North West European Rivers
7. LATEST SAALIAN / WOLSTONIAN TO EARLY WEICHSELIAN / DEVENSIAN (MARINE ISOTOPE STAGES 6-4)
Map 8: Latest Saalian to early Weichselian drainage during sea level lowstand
(Marine Isotope Stages 6 to 4; latest Middle - Late Pleistocene).
Throughout the cold stages of the Pleistocene the rivers overwhelmingly deposited gravels and sands derived either from periglacial weathering or glacial sources. The result is that valley systems contain vast thicknesses of cold-climate sediments deposited by rivers that flowed in a braided or wandering, often multi-channelled, form. These sediment accumulations are generally separated by periods of non-deposition or incision outside subsiding areas. This incision was previously often attributed to interglacial events. It is now, however, clear from modern process studies that incision almost certainly also occurs when river runoff is highly seasonal but when limited supplies of detritus are available, i.e. predominantly under cold climates. Highly peaked discharges characterize modern rivers in periglacial regions. This pattern is caused by storage of water as snow on the land throughout the winter and subsequent rapid melting in spring, giving rise to the nival flood event. Such events may carry much of the year's precipitation in a few days. The result is high flow velocities and therefore strong erosion and transporting power of very short duration. In glacial meltwater streams these discharge peaks may be modified by cycles of glacial melt and discharge of stored water, such as jokulhlaups. Lack of vegetation and the development of permafrost ensure a rapid return of precipitation in catchments to rivers, so that storms may also produce marked flood events in such periglacial streams.