AN ANALYSIS OF ARTIFICIAL RIFFLES AND ENHANCEMENT OF BENTHIC COMMUNITY DIVERSITY BY PHYSICAL HABITAT SIMULATION (PHABSIM) AND DIRECT OBSERVATION

Critical to river rehabilitation decisions is the prediction of the be nefits of certain procedures. In low-order systems, planning should fo cus on flow requirements of lotic organisms, especially benthic specie s. We examined the value of placement of artificial riffles in Holly F ork, a low-order tributary of the West Sandy River (west Tennessee). T he objective was to determine if the instream flow incremental methodo logy (IFIM) and its component computer model, PHABSIM, could adequatel y predict the habitat value of the riffles for benthic macroinvertebra tes and if, after colonization, this 'value' was reflected by increase d diversity of benthic macroinvertebrates. Holly Fork is a severely he ad-cut channel with 2 m high vertical embankments and a substrate domi nated by sand and fines, with occasional gravel riffles. We chose to c onduct the analysis of each stream reach using macroinvertebrate diver sity as the 'target'. Habitat suitability criteria, developed using da ta from adjacent watersheds, predicted a range of suitable physical co nditions that supported highest benthic community diversity. A traditi onal IFIM analysis of a reach of the Holly Fork indicated that, below 0.2 m(3)/s (a flow exceeded 10% of the time), less than 5% of the wett ed area contained adequate habitat for benthic macroinvertebrates. At optimum flows (0.4 m(3)/s and higher), only 15% of the wetted area, pr imarily across small gravel bars, was adequate to support high communi ty diversity. Two artificial riffles, composed of large cobble and bou lder keystones, with leading and trailing aprons of medium cobble and gravel, were placed at 35 m intervals in the test section. After hydro logical stabilization and time for colonization by macroinvertebrates, the artificial riffles were re-analysed. The simulation predicted tha t this reach contained significantly higher amounts of available benth ic habitat at low flows (more than tripled), and over 40% of the total wetted area should support high benthic community diversity at optima l flows. The presence of artificial riffles contributed most of this h abitat enhancement. A plot of cell-by-cell composite habitat suitabili ty and sample diversity from these cells revealed a significant correl ation between PHABSIM predictions and actual community diversity. This technique can be an aid in demonstrating the value of certain restora tion structures during the rehabilitation planning process. Our data s uggest that benthic community diversity is an appropriate target for e valuation of instream flow Values that sustain ecosystem integrity. (C ) 1998 John Wiley & Sons, Ltd.