Longitudinal variations in exposed riverine sediments: a context for the ecology of the Fiume Tagliamento, Italy

1. A key component of physical habitat along braided river systems is the e xposed riverine sediment within the active zone. The relatively unmanaged, gravel-bed Fiume Tagliamento, Italy, provides the focus for exploring two e cologically important properties of exposed riverine sediments: their withi n-patch and between-patch variability in calibre. 2. TO characterize between-patch variation in exposed riverine sediments, r eplicate (within-patch) samples were obtained from three geomorphologically distinct locations along 130 km of the river: bar heads along the margin o f the low-flow channel, the heads of major bars across the exposed surface of the active zone, and floodplain surfaces. A photographic technique enabl ed 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 h eterogeneity in sediment size were observed within bar head sediments along the margin of the low-flow channel. Comparisons between major bar and low- now channel samples revealed greatest within-patch variability in individua l sediment size indices (D-50, A- and B-axes of the larger particles) at he adwater sites, greatest between-patch variability in the three measured ind ices in the central reaches, and lowest between-patch variability at downst ream sites. However, there was a distinct increase in the overall heterogen eity in particle size, which was sustained across all patches, in a downstr eam direction. 4. There was a clear downstream decrease in the size of floodplain sediment s 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 establishm ent, are discussed in relation to the relative relief of the active zone, a nd the highly variable hydrological and climatic regime along the river. Co pyright (C) 2000 John Wiley & Sons, Ltd.