Marcin Słowik

Reconstruction of anastomosing river course by means of geophysical and remote sensing surveys (the Middle Obra Valley, western Poland)

Abstract Intensive hydro technical works were conducted in the middle course of the bra iver (oland) at the beginning of the nineteenth century. The ‘natural’ river course (functioning before the major construction works) was transformed into three artificial canals. Ground‐penetrating radar investigations, ground‐truthed with coring and remote sensing surveys, were conducted to reconstruct the course taken by the river prior to the hydro technical works. This work demonstrated that the bra formerly had an anastomosing planform. Radiocarbon dating indicated that the earliest of the retraced channels were active before 9000 bp. The retraced river system was formed in a proglacial stream valley perpendicularly cut by remains of subglacial tunnels, now partly filled with lakes. The planform consisted of one or two major channels and a number of secondary channels formed by avulsions. During the last 2000 years, some of the avulsions may have been caused by anthropogenic interventions. The sequences of channel fill deposits indicate that particular channels changed from major to secondary ones. Sand deposits filling the bottom and middle parts of the channels point to an important role of upstream sediment supply causing in‐channel aggradation triggering the avulsions. Moreover, spatial variability in river patterns was found within the anastomosing system. Valley sections with meandering anabranches, anastomosing patterns with traces of a lateral migration and traces of a transition from meandering to anastomosing planform were distinguished.

Holocene evolution of meander bends in lowland river valley formed in complex geological conditions (the Obra River, Poland)

Abstract Ground‐penetrating radar surveys, coring and the analyses of satellite and aerial images have been carried out to study differences in the evolution of meander bends formed in various geological conditions. The research was conducted in the lower course of the bra iver (western oland) characterized by a complex geology: particular sections of the valley were formed in glacial, stagnant water and fluvioglacial deposits. The research was conducted in four detailed study sites representing different formation conditions for the meander bends. Four types of meanders were distinguished: laterally migrating bends characterized by frequent changes of migration direction accompanied by river bed avulsions and cutoffs; bends with traces of continuous migration limited by stagnant water basin deposits; confined meanders in a narrow valley formed in glacial till characterized by the occurrence of mid‐channel islands; and meanders with traces of complex changes of the river bed migration influenced by anthropogenic intervention followed by intensive overbank deposition. Moreover, traces of an early development of the bra valley and remains of multi‐channel pattern were discovered. The results also show that despite being formed in different geological conditions, a similar number of the meander migration phases were recorded in the floodplain architecture during the last 7000 years at each of the sites. It is also found that the development of the studied meanders was slow compared with cases of actively migrating meandering rivers.

Anthropogenic transformation of lowland river pattern and changes of spatial extent of lakes inferred from ground-penetrating radar and remote sensing surveys (the Obra River, Poland)

A significant environmental change took place in the middle course of the Obra River (western Poland). The natural river bed pattern was replaced here by three artificial canals in the beginning of the 19th century. Before that time the Obra River bed had been characterized by various river patterns (multi-channel, meandering and disappearing in wetlands). GPR (ground-penetrating radar) surveys and the analysis of historical maps and satellite and aerial images were conducted to retrace the natural river course and changes of spatial extent of Obrzańskie Lake. Two types of the Obra river patterns, which had been functioning here before the period of the hydrotechnical works, were retraced: a large scale meandering pattern and a relatively smaller scale multi-channel pattern indicating features of anastomosing river. The transformation from the meandering to multi-channel pattern might have been caused by climatic changes and/or forming a bifurcation to the Odra River. Historical maps indicated that the multi-channel pattern was active in the second half of the 18th century. Next, the multichannel pattern was artificially transformed into the North Obra Canal. Furthermore, the shorelines of Obrzańskie Lake from the period preceding the hydrotechnical works and from the 19th century were delineated on the basis of the geophysical and remote sensing surveys. Before the anthropogenic intervention, the lake had been directly connected with a network of the Obra channels. Construction of the North Obra Canal resulted in significant decrease of the lakes surface. The presented results may be used in a restoration project aiming to reconstruct the natural river course. Such intervention would increase natural values of the middle Obra valley and influence development of tourism in this area.

