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

Flow restoration in Alpine streams affected by hydropower operations—a case study for a compensation basin

Hydropeaking, resulting from rapid starting and shut-down of turbines, is one of the major hydrological alterations in Alpine streams. The upper Aare River basin in Switzerland comprises a complex high-head storage hydropower scheme. The significant turbine capacities of the two downstream powerhouses produce severe hydropeaking in the Aare River. To reduce the negative impact of the foreseen increase of the turbine discharge, a compensation basin combined with an extended tailrace tunnel downstream of the powerhouses has been designed and is under construction now to facilitate lower flow ramping increasing time for aquatic species to react. The design of the basin and its overall operation had to be defined to reach best ecological as well as economic performance. The retention volume and the operation rules of the basin have been optimized to avoid dewatering of juvenile brown trout. Further, flow ramping has to be limited in order to reduce drifting of macroinvertebrates. The paper presents a consistent approach of a target-oriented process management, including modelling, simulation and comparison of future flow regime without and with mitigation measure. Finally, rules for decision-making as well as the prototype’s final design are addressed

Structural complexity influences the ecosystem engineering effects of instream large wood

Large wood (LW) is an ecosystem engineer and keystone structure in river ecosystems, influencing a range of hydromorphological and ecological processes and contributing to habitat heterogeneity and ecosystem condition. LW is increasingly being used in catchment restoration, but restored LW jams have been observed to differ in physical structure to naturally occurring jams, with potential implications for restoration outcomes. This article examines the structural complexity and ecosystem engineering effects of LW jams at four sites with varying management intensity incorporating natural and restored wood. Our results reveal: (i) structural complexity and volume of jams was highest in the site with natural jams and low intensity riparian management, and lowest in the suburban site with simple restored jams; and (ii) that structural complexity influences the ecosystem engineering role of LW, with more complex jams generating the greatest effects on flow hydraulics (flow concentration, into bed flows) and sediment characteristics (D50, organic content, fine sediment retention) and the simplest flow deflector-style restored jams having the least pronounced effects. We present a conceptual model describing a continuum of increasing jam structural complexity and associated hydromorphological effects that can be used as a basis for positioning and evaluating other sites along the management intensity spectrum to help inform restoration design and best practice