1 BHS Third International Symposium, Managing Consequences of a Changing Global Environment, Newcastle 2010 © British Hydrological Society Disturbance of gravel substrates by signal crayish (Pacifastacus leniusculus) Matthew F. Johnson, Stephen P. Rice and Ian Reid Department of Geography, Loughborough University,UK Email: M.F.Johnson@lboro.ac.uk Abstract This paper focuses on signal crayish, an internationally widespread invasive species, and their impacts on luvial gravel substrates. In a series of laboratory lume experiments, it was found that crayish signiicantly altered the microtopography of substrates by constructing shallow pits within which they sheltered and mounding excavated material into ridges. Crayish also altered the fabric of gravel substrates by brushing past grains when walking, altering grain geometry. This had a signiicant impact on the structural characteristics of water-worked gravels, resulting in a reduction in imbrication of surface grains. The structural and topographic alterations to substrates exposed to crayish had a highly signiicant impact on gravel stability. Nearly twice as many grains were mobilised from substrates disturbed by crayish than from control surfaces which were not exposed to crayish. This was mainly due to increased grain protrusion associated with crayish constructing pits and mounds on the substrate surface. The results of this study suggest that signal crayish could oppose the consolidation and structuring of gravel substrates in rivers, resulting in a signiicant alteration in substrate stability during subsequent high low events. Introduction The reworking of substrates by organisms is considered a fundamental process operating in marine and terrestrial environments (Murry et al., 2002; Gabet et al., 2003) but has been less studied in gravel-bed rivers with a few noticeable exceptions, including the reworking of gravels by spawning salmonid ish (e.g. Gottesfeld et al., 2004; Hassan et al., 2008). The relative coarseness of the substrate and the high energy low environment of gravel-bed rivers may result in organisms being less effective geomorphic agents than in other environments. However, it is known that relatively small alterations to the geometry of surface grains in rivers can have a substantial impact on their entrainment. For instance, the orientation and pivot angle of grains will determine the cross-sectional area exposed to the low and the ease with which one grain will roll over a downstream grain (Li and Komar, 1986; Kirchner et al., 1990; Carling et al., 1992). Grains which protrude high into the low will also undergo greater low forces than those sheltering close to the bed or behind coarser grains (Fenton and Abbott, 1977). The continuous, unidirectional low in rivers alters the location and geometry of surface grains by moving vulnerable grains into less vulnerable positions. As a result, water-worked substrates are more stable than loose, random arrangements of gravel. Typical characteristics of water-working are cluster bedforms and imbrication where grains overlap, locking together and imparting stability to multiple grains (Reid and Hassan, 1992). If the activity of organisms in luvial environments can alter the surface structure of substrates then this may have signiicant implications for the stability of surface grains. This could result in increased transport of bedload and suspended load which would have a substantial impact on the geomorphology of the reach and could also result in the mobilisation of contaminates. Destabilising the substrate surface would also have signiicant implications on benthic organisms which reside on the bed. In this paper we focus on signal crayish (Pacifastacus leniusculus), a large freshwater crustacean which is native to north-western USA, but is now widely present as an invasive species in North America, Japan and western Europe. Aims Crayish have been found to be able rework luvial substrates, particular by winnowing ine material from between coarse grains (Statzner et al., 2000; 2003; Creed and Reed, 2004; Usio and Townsend, 2004). The inluence of abiotic and biotic interactions on substrate reworking by crayish has also been studied (Statzner and Peltret, 2006; Statzner and Sagnes, 2008). The results presented here are part of a larger research project which aims to build on these previous studies by quantifying the impact of signal crayish (P. leniusculus) on luvial gravel substrates and to determine how crayish alter the geomorphic, hydraulic and sedimentary processes operating at the substratum surface. The speciic objectives of this paper are: (1) to establish whether crayish can alter the micro-topography of gravel substrates and determine the mechanisms of disturbance; (2) to determine whether crayish can rework substrates that have been structured by lowing water (i.e. water-worked substrates); and (3) to determine whether gravel stability is modiied due to the reworking of the substratum by crayish. Methodology Overview A series of experiments was undertaken in a laboratory lume to study the impact of crayish on the topography of gravel substrates. Narrowly-graded gravel was placed in an experimental section of the lume and screeded lat. In half of the experiments, the substratum surface was loose with random arrangements of gravel (series 1) and in the other