C. Kasse

River response to variations of periglacial climate in mid-latitude Europe

Abstract The Last Glaciation was characterised by considerable changes in climate. Many European river basins reacted to these changes by initial incision and subsequent pattern change. Earlier research explained this by the time lag of vegetation development after a climatic change, which considerably affected the sediment load. However, since some river basins react differently, or do not react at all, this model needs to be refined. This paper deals with the fluvial evolution of several rivers in northern and central Europe during the Last Glaciation, and two of them, the Weise Elster river in the Leipzig area and the Spree river in the Niederlausitz (Germany), are discussed in more detail. The vegetation cover on the floodplain, in combination with the presence of frozen ground, which affects the discharge characteristics, largely determines the river type. Nevertheless, when the sequences are compared, not only synchronous changes in fluvial development may be observed, but also distinct differences in fluvial development. By analysing the different rivers in their varying geomorphological settings, it appears that grain size, basin configuration and catchment size are important parameters also, which can determine whether a threshold is exceeded or not.

Detection of rapid climate change in Last Glacial fluvial successions in the Netherlands

Climate change during the Last Glacial is considered as a major forcing factor of fluvial system changes. A continuous succession of fluvial sediments, reflecting adaptations to climate change from the Weichselian Middle Pleniglacial (oxygen isotope stage 3) onwards, occurs in lowland river basins in the Netherlands. A comparison of the Pleniglacial and Late Glacial fluvial record in the Netherlands shows that climatic oscillations of similar magnitude did not produce changes in the fluvial sedimentary system of equal magnitude. The Late Glacial fluvial system proves to be highly sensitive to climate change. By contrast, many of the rapid climate changes that have occurred during oxygen isotope stage 3, according to the Greenland ice core record, are not detectable in the fluvial sediments. This can be explained by differences in the impact of the climate variations on drainage basin vegetation. During the Late Glacial, the tree line repeatedly shifted through the Netherlands, whereas the area remained within the tundra zone during the Middle Pleniglacial. Precipitation variations and permafrost aggradation and degradation have played a secondary role. The Weichselian fluvial succession in the Netherlands demonstrates that detection of a change in the fluvial sedimentary system and relating this change to climate change is subject to methodological limitations. The climatic significance of changes in the fluvial record should be carefully evaluated, as well as their chronology. The possibility that climate did not influence the fluvial system should always be considered as a null hypothesis in studies on fluvial successions.

Weichselian Late Pleniglacial and Late-glacial depositional environments, Coleoptera and periglacial climatic records from central Poland (Belchatów).

Environmental conditions during the Weichselian have been studied at the Belchatow brown coal pit in central Poland. Palaeoclimate has been reconstructed by a multiproxy approach based on sedimentary environments, periglacial structures and Coleoptera remains. The Weichselian Middle to Late Pleniglacial sequence reveals a change from lacustrine to fluvial deposition. During the Late Pleniglacial, deposition by ephemeral streams with relatively stable channels was abruptly replaced by braided-river deposition. An increased aridity and the disappearance of the vegetation are held reponsible for this change in river style. The increase of aridity in the second part of the Late Pleniglacial has been inferred from the absence of organic material in the braided deposits and the extension of aeolian environments at the expense of the braided environment. A return towards wetter conditions during the Late-glacial has been inferred from lake marl deposition and local fluvial reworking of dunes. Special attention has been given to the climate reconstruction of the early Late Pleniglacial (26–24 ka). Permafrost conditions during this period, in combination with local strong relief and intensive slope processes, resulted in rapid aggradation by ephemeral streams with relatively stable channels. The climate during the early Late Pleniglacial was extremely harsh. The habitat was in many ways analogous to the true tundra of the present-day Arctic. The periglacial features indicate a mean annual air temperature lower than −4°C. The coleopteran assemblage from Belchatow is one of the most cold-adapted faunas so far recorded from Europe. The mean temperatures of the warmest and coldest months were respectively 8°C and −27°C. It is concluded that a strong temperature decline, especially of the winter temperature, occurred at the transition from the Middle to the Late Pleniglacial. At the onset of the Late-glacial Younger Dryas period a cooling of the climate has been inferred from the presence of small ice-wedge casts at the base of overlying dune sands. A mean annual air temperature between −2 and −5°C is postulated.

