belt and are similar to ‘avulsion belt deposits’ described by Smith et al. [Sedimentology, 36 (1989) 1; Can. J. Earth Sci., 35 (1998) 453]. The lithology of all investigated crevasse–splay deposits ranges from coarse sand to silty clay. Sand is concentrated in crevasse splay channels, and silty and sandy clay occurs as distal deposits and as levee deposits between the channels. The total volume of sand is small, relative to clay and silt. Of a total of 91 avulsion sites, three sites were studied in detail, one each representing the sedimentary architecture of a full nodal avulsion, a failed avulsion, and a crevasse–splay complex that did not lead to avulsion. It is concluded that dating evidence of the abandoned and newly formed paleo-channel is always required to verify if avulsion really occurred, although the presence of large-scale sandy crevasse–splay deposits in combination with two main meandering channels often indicates avulsion. Based on a 2-D outcrop or just one cross section, it is impossible to determine whether a splay complex was associated with an avulsion or not, because the architecture and lithology in both cases can be identical. Not all avulsion locations are characterized by crevasse–splay deposits. There are two possible explanations for this: (1) splay deposits were eroded by the newly formed channel, or (2) splay deposits were never formed. This may be a result of reoccupation of an old channel. In this case, avulsive flow is immediately appropriated by the pre-existing channel and no large-scale splay formation occurs. Alternatively, splay deposits were not formed because avulsion resulted from headward erosion.

Fluvial deposits as a record for Late Quaternary neotectonic activity in the Rhine-Meuse delta,The Netherlands

Neotectonic movements have caused differential subsidence in the Lower Rhine Embayment during the Quaternary. The Late Weichselian and Holocene Rhine-Meuse fluvial archive in the central Netherlands was used to quantify neotectonic movements in a setting that was primarily controlled by sea-level rise and climate change. Evidence for neotectonic activity in the central Netherlands is reviewed. Sedimentary evidence shows that fluvial deposits of Late Weichselian and Holocene Rhine and Meuse (Maas) distributaries are vertically displaced along the northern shoulder of the Roer Valley Graben system. Elevation differences in the longitudinal profiles of Late Weichselian terrace deposits were used to quantify tectonic displacements. New results for the southeastern Rhine-Meuse delta (Maaskant area) show that displacements in the top of the Pleniglacial terrace along the Peel Boundary Fault are up to 1.4 m. The maximum displacement between the Peel Horst and the Roer Valley Graben is 2.3 m. This is equivalent to relative tectonic movement rates of 0.09-0.15 mm/yr, averaged over the last 15,000 years.

The use of GIS in reconstructing the Holocene palaeogeography of the Rhine-Meuse delta, The Netherlands

In Holocene palaeogeographical studies, GIS nowadays is used in various ways, embracing either GIS modelling possibilities or GIS spatial data storage functionality. This paper is an example of the latter type of GIS use for reconstruction of the Holocene Rhine–Meuse delta (128×75 km; part of the Dutch coastal plain). Depositional traces of river courses (channel belts) are major features to map. We digitized mapped channel belt fragments and stored their reconstructed ages in a linked table. The GIS data structure allows checking for internal consistency of the implemented reconstruction, enabling iterative improvement while digitizing and labelling. Palaeogeographical maps for any given time during the Holocene result from GIS‐database queries. Coupled to the GIS are high‐resolution digital elevation data (sub‐decimetre accurate laser‐altimetry data covering the entire study area) and a database of ca 100 000 borehole descriptions, allowing for further mapping improvements. The GIS structure and digital data together greatly improved mapping and enable larger areas to be mapped in greater detail with greater accuracy and in less time.

The TESS-1 Suction Corer: A New Device to Extract Wet, Uncompacted Sediments

Conventional coring can be an expensive and cumbersome technique in uncompacted, waterlogged sediments, especially when large numbers of samples are required rapidly and when vehicular access is restricted. The existing Van der Staay suction corer (Van de Meene et al. 1979) solves economic and other problems (chiefly logistical and environmental issues), but sample extraction has to be done in the field. The original purposes of the Van der Staay suction corer were to enable rapid drilling through sand and allow field sampling. It was not designed to recover intact samples for subsequent laboratory analysis. Cores from the new TESS-1 suction corer, however, are encapsulated in the field. The TESS-1 therefore combines the advantages of the original corer with the possibility of taking samples for laboratory analysis. Samples collected using the TESS-1 suction corer show little deformation, allow coring of soft uncompacted mud and retention of sedimentary structures. Problems due to sediment fluidization and movement inside the core pipe are usually minimal. The TESS-1 suction corer can be used in both wetland areas (beach, tidal flats, and marshes) and in shallow subaqueous environments (lakes, rivers, and nearshore subtidal areas).

