Sediment deposition along the channel margins of a reach of the middle River Severn, UK

This paper investigates vegetational, hydrological and geomorphological inf luences on sedimentation within the river margin of a 1.7 km, sinuous reach of the River Severn, UK. The empirical study was designed to investigate t he impact of the following factors on riparian zone sediment deposition: (i ) land use, (ii) channel planform. location, (iii) the flood stage at which inundation occurs, (iv) submersion time, (v) flood event timing and orderi ng. Sedimentation was observed using sediment traps located on 48 sample pl ots within four sites (one on pasture, one within a poplar plantation and t wo within mixed riparian woodland) during four flood events. A total of 177 observations of sediment deposition were analysed for total dry weight of sediment, particle size and organic content. All sample plots were located within a 30 m wide ribbon of the riparian zone, immediately above the limit of perennial vegetation (i.e. the edge of the low flow channel). Patterns and rates of sedimentation differed between flood events. Whilst significan t differences were found in sedimentation between the four sample sites, th ese were interpreted to be more a function of planform location than land u se. The most marked variations were found in the quantity, particle size an d organic content of deposited sediment according to riparian zone cross-se ctional morphology and elevation. Highest sedimentation occurred within a 1 m elevation band that correlated with the channel-ward face of a morpholog ical bench. Percentage sand content of deposited sediment exhibited a chang e from approximately 35% in a band extending from the low flow channel to a 1 m elevation above the morphological bench edge to approximately 10% abov e this level. Organic matter content was low (less than 5%) in sediment dep osited up to a I rn elevation above the edge of the morphological bench. Ab ove this level, there was a gradual increase in organic matter content. The se observations suggest a link between the form of the riparian zone and th e sedimentation process. The relative influence of vegetational, hydrologic al and geomorphological influences on sedimentation characteristics are als o discussed. Copyright (C) 2001 John Wiley & Sons, Ltd.