Sedimentary architecture and optical dating of Middle and Late Pleistocene Rhine-Meuse deposits – fluvial response to climate change, sea-level fluctuation and glaciation

Eight continuous corings in the west-central Netherlands show a 15 to 25 m thick stacked sequence of sandy to gravelly channel-belt deposits of the Rhine-Meuse system. This succession of fluvial sediments was deposited under net subsiding conditions in the southern part of the North Sea Basin and documents the response of the Rhine-Meuse river system to climate and sea-level change and to the glaciation history. On the basis of grain size characteristics, sedimentological structures, nature and extent of bounding surfaces and palaeo-ecological data, the sequence was subdivided into five fluvial units, an estuarine and an aeolian unit. Optical dating of 34 quartz samples showed that the units have intra Saalian to Weichselian ages (Marine Isotope Stages 8 to 2). Coarse-grained fluvial sediments primarily deposited under cold climatic conditions, with low vegetation cover and continuous permafrost. Finer-grained sediments generally deposited during more temperate climatic conditions with continuous vegetation cover and/or periods of sea-level highstand. Most of the sedimentary units are bounded by unconformities that represent erosion during periods of climate instability, sea-level fall and/or glacio-isostatic uplift.

Process-based modelling of fluvial system response to rapid climate change I: model formulation and generic applications.

A comprehensive model strategy is presented which enables the prediction of catchment hydrology and the dynamics of sediment transport within the alluvial river systems draining these catchments. The model is driven by AGCM-based weather predictions, generalised by using a stochastic weather generator, and by palaeo-climate and palaeo-environment reconstructions. The model consists of a lumped hydrological rainfall-runoff model, calibrated against modern daily discharge data and the AGCM control experiment, combined with simple modules for hillslope erosion, river channel geometry, sediment transport and fluvial planform type. We apply the model to a conceptualised climatic cycle, and investigate the response to brief climatic events. Model predictions are discussed and compared to reconstructed river behaviour.

Process-based modelling of fluvial system response to rapid climate change II. Application to the River Maas (The Netherlands) during the Last Glacial-Interglacial Transition

A comprehensive process-based numerical model of catchment hydrology and alluvial channel dynamics is applied to the evolution of the river Maas during the Last Glacial-Interglacial Transition. Palaeo-climatological reconstructions based on a number of climatic and environmental proxies are combined with atmospheric circulation model predictions to yield continuous time series for temperature, precipitation and vegetation cover for the period of 14-9 C-14 kyr BP. These climatic data are used as input for the numerical model. Predictions are made for discharge statistics, hillslope erosion potential, river channel sediment transport, channel pattern and incision potential. These predictions are compared with reconstructed fluvial dynamics of the Maas during this time period. We find that the major fluvial morphodynamical events can be explained by our model as a result of climate change induced affects. However, relatively high precipitation amounts during GS-1 must be applied in order to correctly predict floodplain wide incision during this period. We further show by means of a number of sensitivity analyses that the uncertainty in the adopted climate reconstructions do not have a large impact on modelled morphodynamics

Climate-driven fluvial development and valley abandonment at the last glacial-interglacial transition (Oude IJssel-Rhine, Germany)

