A MESOHABITAT METHOD USED TO ASSESS MINIMUM FLOW CHANGES AND IMPACTS ON THE INVERTEBRATE AND FISH FAUNA IN THE RHO ˆ NE RIVER, FRANCE ATLE HARBY, a * JEAN-MICHEL OLIVIER, b SYLVIE MERIGOUX b and EMMANUEL MALET b a SINTEF Energy Research, Water resources, 7465 Trondheim, Norway b Ecology and Fluvial Hydrosystems Laboratory, University Claude Bernard-Lyon 1, France ABSTRACT At the 8 km bypass section of Chautagne in the Rho ˆne River, most of the flow is diverted due to hydropower production. The environmental flow to the bypass section of the Rho ˆne River was increased from 10 m 3 s 1 (winter) and 20 m 3 s 1 (summer) to 50 m 3 s 1 (winter) and 70 m 3 s 1 (summer) in July 2004. A Norwegian mesohabitat method of classifying the river into physical mesoscale morphological (mesohabitat) classes by visual observation was applied at 10 and 70 m 3 s 1 . The results show that the dominating classes at both flows are deep and low velocity pools, but a higher physical diversity occurs at 70 m 3 s 1 , quantified by indices from Simpson (1949) and Shannon and Weaver (1962). In total, 6 per cent of the depth measurements and 0.3 per cent of the velocity measurements were outside the expected range at low flow. At high flow, 16.2 per cent of the depth measurements and 19.6 per cent of the velocity measurements were outside the expected range. The change in mesohabitats gives impacts on the composition and abundance of fish and invertebrates. Rheophilic taxa are favoured by the increased flow while more limnophilic species will find less amount of suitable habitat at 70 m 3 s 1 . Other studies show that population densities of grayling, brown trout, nase and dace have decreased greatly and become endangered since hydropower development, while densities of gudgeon, minnow and stone loach have increased. Fish habitat preferences from Lamouroux et al. (1999a) and Mallet et al. (2000) were used. Most of the endangered species need high velocity mesohabitats and both high and low depths during all life stages. The increase of such mesohabitat proportions at 70 m 3 s 1 indicates that their population should recover soon. The method may be a useful tool to compare different flow situations and their impact on the invertebrate and fish population structure even in large rivers. Copyright # 2007 John Wiley & Sons, Ltd. key words: mesohabitats; environmental flow; flow change; invertebrates; fish; population changes; large rivers Received 19 December 2005; Revised 18 October 2006; Accepted 21 December 2006 INTRODUCTION Until the early 19th century, the human pressure on the Rho ˆne, like the Rhine or the Danube, has progressively increased from canalization for navigation improvement to hydropower production (Bravard et al., 1992; Persat et al., 1995). Downstream from Lake Geneva, the Rho ˆne River was one of the first rivers used for hydropower development (years 1871, 1886, 1892). Presently, there are 22 dams along the river, and among them, 18 are low-fall power plants. After more than 50 years of intensive river regulation, a large restoration programme has recently been initiated. This 10-year programme has two main objectives: to increase the minimum flow in bypass sections, and to restore several side-arms with varying connections to the main channel. Currently, the programme will handle only eight sections of the upper and lower parts of the French Rho ˆ ne. One early task was to determine the ecological value of the minimum flow within economic constraints. Hydraulic and biological models have been used to answer this question, focusing on long-term data on fish community and habitat preferences. These studies aimed to predict changes in fish community composition related to changes in river width, depth and velocity associated with discharge (Lamouroux et al., 1999b). River managers have appealed to scientists (including ecologists, RIVER RESEARCH AND APPLICATIONS River. Res. Applic. 23: 525–543 (2007) Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/rra.997 *Correspondence to: Atle Harby, SINTEF Energy Research, Water resources, 7465 Trondheim, Norway. E-mail: atle.harby@sintef.no Copyright # 2007 John Wiley & Sons, Ltd.