River behaviour and Holocene alluviation: The River Severn at Welshpool, mid-Wales, U.K.

Abstract

A combination of archaeological evidence, 14C dates, terrace mapping, heavy metal analysis, grain size analysis and historical maps is used in a detailed analysis of the alluvial history of the River Severn floodplain around Welshpool in mid-Wales, U.K. ‘Welshpool Gravels’ underlie a higher terrace surface up to 6–7 m above the present channel. They form a sequence of gravels at least 30 m in thickness. The upper surface is characterized by a series of braided palaeochannel patterns. These sediments were probably deposited at the end of the last glaciation as outwash, and are contemporaneous with other high, gravelly terrace deposits found in the Severn and other mid-Wales basins. Overlying the Welshpool Gravels on the contemporary floodplain are a variable thickness of finer sediments, the ‘Leighton Silts’. Morphological mapping and dating of two cut-offs to 2850 ± 60 a BP and 1190 ± 70 a BP indicates that a channel pattern similar to the present planform had formed by the mid to late Holocene. From this period, floodplain development has been dominated by a single-thread meandering channel with fine vertical sedimentation and limited lateral gravel accretion. Abandonment of extended lengths of channel formed by an avulsion mechanism is apparent. A combination of historical map data, 14C dates and the analysis for heavy metals in fine sediments, which were washed into the river system during mining, indicates that there has been at least 4 m of sedimentation since the early 17th century, but only in a central belt of varying width. Metal-rich waste, identified in the fine sediments of this zone of ‘Trehelig Silts’, indicates those areas which were most heavily sedimented during the peak of metalliferous mining in the 18th and 19th centuries. Although the near-channel margins appear to be superficially similar to the older floodplain, the spatial and vertical pattern of historic sedimentation is complex, and is not reflected in marked elevation differences. The division of sedimentation periods into these three broad time-spans (Late Quaternary Terraces, Late Holocene alluviation and avulsion, and the historical metal-mining period) shows that an apparently simple planar floodplain is in reality underlain by complex sedimentation units. Floodplain construction has involved the development of inset units, in cut-offs and adjacent to migrating channels, as well as the expected contrasts between in-channel and overbank environments. This has implications both for alluvial sedimentation modelling and for the identification of high-pollution zones on the floodplain. These cannot be predicted on the basis of simple ‘in-channel’ and ‘overbank’ environments given the historically complex evolution.