Changes in Sediment Flux and Storage Within a Fluvial System: Some
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HYDROLOGICAL PROCESSES Hydrol. Process. 17, 3321–3334 (2003) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/hyp.1389 Changes in sediment flux and storage within a fluvial system: some examples from the Rhine catchment Andreas Lang,1* Hans-Rudolf Bork,2 Rudiger¨ Mackel,¨ 3 Nicholas Preston,1 Jurgen¨ Wunderlich4 and Richard Dikau1 1 Geographisches Institut, Universit¨at Bonn, Meckenheimer Allee 166, D-53115 Bonn, Germany 2 Okologie-Zentrum¨ Kiel, Christian-Albrechts-Universit¨at zu Kiel, Schauenburger Str. 112, D-24118 Kiel, Germany 3 Institut f¨ur Physische Geographie, Albert-Ludwigs-Universit¨at, Werderring 4, D-79085 Freiburg, Germany 4 Institut f¨ur Physische Geographie, Johann Wolfgang Goethe-Universit¨at, D-60054 Frankfurt, Germany Abstract: The Rhine river system can look back on a long history of human impact. Whereas significant anthropogenic changes to the river channel started only 200 years ago, the impacts of land use have been felt for more than 7500 years. Here, we review results from several case studies and show how land-use change and climate impacts have transformed the fluvial system. We focus on changes in sediment delivery pathways and slope–channel coupling, and show that these vary in time and depend on the magnitude of a rainfall event. These changes must be accounted for when trying to use sediments as archives of land-use change and climatic impacts on fluvial systems. For example, human impact is recorded in slope deposits only as long as rainfall intensity and runoff generation do not exceed the threshold for gullying. Similarly, climatic impacts are only recorded in alluvium when both the landscape is rendered sensitive by human activities (e.g. deforestation) and rainfall thresholds for gullying are exceeded. Copyright 2003 John Wiley & Sons, Ltd. KEY WORDS Rhine river; land-use change; climate change; sediment delivery; system evolution INTRODUCTION The River Rhine drains large parts (189 700 km2 of central Europe. The main river channel stretches 1320 km and drains into the North Sea. Along its course the hydrological regime of the Rhine changes from glacio-nival in the Alps and upland areas to pluvially dominated lowlands in the Netherlands. Here, the mean discharge is 2500 m3 s1, the mean flood discharge is 6000 m3 s1 and the mean discharge at low flow is 1000 m3 s1 (IHP/OHP, 1996). A detailed description of the physiographic setting and the present-day hydrology of the Rhine is presented by Middelkoop and Asselmann (2003). The Rhine river system has probably been studied in greater detail than any other drainage basin in the world. A simple literature search returned more than 800 scientific articles and books, not including publications dealing with the cultural and political importance of the Rhine. Agricultural activities in the Rhine drainage basin date back to the Early Neolithic (¾7500 years ago). The loess landscapes of northern Switzerland, southern Germany, and France were especially favourable for settlement, due to their fertile soils and relatively mild climate. By medieval times the whole Rhine catchment had been settled, with only a few exceptions in remote mountain environments. Today, the Rhine catchment can be characterized as ‘advanced industrial’ according to Wasson (1996). Until the beginning of the 19th century the channel dynamics of the larger rivers within the Rhine drainage basin were dominantly climatically driven. While changes in land use resulted in strongly increased * Correspondence to: Andreas Lang, Department of Geography, University of Liverpool, Liverpool L69 72T, UK. E-mail: [email protected] Copyright 2003 John Wiley & Sons, Ltd. 3322 A. LANG ET AL. sedimentation (floodloams), the river form itself remained largely unchanged. The impact of anthropogenic structures, like water-mills and small dams, can be traced back to the Iron Age. Their influence, however, seems to have been only local. During the 19th century more dramatic changes were initiated by engineering work to permit the passage of ships, hydro-electric power plants, and flood protection. The most prominent of these changes was the channelizing of the course of the Rhine river in the Upper Rhine graben, starting with the work of Tulla in the early 19th century. In addition to these major impacts, the combination of increasingly numerous small impacts (e.g. surface sealing due to construction of roads and buildings, draining of wetlands and channelizing of smaller creeks and ditches) began to change the fluvial regime of the Rhine drastically. However, influences on the fluvial regime have not been restricted to engineering works. Many of the high flood events recorded during the Middles Ages and early modern times were related to ice dams and their failure. Owing to the impact of power plants using river water for cooling and releasing warm water back into the river, and the impact of urban waste water, ice has been absent from