Evolution of river planforms downstream of dams: Effect of dam construction or earlier human-induced changes?: Evolution of river planforms downstream of dams

When studying the evolution of landscape, it is difficult to discriminate the influence of anthropogenic from natural causes, or recognise changes caused by different sources of human action. This is especially challenging when the influence of certain sources is overprinted. For instance, although dam closure is the most common method of altering river courses, dam construction is often preceded by hydro-technical works such as channel straightening, embankment construction or sediment mining. Both dam construction and the hydro-technical works that precede dam closure can result in changes in the balance between sediment supply and transport capacity, and often, changes in river planform. The main objective of this study was to verify whether the works preceding dam closure are an important driver of river planform changes on the lower Drava River (Hungary). The case study is based on geological and geophysical surveys, as well as the analysis of historical maps covering an anabranching, 23 km long valley section. We show that channel straightening conducted prior to dam closure resulted in a transition from a meandering to sinuous planform with channel bars. Dam construction itself then caused enhanced incision, exposure of bar surfaces, vegetation encroachment and the formation of an anabranching planform. Based on this study, we developed models of alluvial island and channel planform evolution downstream of dams. Dam construction enhances channel incision, narrowing, and the reduction of flow caused by earlier hydro-technical works. Many rivers downstream of dams experience episodes of anabranching or wandering, with a multi-thread pattern replacing sinuous, braided and meandering courses. When incision continues, river patterns evolve from anabranching to sinuous via the attachment of alluvial islands to floodplains. However, the timing and sequence of these changes depend on hydrological and sediment supply regimes, geomorphic settings and anthropogenic actions accompanying dam construction. Copyright

THE FORMATION OF AN ANABRANCHING PLANFORM IN A SANDY FLOODPLAIN BY INCREASED FLOWS AND SEDIMENT LOAD

Anabranching rivers evolve in various geomorphic settings and various river planforms are present within these multi-channel systems. In some cases, anabranches develop meandering patterns. Such river courses existed in Europe prior to intensive hydro-technical works carried out during the last 250 years. Proglacial stream valleys, inherited from the last glaciation, provided a suitable environment for the development of anabranching rivers (wide valleys floors with abundant sand deposits). The main objective of the present study is to reconstruct the formation of an anabranching river planform characterized by meandering anabranches. Based on geophysical and geological data obtained from field research and a reconstruction of palaeodischarges, a model of the evolution of an anabranching river formed in a sandy floodplain is proposed. It is demonstrated that such a river system evolves from a meandering to an anabranching planform in periods of high flows that contribute to the formation of crevasse splays. The splay channels evolve then into new meandering flow paths that form “second-order” crevasses, avulsions and cutoffs. The efficiency of the flow is maintained by the formation of cutoffs and avulsions preventing the development of high sinuosity channels, and redirecting the flow to newly formed channels during maximum flow events. A comparison with other anabranching systems revealed that increased discharges and sediment loads are capable of forming anabranching planforms both in dryland and temperate climate zones. The sediment type available for transport, often inherited from older sedimentary environments, is an important variable determining whether the channel planform is anabranching, with actively migrating channels, or anastomosing, with stable, straight or sinuous branches.

Infuence of GPR measurement conditions on depth penetration and resolution of radar images illustrating lowland valley alluvial fill (field experiment)

There are many studies referring to the analysis of floodplain architecture using GPR (ground-penetrating radar). However, the geophysical surveys are usually conducted at a single set of land surface and groundwater level conditions. The main question is whether an optimal set of such conditions resulting in optimal (highest possible) resolution and depth range can be determined? A field experiment, based on the GPR surveys conducted at various groundwater levels and parameter settings, was carried out to study their influence on depth range and resolution of the GPR surveys. The test was conducted in the lower course of the Obra River (western Poland). Three sets of the GPR measurement were done: at low groundwater level, at the groundwater level situated close to the land surface and when the surface of the floodplain was inundated and frozen. The results indicated significant differences in the depth range and resolution of particular surveys. The GPR images from the survey conducted on the frozen floodplain were featured with a low depth range in comparison with the remaining experimental measurement. Despite the smooth surface of the frozen floodplain that seemed suitable for the geophysical surveys, the images were obscured by numerous diffractions originating from a layer of water underlying the ice cover. The presented results are the first step to create an atlas of georadar images illustrating various depositional environments. The research showed that such database should consist of GPR images illustrating given sedimentary environment by at least three different sets of hydrogeologic conditions and parameter settings.