SUMMARY1. This review is presented as a broad synthesis of riverine landscape diversity, beginning with an account of the variety of landscape elements contained within river corridors. Landscape dynamics within river corridors are then examined in the context of landscape evolution, ecological succession and turnover rates of landscape elements. This is followed by an overview of the role of connectivity and ends with a riverine landscape perspective of biodiversity. 2. River corridors in the natural state are characterised by a diverse array of landscape elements, including surface waters (a gradient of lotic and lentic waterbodies), the fluvial stygoscape (alluvial aquifers), riparian systems (alluvial forests, marshes, meadows) and geomorphic features (bars and islands, ridges and swales, levees and terraces, fans and deltas, fringing floodplains, wood debris deposits and channel networks). 3. Fluvial action (erosion, transport, deposition) is the predominant agent of landscape evolution and also constitutes the natural disturbance regime primarily responsible for sustaining a high level of landscape diversity in river corridors. Although individual landscape features may exhibit high turnover, largely as a function of the interactions between fluvial dynamics and successional phenomena, their relative abundance in the river corridor tends to remain constant over ecological time. 4. Hydrological connectivity, the exchange of matter, energy and biota via the aqueous medium, plays a major though poorly understood role in sustaining riverine landscape diversity. Rigorous investigations of connectivity in diverse river systems should provide considerable insight into landscape-level functional processes. 5. The species pool in riverine landscapes is derived from terrestrial and aquatic communities inhabiting diverse lotic, lentic, riparian and groundwater habitats arrayed across spatio-temporal gradients. Natural disturbance regimes are responsible for both expanding the resource gradient in riverine landscapes as well as for constraining competitive exclusion. 6. Riverine landscapes provide an ideal setting for investigating how complex interactions between disturbance and productivity structure species diversity patterns.

The inputs and breakdown of terrestrial leaf litter in streams is a fundamental ecological process that sustains in-stream foodwebs and secondary production. In temporary rivers, litter breakdown is reduced during dry phases, but the long-term effect of alternating drying and wetting cycles on litter breakdown is still poorly understood. We tested the hypothesis that leaf litter breakdown (LLB) in temporary rivers is primarily controlled by flow permanence (the number of flowing days over a given period expressed in %), and that drying events affect LLB during leaf fall periods through reduction of microbial activity and the modification of aquatic invertebrate assemblages. LLB rates (k), microbial activity and invertebrate assemblages were determined in winter at ten crosssections scattered along a flow permanence gradient on the temporary Albarine River, France. Results demonstrated that summer drying events affected the breakdown process for up to 6 months after flow has resumed in the river. LLB rates decreased exponentially with decreasing flow permanence, and with increasing drying event duration and frequency. These exponential relationships were observed for flow permanence variables calculated for the river for both 24-years and 1-year time periods prior to the experiment. A decrease in flow permanence from 100 to 85% led to a four-fold decrease in leaf litter breakdown rate. Microbial activity, which typically did not differ between cross-sections, failed to explain the betweencross-section differences in k. By contrast, invertebrate assemblages and, shredders, in particular, decreased exponentially with decreasing flow permanence and with increasing drying event duration and frequency.

Braided gravel-bed rivers are widespread in temperate piedmont and mountain-valley areas. In their pristine state, braided rivers are characterized by a shifting mosaic of channels, ponds, bars, and islands, since both flow and flood pulses create a diversity of habitats with fast turnover rates. Large wood has a major role in determining the geomorphology and ecological functioning of these rivers. Braided river habitats are colonized by a diverse fauna and flora adapted to their dynamic nature, including a significant proportion of highly endangered species. Animals exhibit high mobility, short and asynchronic life cycles, and ethological and phenological plasticity. Braided gravelbed rivers also offer various categories of refugia such as shore areas, hypogeic and hyporheic habitats that are pivotal for maintaining diversity in the face of frequent disturbances. Today, however, most gravel-bed rivers bear little resemblance to their highly dynamic natural state due to anthropogenic modifications, and most braided rivers have been converted into incised single-thread channels. Gravel bars and vegetated islands are among the most endangered landscape elements worldwide. They are very sensitive to channelization, gravel extraction, and flow regulation. Therefore, more than for most other ecosystems, restoring braided rivers and their landscape elements means restoring their underlying hydrogeomorphological dynamics.

-Fifty years after the hyporheic zone was first defined (Orghidan, 1959), there are still gaps in the knowledge regarding the role of biodiversity in hyporheic processes. First, some methodological questions remained unanswered regarding the interactions between biodiversity and physical processes, both for the study of habitat characteristics and interactions at different scales. Furthermore, many questions remain to be addressed to help inform our understanding of invertebrate community dynamics, especially regarding the trophic niches of organisms, the functional groups present within sediment, and their temporal changes. Understanding microbial community dynamics would require investigations about their relationship with the physical characteristics of the sediment, their diversity, their relationship with metabolic pathways, their interactions with invertebrates, and their response to environmental stress. Another fundamental research question is that of the importance of the hyporheic zone in the global metabolism of the river, which must be explored in relation to organic matter recycling, the effects of disturbances, and the degradation of contaminants. Finally, the application of this knowledge requires the development of methods for the estimation of hydrological exchanges, especially for the management of sediment clogging, the optimization of self-purification, and the integration of climate change in environmental policies. The development of descriptors of hyporheic *Corresponding author: pierre.marmonier@univ-lyon1.frArticle published by EDP Sciences Ann. Limnol. -Int. J. Lim. 48 (2012) [253][254][255][256][257][258][259][260][261][262][263][264][265][266] Available online at: Ó EDP Sciences, 2012 www.limnology-journal.org DOI: 10.1051/limn/2012009 zone health and of new metrology is also crucial to include specific targets in water policies for the long-term management of the system and a clear evaluation of restoration strategies.