Large-amplitude meanders may form in low-energy rivers despite generally limited mobility in theses systems. Exceptionally large meanders which even extend beyond the valley sides have developed in ...the Overijsselse Vecht river (the Netherlands) between ca. 1400 CE (Common Era) and the early 1900s, when channelization occurred. Previous studies have attributed the enhanced lateral dynamics of this river to changes in river regime due to increased discharges, reflecting climate and/or land-use alterations in the catchment. This paper focuses on local aspects that may explain why exceptionally large meanders developed at specific sites. Through an integrated analysis based on archaeological, historical, and geomorphological data along with optically stimulated luminescence dating, we investigated the relative impact of three direct and indirect anthropogenic causes for the local morphological change and enhanced lateral migration rates: (1) lack of strategies to manage fluvial erosion; (2) a strong increase in the number of farmsteads and related intensified local land use from the High Middle Ages onwards; and (3) (human-induced) drift-sand activity directly adjacent to the river bends, causing a change in bank stability. Combined, these factors led locally to meander amplitudes well beyond the valley sides. Lessons learned at this site are relevant for management and restoration of meandering rivers in similar settings elsewhere, particularly in meeting the need to estimate spatial demands of (restored) low-energy fluvial systems and manage bank erosion.
Inflow of nutrient poor alkaline groundwater is generally seen as a prerequisite for the development of species rich fen vegetation. Drainage and groundwater abstraction may lead to the development ...of so-called rainwater lenses in the upper groundwater of fens, which prevent upward seeping alkaline groundwater from reaching the fen root zone. This threatens the conservation of the species rich vegetation. To provide insight in rainwater lens dynamics as a basis for conservation and restoration of fen biodiversity, numerical groundwater flow model simulations were performed for a hypothetical case area in the Netherlands.
The simulations indicate that drainage canal surface water levels mainly affect the phreatic level in the fens, while groundwater inflow fluxes mainly affect the thickness of the rainwater lens. Increasing thickness of a semi-confining surface layer results in a rise in phreatic level. As the phreatic level reaches the soil surface, the rainwater lens may split in two as a result of groundwater discharge windows, which come into existence in the centre of the parcel. The effect of seasonal recharge fluctuations on the rainwater lens appears small. Spells of dry and wet years have a considerably larger effect especially on the lower boundary of the rainwater lens, resulting in thinner lenses during spells of dry years.
Most simulations show rainwater lens thicknesses well exceeding rooting depths of fen plants. This is inconsistent with the fact that fen vegetation is found in nature reserves for which the simulations are considered representative. Chemical buffer processes in the soil may explain this inconsistency, masking an acidification process under way. As buffer capacities become exhausted over time, acidification and fen deterioration will occur at a certain future moment. To arrive at more definite conclusions about this hypothesis, our simulations of convective groundwater flow need to be supplemented by modelling of hydrochemical processes in rainwater lenses, followed by verification at actual field locations.
This paper presents a geoarcheological study on potential canal subsections present in the Roman-age Vecht branch of the Rhine-Meuse delta (the Netherlands).The first Roman canals in this delta were ...dug around 12 BC by Drusus, but their location has been the subject of debate since the 16th century, with various hypotheses proposed. Based on actual palaeogeographical knowledge of the Rhine-Meuse delta, the Utrechtse Vecht hypothesis is considered the most plausible. Within the study area, in the northern part of the Vecht system, natural sections of this river may alternate with possible artificial reaches, created at the time of Drusus. Such artificial canals, being part of an otherwise natural channel belt system, can widen and deepen overtime, eroding all or most of the recognizable features associated with their original construction. As study area was chosen a relatively straight section of the Vecht between two former lakes. Two approaches were used. The first approach centred upon mapping channel morphology and recording sediment stratigraphy of the river deposits through detailed auger coring. Results corroborated the hypothesis of an originally straight feature (landform), confirming that it might have started life as a dug course, but not providing preserved archaeological remains of this stage. The second approach was chronological, whereby a programme of 14C dating was undertaken to refine the understanding of the origin and development of this reach of the Vecht, allowing earlier chronological investigations to be further contextualised and reassessed. A significant challenge to understand age control and floodplain evolution is the degradation of the top of the clayey peat that was observed below the levee deposits; this degradation is due to the lowering of groundwater levels and causes the end of peat growth to be dated as older than it actually is. Using new radiocarbon dates we have reconstructed that the Overmeer-Nigtevecht reach of the Vecht between two former lakes started life as a straight channel. We have constrained its age to be closer to the time of Drusus’ activities (early Roman age). Although we have not found in situ remains of Drusus canal(s), these two new insights make the Vecht option, effectuated by a series of short canals, more likely to be the Drusus canal(s).
