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Methods for defining and retrieving reference conditions for large rivers were explored with emphasis on hydromorphologic and biologic quality indicators. For a set of four large rivers in the European Western Plains ecoregion, i.e., the rivers Meuse, Loire, Allier, and Dordogne, reference reaches were selected based on geomorphologic characteristics. A survey of riparian land use, vegetation, and bed geometry was done for the selected reaches. Responses of the riparian landscape to hydromorphologic conditions were determined with a set of existing and newly developed measures of riparian dynamics and forest development. Strong correlations were observed at the reach and local levels between the ratios of width to depth and embankment and the developed measures of riparian dynamics and forest. Boundary conditions for riparian forest development were determined for the hydromorphologic and biologic indicators of riparian dynamics and vegetation structure. These conditions also proved useful for determining the presence of sustainable populations of Populus nigra and Salix purpurea. From this agreement between abiotic and biotic boundary conditions, a set of useful reference conditions was determined, and a framework for the definition of reference and good status conditions subsequently evolved. Finally, a proposal for assessment and monitoring the proposed indicators is discussed for its applicability. [ABSTRACT FROM AUTHOR]

Integrated management of aquatic ecosystems is a common concern for water resources managers as well as for researchers. The 1992 French Water Act aims to preserve the biotic and ecological integrity of aquatic ecosystems as part of water management schemes, creating needs for new methodologies for studying these systems. This paper describesthe partitioning of the Loire basin (105 000 km2, France)into hydro-ecoregions tested at the mesohabitat scale. The followingnull hypotheses were examined in the four largest hydro-ecoregions of the basin: (1) differences in mesohabitat types distribution do notexist between regions; (2) the longitudinal structure of mesohabitattypes distribution is not different between regions; and (3) the factors governing distribution and longitudinal evolution of mesohabitats distribution are not different between regions. It was found that the four regions behaved in different ways in terms of distribution and longitudinal evolution of mesohabitats. Valley slope and stream order, the two tested control variables, do not play the same role in each region. If the region contains mainly alluvial rivers, slope and/or order do explain or predict mesohabitat distributions. If the region contains cohesive rivers, these factors do not, or poorly explain, mesohabitat distributions. Since predictive models cannot be developed in most regions for mesohabitat distributions, it is necessary to build descriptive models at the regional scale. In addition to one regional predictive model, this paper provides such results for three regions of the Loire basin. [ABSTRACT FROM AUTHOR]