In the Weichselian, the Lower Rhine in the Dutch-German border region has used three courses, dissecting ice-marginal topography inherited from the Saalian. In the Late Weichselian, the three courses functioned simultaneously, with the central one gaining importance and the outer ones abandoning. This study aims to reconstruct the fluvial development and forcings that culminated in abandonment of the northern branch ‘Oude IJssel-Rhine’, at the time of the Lateglacial to Holocene transition. The fluvial architecture is studied using a cored transect over the full width of the valley, detailed cross-sections over palaeochannels and geomorphological analysis using digital elevation and borehole data. Biostratigraphy, radiocarbon dating and OSL dating provide a timeframe to reconstruct the temporal fluvial development. In its phase of abandonment, the fluvial evolution of the Oude IJssel-Rhine course is controlled by the ameliorating climate and related vegetation and discharge changes, besides by intrinsic (autogenic) fluvial behaviour such as the competition for discharge with the winning central branch and the vicinity of the Lippe tributary confluence. The rapid climate warming at the start of the Late Glacial resulted in flow contraction as the initial response. Other fluvial geomorphic adjustments followed, with some delay. An aggrading braided or transitional system persisted until the start of the Allerod, when channel patterns finally changed to meandering. Floodplain incision occurred at the Allerod - Younger Dryas transition and a multi-channel system developed fed by Rhine discharge. At the start of the Holocene, this system transformed into a small-scale, local meandering system, which was abandoned shortly after the start of the Holocene. The final abandonment of the Oude IJssel-Rhine and Niers-Rhine courses can be attributed to deep incision of the Central Rhine course in the earliest Holocene and is considered to be controlled by flow contraction induced by climate and related vegetation and discharge changes.

Sensitivity of floodplain geoecology to human impact: A Holocene perspective for the headwaters of the Dijle catchment, central Belgium

Floodplain deposition rates have increased markedly under influence of human impact throughout the late Holocene in many western and central European catchments. Consequently the geomorphology and ecology of many floodplains changed. In this study we discuss this human impact and its influence on the floodplain geoecology during the middle and late Holocene for the headwaters of the Dijle catchment, located in the Belgian loess belt. The floodplain geoecology and the regional vegetation was reconstructed from sedimentological and palynological analyses. An age–depth model for the studied sequences was obtained using 17 radiocarbon dates. Statistical analyses of the pollen data (cluster analysis and canonical correspondence analysis) were used to detect changes in the pollen record. Our data show that until c. 2500 cal. BP, human impact was nearly absent or localized with no discernible influence on the floodplain geoecology. The floodplain was in a stable phase and consisted of a marshy environment where organic material could accumulate, which is interpreted as the natural state of the floodplain. From c. 2500 cal. BP onwards, human impact gradually increased. However, only when human impact in the catchment crossed a threshold around 500 cal. BP, the floodplain geoecology changed with clearing of the Alder carr forest, the establishment of a single channel river and the dominance of minerogenic overbank sedimentation. Spatial variability in the coupling between increasing human impact and changes in floodplain geoecology can be attributed to differences in hillslope–floodplain connectivity and local differences in human impact.

Periglacial environments during the early Pleistocene in the Southern Netherlands and Northern Belgium

Abstract The palaeoenvironment of three Early Pleistocene glacials in The Netherlands and Belgium are studied: the intra-Tiglian “Beerse glacial” and the Eburonian and Menapian cold stages. Climatic conditions are expressed by specific periglacial phenomena and by palaeobotanical data. The three cold periods are characterized by a nearly treeless tundra vegetation and a continuous or discontinuous permafrost. Mean summer temperatures are estimated around or slightly below 10°C, while mean annual temperatures were below −6 to −4°C.

Process-based modelling of the climatic forcing of fluvial sediment flux: some examples and a discussion of optimal model complexity

Abstract During the complex and highly dynamic climate of the Late Quaternary, precipitation and temperatures were highly variable and generally did not change synchronously. Reconstructed fluvial sedimentological response to this climatic forcing was previously shown to be complex. This response can be understood only by accounting for all the relevant catchment processes. This paper reviews a number of simple numerical models that aim to enhance our understanding of the role of different processes in this climate-sedimentology system. The applicability of these models depends on the scale of the models involved, the required input data and the way that parameters are estimated. Simple process-based models perform best on Quaternary timescales because their data needs are low and parameters can be interpreted in physical terms, which enables an a priori estimation. Process-based models of fluvial dynamics may provide a methodology for the understanding of the mechanisms responsible for the dynamics of sediment flux to basins.