After more than 300 years of river management, scientific knowledge of European river systems has evolved with limited empirical knowledge of truly natural systems. In particular, little is known of the mechanisms supporting the evolution and maintenance of islands and secondary channels. The dynamic, gravel-bed Fiume Tagliamento, Italy, provides an opportunity to acquire baseline data from a river where the level of direct engineering intervention along the main stem is remarkably small. Against a background of a strong alpine to mediterranean climatic and hydrological gradient, this paper explores relationships between topography, sediment and vegetation at eight sites along the active zone of the Tagliamento. A conceptual model of island development is proposed which integrates the interactions between large woody debris and vegetation, geomorphic features, sediment calibre and hydrological regime. Islands may develop on bare gravel sites or be dissected from the floodplain by channel avulsion. Depositional and erosional processes result in different island types and developmental stages. Differences in the apparent trajectories of island development are identified for each of the eight study sites along the river. The management implications of the model and associated observations of the role of riparian vegetation in island development are considered. In particular, the potential impacts of woody debris removal, riparian tree management, regulation of river flow and sediment regimes, and changes in riparian tree species' distribution are discussed.

Abstract:The stream hydrograph is an integration of spatial and temporal variations in water input, storage and transfer processes within a catchment. For glacier basins in particular, inferences concerning catchment-scale processes have been developed from the varying form and magnitude of the diurnal hydrograph in the proglacial river. To date, however, such classi®cations of proglacial diurnal hydrographs have developed in a relatively subjective manner. This paper develops an objective approach to the classi®cation of diurnal discharge hydrograph shape' and`magnitude' using a combination of principal components analysis and cluster analysis applied to proglacial discharge time-series and to diurnal bulk¯ow indices. The procedure is applied to discharge timeseries from two dierent glacier basins and four separate ablation seasons representing a gradient of increasing hydrological perturbation as a result of (i) variable water inputs generated by rainstorm activity and (ii) variable location and response of hydrological stores through a systematic decrease in catchment glacierized area. The potential of the technique for application in non-glacial hydrological contexts is discussed.

No abstract

ABSTRACT1. High climatic sensitivity and lack of significant human impact make alpine river basins important environments for examining hydrological and ecological response to global change.2. This paper is based upon previous and ongoing research within a glacierized, alpine river system (Taillon-Gabie´tous basin, French Pyrenees), which adopts an interdisciplinary approach to investigate the climate-hydrology-ecology cascade. Data are used to advance hypotheses concerning impacts of climate change/variability upon alpine river system hydrology and ecology.3. The snowpacks and glaciers that are the source of Pyrenean streams are climatically sensitive, with glaciers in retreat for most of the historical period. Given anticipated changes in summer airmass frequencies, the volume of meltwater may decrease, the nature and rate of glacier drainage may alter, and the timing of peak snow-and ice-melt may shift. However, rainfall-runoff and groundwater may increase their relative contributions to stream flow.4. The influence of changing water source contributions on physico-chemical habitat and, in turn, on benthic communities is assessed using an alternative alpine stream classification. This model predicts more rapid downstream change in benthic communities in the future as meltwater contributions decline and, at the basin-scale, biodiversity may be reduced owing to less spatiotemporal heterogeneity in water sources contributions and, thus, physico-chemical habitat. However, predictions are complicated by potential changes in biotic interactions with altered species' distributions.5. Integrated, long-term research into the climate-hydrology-ecology cascade in other alpine river basins is vital because interdisciplinary science is fundamental to predicting stream hydrology and ecology under scenarios of future climate/variability, to assessing the utility of alpine river

1. This paper presents a framework for the analysis of glacial stream ecosystems based upon the hierarchy of physical processes that operate over timescales varying from diel to millennial. Linked conceptual models propose interrelationships between climate, hydrology and fluvial geomorphology in alpine catchments. 2. These conceptual models are illustrated using data from the Taillon/Gabiétous catchment, French Pyrénées. Secondary sources provide information concerning the long‐term sequence of climatic and geomorphological controls on contemporary catchment processes. Detailed hydrogeomorphological field data collected over three consecutive summer melt seasons (1995–97) permit identification of marked changes in shorter‐term (diel, seasonal and inter‐annual) physical processes. 3. Clear differences in the response of water quality and quantity variables were observed between years as climatic conditions varied. In two of the three study years, a precipitation‐driven regime was imposed upon the typical ablation‐driven river discharge pattern in alpine streams. Clear changes in water quality and quantity were evident with increasing distance from the glacier: (i) discharge increased although specific discharge decreased markedly, (ii) the mean and variability in water temperature increased, and (iii) base concentrations of suspended solids decreased. 4. The physical processes incorporated in the conceptual models presented in this paper have ecological implications because they underpin a nested suite of disturbance regimes operating over timescales from diel to millennial.