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

Abstract

1. Data are presented on the sedimentological characteristics of 51 river reaches which include trout (Salmo trutta) spawning grounds from three different 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, five 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. The 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 characterized by relatively high silt–clay contents (sub-0.063 mm) which exceeded 10% of 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 surface 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 distributions for trout streams in (a) upland and (b) lowland areas are presented for use in assessing levels of siltation and in channel restoration in southern England. Copyright © 2000 John Wiley & Sons, Ltd.