The morphodynamics of the lower River Vecht, the Netherlands, and the influence of geomorphological setting and bank composition on meander migration were studied by means of reconstructing the ...pre-channelization landform configuration on a scale of 1 : 25,000, using historical maps from 1720, 1850 and 1890 A.D. and other data. A downstream sequence of reaches was observed, each with a typical fluvial style and channel migration rate: (a) a narrow meander belt and a highly sinuous channel with intermediate migration rate, in the middle of an extensive floodbasin; (b) a wide meander belt and high rates of lateral channel migration, especially where large meanders impinged upon valley bluffs, as part of an incised setting; (c) a low sinuosity, embanked channel with low rates of downstream migration because of confinement by dikes, occurring in an inland delta with sandy sediments. Local variation in meander migration rates was observed within reach B. This was caused by the spatial variability of bank resistance as reflected by the width-depth ratio of the channel and the silt-clay ratios of deposits. River banks are: 1) very erodible when consisting of channel deposits, aeolian dune deposits or when coarse fluvio-periglacial deposits occur at their base; 2) erodible when dominated by overbank deposits or aeolian sand sheet deposits; 3) resistant when a plaggen-layer is exposed; and 4) very resistant when dominated by floodbasin deposits. These implications of meander variability enable to assess the effects of the rehabilitation of the meandering process.
Around 800 AD the peatlands around Amstelland were drained by two rivers. The ‘northern’ Amstel discharged surplus water directly into Lake Almere, which from the 12th century onwards was enlarged to ...form the Sudersee (in Dutch: Zuiderzee). The ‘southern’ Amstel was a near-abandoned westernmost arm of the River Vecht discharging the poorly drained borderland around the river Vecht into Lake Almere/the Sudersee. As part of the reclamation of peatlands associated with agricultural activities between the end of the 10th and the mid-13th centuries, the rivers were connected via a canal, thus creating the river Amstel, as it is known today.
Since 2003 extensive archaeological research has been conducted in Amsterdam, the Netherlands, connected with the initial phase of the new underground system (Noord/Zuidlijn). Research has mainly ...focused on two locations, Damrak and Rokin, in the centre of Medieval Amsterdam. Both sites are situated around the (former) River Amstel, which is of vital importance for the origin and development of the city of Amsterdam. Information on the Holocene evolution of the river, however, is relatively sparse. This project has provided new evidence combining archaeological and geological data, and allowed the reconstruction of six consecutive landscape phases associated with the development of the River Amstel. The course of the present-day Amstel is the result of a complex interaction of processes that started with an early prehistoric tidal gully within the Wormer Member of the Naaldwijk Formation, including Late Neolithic (2400–2000 BC) occupation debris in its fill that was subsequently eroded. Next, this system developed into a later prehistoric Amstel river course that was part of the Angstel–Vecht–Oer-IJ system (1020–350 BC), meandering through a peat-dominated landscape. Later on the processes included intensive reclamation of land, drainage and canalisation, although the Amstel was also strongly influenced by natural storm tides. After intense land reclamation, starting around 1200 AD, the meandering Amstel from Nes to Kalverstraat, which was originally 150 m wide, became the rather straight 20–50 m wide tamed, canalised river of today.
