John Lewin

Initiation of bed forms and meanders in coarse-grained sediment

In a straight plane-bed channel in coarse sediment under natural flow conditions, primary transverse bars were rapidly formed during infrequent high flows, and the accompanying flow modifications led to bank erosion. Primary bars were subsequently incorporated as the cores of point-bar complexes, with additional lateral and tail accretion, chute formation, and lesser erosional and sedimentary modifications. The range of natural flows produced a variety of bed forms; therefore, the channel form at any point in time was not strictly attributable to any single controlling discharge, but the broad features of channel development in general relate to the forms of the pseudo-meandering model. A three-phase model of meander development, with these observations exemplifying the first phase, is adequate and useful.

River response to high-frequency climate oscillations in southern Europe over the past 200 k.y.

A 200 k.y. chronology of river response to climate-related environmental change has been established for northeast Spain using newly developed luminescence dating techniques. This constitutes the best-documented record of late Quaternary river behavior currently available for the North Atlantic region and enables fluvial stratigraphies to be compared with high-resolution ice core and marine oxygen isotope climate series. Pleistocene and Holocene river aggradational episodes coincide with stadial or neoglacial events, while phases of river incision occur during interstadial or interglacial periods. Alluviation and erosion cycles would appear to track variations in sediment supply controlled by vegetation cover and winter storm frequency.

Source identification for suspended sediments

Abstract Studies of suspended sediment in the 170-km 2 River Ystwyth catchment (mid-Wales) for 1973–1975 suggest two complementary methods of identifying their sources. The colour of yielded sediments, and the details of sediment-discharge relationships with respect to time, allow a separation into “channel” and “non-channel” sources. In two successive years, yields of 43,439 and 12,233 t are estimates, with “channel” sediments contributing 40.5 and 53.3%, respectively. Explanations for such yields and the more general applicability of the methods used, are discussed.

The fluvial record of climate change

Fluvial landforms and sediments can be used to reconstruct past hydrological conditions over different time scales once allowance has been made for tectonic, base-level and human complications. Field stratigraphic evidence is explored here at three time scales: the later Pleistocene, the Holocene, and the historical and instrumental period. New data from a range of field studies demonstrate that Croll–Milankovitch forcing, Dansgaard–Oeschger and Heinrich events, enhanced monsoon circulation, millennial- to centennial-scale climate variability within the Holocene (probably associated with solar forcing and deep ocean circulation) and flood-event variability in recent centuries can all be discerned in the fluvial record. Although very significant advances have been made in river system and climate change research in recent years, the potential of fluvial palaeohydrology has yet to be fully realized, to the detriment of climatology, public health, resource management and river engineering.

Distribution of metal pollutants in floodplain sediments

Summary Metal concentrations in the polluted floodplain sediments of the River Rheidol are analysed. These vary according to the date of floodplain reworking in relation to that of former mining activity, according to sedimentary environment, and with metals present. Variability between and within sedimentary environments is discussed, together with the differential behaviour of Pb, Cu and Zn.

Distribution of metal pollutants in active stream sediments

Summary The spatial distribution of metal concentrations in stream sediments of the former metal mining areas of mid-Wales and the Colorado Front Range are discussed. Concentration patterns vary in a complex manner and between environments studied, but some systematic relationships between concentration and particle size and with distance downstream are apparent. A negative exponential curve of the form loge Y = a′−b′X was fitted to the latter relationship: most situations show too complex a pattern of input and dispersal for this to be attempted, but further sampling elsewhere should allow comparison of dispersal patterns in different mining environments.

Late quaternary fluvial sedimentation in the voidomatis basin, Epirus, Northwest Greece

Abstract Detailed morpho- and lithostratigraphic investigations, allied with radiometric dating, in the Voidomatis basin, Epirus, northwest Greece, have identified four Quaternary terraced alluvial fills that range from middle Pleistocene to historic in age. Major-periods of alluviation during the late Quaternary were associated with valley glaciation (ca. 26,000–20,000 yr B.P.) and subsequent deglaciation (ca. 20,000–15,000 yr B.P.) in the Pindus Mountains during Late Wurmian times, and more recently linked to overgrazing sometime before the 11th century AD. The late Quaternary alluvial stratigraphy of the Voidomatis River is more complex than the “Older Fill” and “Younger Fill” model outlined previously, and it is suggested that these terms should no longer form the basis for defining alluvial stratigraphic units in the Mediterranean Basin.