Impacts of 25 years of groundwater extraction on subsidence in the Mekong delta, Vietnam

Many major river deltas in the world are subsiding and consequently become increasingly vulnerable to flooding and storm surges, salinization and permanent inundation. For the Mekong Delta, annual subsidence rates up to several centimetres have been reported. Excessive groundwater extraction is suggested as the main driver. As groundwater levels drop, subsidence is induced through aquifer compaction. Over the past 25 years, groundwater exploitation has increased dramatically, transforming the delta from an almost undisturbed hydrogeological state to a situation with increasing aquifer depletion. Yet the exact contribution of groundwater exploitation to subsidence in the Mekong delta has remained unknown. In this study we deployed a delta-wide modelling approach, comprising a 3D hydrogeological model with an integrated subsidence module. This provides a quantitative spatially-explicit assessment of groundwater extraction-induced subsidence for the entire Mekong delta since the start of widespread overexploitation of the groundwater reserves. We find that subsidence related to groundwater extraction has gradually increased in the past decades with highest sinking rates at present. During the past 25 years, the delta sank on average ∼18 cm as a consequence of groundwater withdrawal. Current average subsidence rates due to groundwater extraction in our best estimate model amount to 1.1 cm yr−1, with areas subsiding over 2.5 cm yr−1, outpacing global sea level rise almost by an order of magnitude. Given the increasing trends in groundwater demand in the delta, the current rates are likely to increase in the near future.

The alluvial architecture of the Coevorden Field (Upper Carboniferous), the Netherlands

A detailed reconstruction of the alluvial architecture of the Coevorden gas Field (Tubbergen Formation, Upper Carboniferous), which is located in the northeastern part of the Netherlands, is presented. This reconstruction is based on well logs, cross-sections and paleogeographic maps. Sedimentological analysis of a 93 m long core allowed to refine the interpretation of the depositional environment. Accurate width determinations are necessary to correctly correlate fluvial sandbodies and reconstruct alluvial architecture. Without using sedimentological information, sandbody width is likely to be overestimated. A method developed by Bridge and Tye (2000) was used to calculate the width of one sandstone body from cross-set thicknesses. On the basis of this calculation and the paleogeographic reconstructions, it may be stated that on average the width of the channel belts we studied in the Coevorden field does not exceed 4 km. Moreover, our paleogeographic reconstructions, which point to a northwestern direction of paleoflow, are in accordance with earlier observations from the study area. The Tubbergen Formation and timeequivalent sediments in Germany are reviewed briefly to put the Coevorden Field in a regional context. The thickness of the Tubbergen Formation is ~450 m in our study area. In the adjacent German area, time-equivalent sedimentary sequences reach higher thicknesses. This may be attributed to tectonic movements along the Gronau Fault zone and the coming into existence of the Ems Low, of which the Coevorden Field is the westernmost part.

The impact of avulsion on groundwater level and peat formation in delta floodbasins during the middle-Holocene transgression in the Rhine-Meuse delta, The Netherlands

By redistributing water and sediment in delta plains, avulsions of river branches have major environmental impacts, notably in changing hydrological and peat-forming conditions in floodbasins. The central part of the Rhine-Meuse delta, with its extensive databases including detailed lithological data and high-resolution age control, offers a unique opportunity to study middle-Holocene avulsion impacts on floodbasin groundwater level and peat formation. Avulsion has caused local accelerations of rising groundwater tables to be superimposed on decelerating base-level rise. This is evident from comparing single-site groundwater rise for multiple floodbasins in the river-dominated part of the delta, with regionally averaged groundwater-rise reconstructions. Floodbasin type (lacustrine versus terrestrial wetland), size and openness, partly through effects on discharge dispersal, affect how strongly the floodbasin groundwater tables respond to avulsion-diverted discharge. Cross-sectional lithology repeatedly indicates a shift from high-organic wood peat to low-organic reed peat in the vicinity of the avulsed channel, resulting from changes in water-table regime and nutrient status. Avulsive impact on the floodbasin groundwater table was most pronounced during the transition from transgressive to high-stand stage (between ca. 6000 and 4000 years ago), owing to developing floodbasin compartmentalization (size reduction, confinement) resulting from repeated avulsion. By way of environmental impacts on groundwater tables and vegetation, avulsions thus affect the heterogeneity of floodbasin facies.