Limaria hians (Mollusca : Limacea): a neglected reef-forming keystone species

1. A key component of physical habitat along braided river systems is the exposed riverine sediment within the active zone. The relatively unmanaged, gravel-bed Fiume Tagliamento, Italy, provides the focus for exploring two ecologically important properties of exposed riverine sediments: their within-patch and between-patch variability in calibre. 2. To characterize between-patch variation in exposed riverine sediments, replicate (within-patch) samples were obtained from three geomorphologically distinct locations along 130 km of the river: bar heads along the margin of the low-flow channel, the heads of major bars across the exposed surface of the active zone, and floodplain surfaces. A photographic technique enabled rapid and consistent field sampling of the coarse sediments at bar heads along the low-flow channel margin and on major bars across the dry bed. 3. A downstream decrease in particle size and an increase in within-patch heterogeneity in sediment size were observed within bar head sediments along the margin of the low-flow channel. Comparisons between major bar and low-flow channel samples revealed greatest within-patch variability in individual sediment size indices (D50, A- and B-axes of the larger particles) at headwater sites, greatest between-patch variability in the three measured indices in the central reaches, and lowest between-patch variability at downstream sites. However, there was a distinct increase in the overall heterogeneity in particle size, which was sustained across all patches, in a downstream direction. 4. There was a clear downstream decrease in the size of floodplain sediments in the headwaters, but thereafter there was no distinct downstream trend in any of the calculated particle size indices. 5. The geomorphological controls on the observed patterns and the potential ecological significance of the patterns, particularly for plant establishment, are discussed in relation to the relative relief of the active zone, and the highly variable hydrological and climatic regime along the river

A hydrogeomorphological context for ecological research on alpine glacial rivers

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

Riparian vegetation and island formation along the gravel-bed Fiume Tagliamento, Italy

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

Wood storage within the active zone of a large European gravel-bed river

Wood storage within the active zone of the dynamic, gravel-bed, Fiume Tagliamento, Italy, was investigated at eight sites along the river's main stem. The quantity, nature, and mode of wood storage revealed a number of trends related to active zone morphology, cover type, and distance from the river's source. Relatively small quantities of wood were stored on open-gravel surfaces (estimates ranged from 1 to 21 t ha−1), intermediate quantities were associated with established islands (24–186 t ha−1), and large quantities were associated with pioneer islands (293–1664 t ha−1). Thus, variations in the geomorphological style of the river, which are associated with changes in these three cover types, are reflected in variations in the amount of wood that is stored in different reaches. In addition, although wood was found in many locations within the active zone, it was preferentially stored in three specific locations: (i) bar crests (the main open-gravel location for wood accumulations and pioneer islands); (ii) the margins and (iii) surfaces of established islands. The proportion of the stored wood that was living (sprouting) increased downstream and was higher on the open gravel than in association with established islands. There was a downstream gradient in the dominant type of wood accumulation. Individual logs predominated at the most upstream site. Thereafter, on the open gravel, whole shrubs and trees dominated the more confined sites in the headwaters and middle reaches, whereas, jams were the most frequent form of accumulation in the downstream reaches. Jams were the most frequent type of accumulation associated with established islands throughout the river. In contrast to small streams, where debris dams constitute the major type of wood accumulation, complex patterns and trends of wood storage were revealed along the Tagliamento. Although further studies are needed, it is clear that erosion of woody vegetation, its subsequent transport and deposition, play a major role in structuring the geomorphological and ecological character of this relatively natural, large European river-system. Insight into the mechanisms underlying the observed spatial patterns will contribute to a better understanding of the dynamic processes involved, and is essential for more effective management of river ecosystems