Planform cyclicity in an unstable reach: complex fluvial response to environmental change

Long- and short-term channel changes are documented and analysed for a historically unstable reach of the River Severn at Llandinam, mid-Wales. Long-term changes (the last 150 years), reconstructed from 10 archival sources, are characterized by channel planform switching between meandering (1836–1840 and 1948–1963) and braided (1884–1903 and 1975–present) phases. Short-term changes, monitored by detailed planform surveys over a 2·5 year period, showed smaller-scale channel adjustments involving channel switching, bar accretion and channel expansion. Phases of braiding at Llandinam have been triggered by extrinsic controls, primarily flooding, but intrinsic controls (floodplain sediments, planform evolution and channel gradient) have been influential in priming the reach prior to destabilization. Flow regulation on the River Severn since 1968 has partly frozen the planform of the contemporary braid zone. Management of channel planform adjustments, where environmental change is phased in over time, must be informed by a knowledge of the potential for triggered planform switches. In addition, the effects of environmental change on fluvial systems are often historically contingent upon the state of the channel at the time of impact.

The history of the major rivers of southern Britain during the Tertiary

The evolution of the drainage system of lowland Britain is discussed on the basis of available geological evidence, including that from terrestrial sites and that which has more recently become available from offshore exploration of the North Sea, the English and Bristol Channels, and the Irish Sea. Tertiary stratigraphy throws considerable light on landform and river development. Paleocene destruction of a chalk cover, which seems to have been incomplete in western Britain, was accompanied by basin sedimentation under a tropical climate. The major elements, the Thames, Solent, Hampshire (?proto-Avon) river, Irish Sea river and possibly an early Trent river, existed almost throughout the Cenozoic. The influence of Atlantic rifting and thermal doming in NW Britain appears to have been stronger and more temporally focused than the persistent flexuring that determined and maintained Tertiary drainage lines in the SE. Here also the folded Mesozoic terrains on the surface contrast with the more dominant block-faulted relief of the Palaeozoic ‘oldlands’. The rivers of the SE can be shown to have extended or reduced their lengths in response to relative sea-level change and gentle warping. Drainage antecedence, the destruction of the Solent system and the breaching of the English Channel are also evident. By contrast, the major river systems of the west are now entirely submerged. Long-term stability of the drainage pattern reflects a persistent tectonic regime in the south, with a subdued low-relief landscape having a weathered regolith and dense vegetation cover. Meandering river channels and alluvial styles predominated, although channel forms varied according to sediment load, slope and discharge variability. Coarse gravel-dominated accumulations are rare and localized. Chemically stable lithologies dominate the clastic component throughout. It is apparent that the deeply incised river valleys seen today are related to high, predominantly coarse sediment yields, encouraged by substantial, rapid climate changes in the Pleistocene. This emphasizes the significance of mechanical compared with chemical weathering for the rate and nature of landscape dissection, and the modifications that have arisen as a result of glaciation, frost-climate weathering, rapidly changing climates and sea levels. The stratigraphical evidence here reviewed is at variance with older, largely geomorphologically based landform evolution models (‘denudational chronology’), but gives considerable support to the recent proposals emphasizing the significance of Paleocene erosion, and enduring low-relief landscapes and drainage systems evolving alongside fold development during the Paleogene. Given the depobasin evidence now available, postulated fluvially active episodes can, and must, be linked to contemporaneous deposition. Some at least of the many controversies involving the identity of erosion surfaces, the dating of them using only residual deposits and weathering mantles, and the selection of particular Tertiary episodes as ones of landscape development can now be resolved.

Welsh floodplain studies: II. Application of a qualitative inundation model

Abstract The relationship of inundating waters to patterns of floodplain relief is assessed in terms of an inundation model involving types of input, output and transfer processes. This model is applied to floods observed in the rivers Dyfi and Teifi in mid-Wales from 1973–1977. These show complex patterns of actual inundation sequences, involving hysteretic effects in the relationship between river stage and flood extent, and varying effects arising from human floodplain modifications, different rates of stage fluctuation, and the local incidence of floodplain forms. Further sequential studies of inundation for contrasted floodplain types should allow better understanding of floodplain conductivity as an aid to flood-routing procedures and as a help in understanding discharge-inundation relationships.