the larger rivers during recent decades. The anthropogenic impact of large engineering structures is rather obvious compared with the impacts of land use. Over the longer term, however, land use and land-use change also produce significant changes through the summed effect of small but more frequent events. Here, we report results of several case studies from the German part of the Rhine catchment, exemplifying the long-term development of a fluvial system undergoing human impact, and with these results we address some of the LUCIFS research questions. CASE STUDIES Upper Rhine lowlands The first case study is located between the towns of Neuenburg and Offenburg in the southern part of the Rhine graben (Figures 1 and 2). The Upper Rhine graben system is a dominant tectonic feature in the course of the Rhine. The graben is filled with Tertiary and Quaternary sediments to depths of up to several hundreds of metres. During the Wurm¨ Glacial, the Rhine was a braided river system, fully in accordance with the periglacial climate conditions. At least one channel was located east of the Kaiserstuhl, which is an upland area within the alluvial flood plain of the Rhine river. At the beginning of the Late Glacial period the fluvial system changed and a meandering river developed. This change was associated with a decrease of gravel load and higher and more continuous runoff. The new channels incised into the Wurmian¨ gravels and a distinct erosional scarp was formed (Hochgestade), which today still forms a pronounced boundary between the higher Wurmian¨ terrace (Niederterasse) and the lower Holocene alluvial plain. According to pollen analysis and 14C data, the eastern course of the Rhine existed as a perennial stream until the end of the Late Glacial (Friedmann, 1999). However, the eastern Rhine channels were also partially reactivated during the Holocene in response to exceptional floods. The Late Glacial environment was largely unaffected by human activity, and changes in the fluvial system were caused by climatic changes. During the Atlantic Period and the Holocene climatic optimum (warmer temperatures, especially during summer), the Neolithic Revolution introduced widespread human impacts on the natural environment for the first time. The first settlements and agricultural activities in the southern Upper Rhine Valley are reflected in pollen diagrams, as for example in the Wasenweiler Ried (Figure 2). A period of geomorphic stability during the early Atlantic Period (about 8000 years ago) is indicated by the development of forests (mixed oak and hazel) and the black lowland (or floodplain) soil (Schneider, 2000). About 6000 years ago the situation changed: pollen from cereals and agricultural weeds appears (Friedmann, 1999). The pollen records show declining percentages of woodland pollen, whereas percentages of pollen indicating open land increase. This is due to woodland grazing and the first existence of range land, as can also be seen by the increase in grass pollen. The earliest occurrence of soil erosion in the loess areas took place during this period, and led to the accumulation of colluvial and fluvial sediments (Mackel¨ and Copyright 2003 John Wiley & Sons, Ltd. Hydrol. Process. 17, 3321–3334 (2003) CHANGES IN SEDIMENT FLUX AND STORAGE 3323 7°E9°E Elevation / m a.s.l Sieg < 220 Vogels- 220-340 3 berg 340-450 Lahn 450-570 Frankfurt Mosel Rhein 570-730 730-950 Main 50°N 950-1280 >1280 Mannheim Neckar Nürnberg 2 Stuttgart Upper Rhine Graben Donau München 48°N Rhein 1 Schwäbische Alb Schwarzwald N Bodensee Thur W E Rhein Alps S Rhein 0 20 40 60 80 km 1 Case study ‘Upper Rhine Lowlands’ Town 2 Case study ‘Vaihingen/Enz’ River Rhein 3 Case study ‘Amöneburger Becken’ Figure 1. Map of southern central Europe showing the location of the study sites Friedmann, 1999; Schneider et al., 2000). A more noticeable human impact on the environment is documented from the Iron Age: the La Tene` culture (4th to 1st centuries BC) and the Roman period (1st to 4th centuries AD). A further decrease in woodland and an increase in open land during this period is apparent in pollen diagrams from the Upper Rhine Lowlands. Intensive agricultural land use can clearly be seen by an increase in pollen of cereals, weeds and pasture grasses. The growing human population resulted in an increase in the number of settlements and an extension of the farming area. In addition, mining activities caused a further reduction of the woodlands. For this period, higher rates of erosion are clearly documented in fluvial sediments and changed morphodynamic responses of the river systems. With the retreat of the Romans, the population density declined and many settlements and agricultural fields were abandoned. A significant impact on the environment of the climatically favoured and fertile loess-covered areas of the Upper Rhine Lowlands occurred once again during the Alamannic land acquisition and consolidation phase (5th to 7th centuries AD), as documented in alluvial sediments from the Elz and Rhine channels (Schneider, 2000).