Based on historical evidence a description in broad outline is provided of landscape and settlement development in the Vecht area north of Utrecht for the period 722–1122 AD. Although the fluvial ...activity of the Vecht channel was reduced from about 2300 BP onwards, the river retained its economic importance as a shipping route in Early Medieval times according to the appearance of mintage and the spread of tollhouses. In the 8th century the Vecht still was seen as a river and as a branch of the Rhine. At that time the Vecht river discharged into a stretch of water that was named Almeer and was characterised as stagnant. The Almeer is regarded as the successor of Lake Flevo, mentioned by classical authors. Since this body of water is indicated as stagnum, it is unlikely that the water level in the southern part of the Almeer was affected by the Vlie tidal inlet in the north at that time. At the end of the 10th century the Almeer already had substantial dimensions. The building of dikes on the southern forelands of the Zuiderzee and the IJ-Lake started at least about 1200 AD. At the beginning of the timeline, settlement was limited to the natural levees of the river Vecht and its distributaries. The reclamation of the vast peatlands on both sides of the river started around the middle of the 11th century. The opening up of new areas for agriculture and settlement was accompanied by a transformation of the social fabric. In association with the pattern of land division historical information is used to indicate some of the changes that occurred in the Vecht area during that period. The drive to further intensify land use resulted in 1122 AD in the decision to build a dike on the northern bank of the Lower Rhine, since the Vecht is a blind arm of the river Rhine.
The relations between groundwater composition, land use, soil conditions and flow patterns on a regional scale are studied for the Gooi and Vechtstreek area in the Netherlands. This densely populated ...area consists of a glacier-created ridge with dry sand soils bordered by the Vecht and Eem River plains with wet peat and clay soils. R-mode factor analysis and Q-mode cluster analysis were applied to a set of 1349 groundwater analyses to determine the factors controlling groundwater composition and the main resulting water types. The results indicate that groundwater composition in the study area is affected on a regional scale by human activities through changes in land use and intervention in natural flow patterns. On the ridge, ground water is recharged by precipitation, which dissolves carbonates from the matrix of the sandy aquifer. Increased solute concentrations in shallow ground water, especially of nitrate, sulphate and potassium, indicate increased pollution resulting from urbanization and increasingly intensive agricultural activity over the past decades. In the Vecht River plain infiltration occurs as a result of drainage of polders and groundwater extraction on the ridge. Recharge occurs by precipitation and from polluted surface water to which ammonium, organic complexes and carbonic acid are added through decomposition of organic matter in the peat and clay soils. The carbonic acid results in enhanced dissolution of carbonates present in the soil and the underlying sandy aquifer. Oxygen depletion and subsequent low redox potentials result in denitrification, dissolution of manganese and iron oxides, and sulphate reduction. The flow of ground water from high-level to low-level polders causes displacement of a former stagnant brackish groundwater body under the Vecht River plain accompanied by increased mixing of fresh and brackish ground water.
On the Vecht river plain (western Netherlands), small fens, remnants of a large mesotrophic wetland bordering a moraine, of 1 to 5 ha are found in a man-made matrix of lakes and pastures. The ...regional position of the fens, local position of sampling sites, composition of the vegetation and local hydrological variables were measured. Polders in the river plain produce a complex hydrology obscuring the regional zonation between moraine and river. Water supply and species composition are determined more by a site's regional than local position. High-productivity reedlands are abundant close to the river. Carex paniculata reedlands receive large amounts of river water, which gives their fen water a high K+ concentration. Low-productivity C. diandra fens and litter fens have their optimum closer to the moraine. C. diandra fens are fed mainly by inflowing nutrient-poor ground- or surface water; litter fens receive primarily rainwater. Nutrients in fen water and in peat are lowest in C. diandra and C. lasiocarpa fens, but do not differ significantly between the communities. In both, iron seems to be more important than calcium in reducing phosphate solubility. Iron richness in the C. diandra fens is caused by present inflows of ground- or surface water, while in C. lasiocarpa fens, which succeed the former, iron richness is the result of historical inflows.
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