Regional variations in the sediment structure of trout streams in southernEngland: benchmark data for siltation assessment and restoration

1. Data are presented on the sedimentological characteristics of 51 river r eaches which include trout (Salmo trutta) spawning grounds from three diffe rent hydrological regions of southern England: I, upland, hard-rock of the south west (20 sites); soft-rock of the east with, II, chalk (11 sites) and III, sandstone or limestone (20 sites). 2. Data were obtained using a standard sampling protocol: at each site, fiv e randomly distributed freeze-cores to a depth of 30 cm were taken to yield a composite weight of at least 20 kg 3. All sites had a well-developed armour layer and at more than 70% of the sites the armour layer had a median b-axis of more than 35 mm and included some cobbles (stones larger than 64 mm. 4. The substratum at 65% of the Type I sites had less than 10% finer than 1 mm contrasting with all sites of Type II having more than 25% sub-1 mm. Th e Type II sites contained high levels of sands (0.125-1 mm). Type III sites had a wide range of fine sediments (15-53% sub-1 mm) but were characterize d by relatively high silt-clay contents (sub-0.063 mm) which exceeded 10% o f the sample weights at six sites. At most sites, lower (15-30 cm) sections contained about twice the loading of fine sediments compared with the surf ace layer (0-15 cm). 5. A target loading for the substratum of a healthy stream of the important sub-1 mm size fraction is set at 14%. Benchmark particle-size distribution s for trout streams in (a) upland and (b) lowland areas are presented for u se in assessing levels of siltation and in channel restoration in southern England. Copyright (C) 2000 John Wiley & Sons, Ltd.