Sediment storage in the shallow hyporheic of lowland vegetated river reaches

Excessive fine sediment deposition oil the river channel bed together with colmation of finer sediments within the hyporheic are now linked to the degradation of the aquatic habitats of gravel bed rivers in permeable catchments. Previous studies of chalk rivers (associated with Outcrops of calcareous rock) have demonstrated the important role of aquatic vegetation in trapping fine sediment on the river channel bed. This research investigated the spatio-temporal patterns and composition of fine sediment stored in two vegetated river reaches, in the Frome and Piddle catchments, Dorset (UK), with contrasting hydrological regimes, in order to establish the importance of aquatic vegetation in controlling the magnitude and timing of sediment storage in chalk rivers. Monthly mapping of macrophyte and sediment cover at the two sites (Maiden Newton and Snatford Bridge, 2003-2004) revealed a cyclical pattern of sediment storage related to the growth and die-back of aquatic vegetation peaking at 66.8 kg m(-2) in July 2003 at Maiden Newton, and 23.5 kg m(-2) in October 2003 at Snatford Bridge. Sediment was stored within gravels and beneath vegetation in the margins and mid-channel locations at both sites. Significantly more sediment was stored beneath vegetation than within gravels. The spatio-temporal pattern of sediment storage at the reach scale and the composition of the stored sediments reflected the growth patterns and functional form (flexibility) of the dominant macrophytes Ranunculus penicillatus subsp. pseudofluitans (water crowfoot) and Rorippa nasturtium aquaticum (watercress). Finally, the paper discusses the implications of reach-scale patterns in sediment storage for contaminant storage. Copyright (C) 2009 John Wiley & Sons, Ltd.