Peter Houben

Holocene floodplain sediment storage and hillslope erosion within the Rhine catchment

The response of fluvial systems to land use and climate change varies depending on catchment size. While forcing-response mechanisms of small catchments are reasonably well understood, the response of larger drainage basins is less clear. In particular, the impact of land use and climate change on the Rhine system is poorly understood because of the catchment size (185 000 km2) and the long history of human cultivation, which started approximately 7500 years ago. A sediment budget is calculated to specify the amount of alluvial sediment that was deposited during the Holocene and to estimate long-term soil erosion rates. The results suggest that 59±14 X 109 t of Holocene alluvial sediment is stored in the non-alpine part of the Rhine catchment (South and Central Germany, Eastern France, The Netherlands). About 50% of Holocene alluvial sediment is deposited along the trunk valley and the delta (Upper Rhine, Lower Rhine, coastal plain), while the rest is stored along the tributary valleys. The floodplain sediment storage corresponds to a mean erosion rate of 0.55±0.16 t/ha per yr (38.5±10.7 mm/kyr) across the Rhine catchment outside the Alps. This Holocene-averaged estimate amounts for sediments that were delivered to the channel network and is at the lower limit of erosion rates from other studies of different methodology.

Spatio-temporally variable response of fluvial systems to Late Pleistocene climate change: a case study from central Germany

Abstract The change of the Earths climate and anthropogenic forcing challenge catchment management as the deliberate use of the fluvial systems natural resources depends to some extent on the predictability of the systems variable sensitivity to external impacts. The alluvial history of the Wetter valley in central Germany provides an example of the occurrence of a spatio-temporally variable catchment sensitivity to the Late Pleistocene climate change. Empirical models that are based on sediment flux controls such as sediment supply, transport capacity, and gradient serve to explain differential floodplain construction. During the early Younger Dryas the large-scale basin-upland morphology caused localised divergent cooling impacts and vegetation changes, which generated reach-scale variable sediment transport system states in the Wetter valley. The spatial arrangement of reach-scale variable response is ascribed to the tectonically driven morphological configuration of the Wetter catchment. The crucial amplification of differential floodplain construction can be assigned to the spatially variable availability of a surplus of Laacher See Tephra particles in manner of a pulsed sediment input. The ubiquitous tephra cover was previously deposited by the Laacher See Volcano event in the upper Allerod. Thus, the reach-scale variable response during the early Younger Dryas can be deemed to have been climatically triggered but superimposed by an internally driven specific upstream catchment response. The documented processes of Late Quaternary alluviation in the Wetter valley imply the need for whole-catchment approaches. The latter are a prerequisite to reveal the occurrence and causalities of variable fluvial response in relation to external impacts that interact with internal morphological controls. This is seen to be crucial to achieve an improved predictability of future environmental change in fluvial landscapes.

Environmental change and fluvial activity during the Younger Dryas in central Germany

Abstract Late Pleistocene climatically induced environmental changes are reconstructed by applying a multidisciplinary approach on floodplain sediments of small- to medium-sized catchments in central Germany. Radiocarbon dating, pollen analysis and the presence of an accurately dated tephra layer allow the establishment of a reliable chronology. The beginning of the Younger Dryas is marked by a change in fluvial activity that lasts for several hundred years. During this period gravels and sands were deposited by a braided river system. Fluvial systems at this time were predominantly controlled by the climatic conditions of the surrounding uplands, where the climatic deterioration led to a lowering of the forest limit and enhanced periglacial slope processes. An open pine forest prevailed in the basin areas and no evidence of slope wash and solifluction was found. The second part of the Younger Dryas is characterised by a meandering fluvial system and the deposition of overbank fines. The rapid transition from Younger Dryas to Preboreal coincides with an increase in organic deposition.

Asynchronous Holocene colluvial and alluvial aggradation: A matter of hydrosedimentary connectivity

Based on Optically Stimulated Luminescence (OSL) and radiocarbon dating we establish chronologies of colluviation and alluviation in different floodplain sections of the northwestern Wetterau loess basin (Germany). Similar to some other European valley floors, Holocene floodplain aggradation is marked by two important breaks: (1) a millennial-scale delay between the Neolithic agricultural colluviation and floodplain aggradation. In loess catchments agricultural colluviation started at about 7000 cal. BP and anthropogenic floodplain aggradation only at about 2200 ± 200 cal. BP; (2) a centennial-scale variability in a temporary rise in rates of anthropogenic floodplain aggradation (up to 3.6 ± 1.7 mm/yr) during the High Middle Ages in directly neighbouring reaches. Independent archaeologic, historic, and vegetation records document distinct agricultural histories of hillsides and floodplains and highlight the importance of hydrosedimentary connectivity as compared with land use intensity. The late Iron Age start of alluviation can be linked to the introduction of an integrated land use system with intense cultivation on hillsides and immediate neighbouring floodplains. The centennial-scale variability of medieval peak aggradation is a result of the successive introduction (or temporal failure) of hydraulic water milling infrastructure. Using palaeoecological and geomorphological information for reconstructing cause and consequence of sediment redistribution in coupled human–natural systems requires firm information about the spatial organisation and technological abilities that are associated with socio-agricultural transformations.