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Sedimentation and clogging (colmation) of interstitial pore spaces with fine sediment particles is widely considered to be one of the most significant threats to lotic ecosystem functioning. This paper presents the results of a running water mesocosm study examining the effect of benthic and hyporheic fine sediment loading and particle size on the vertical movement and distribution of the freshwater amphipod Gammarus pulex.A gradient of fine sediment loading and different particle sizes were used to examine the ability of G. pulex from two body size classes to access and migrate vertically within subsurface sediments.We tested three hypotheses: (a) sediment loading would modify the distribution of G. pulex by limiting vertical movement; (b) the deposition of large particles and heterogenous sediments would limit the vertical movement of individuals more than homogeneous fine‐grained sediments; and (c) large bodied individuals would be prevented from migrating vertically with increasing sediment loading and particle size/heterogeneity.Sediment loading, particle size and heterogeneity of deposited sediment had a significant effect on the vertical movement of individuals, with heterogeneous sand (0.125–4 mm) acting as the strongest barrier to the vertical movement of individuals through the infilling and clogging of interstitial spaces followed by coarse (1–4 mm) and fine sand (0.125–4 mm).Fine sediment loading and particle size acted as a filter on body size and limited the ability of large bodied individuals to migrate vertically to a greater extent than small bodied individuals.This study demonstrates that the effects of fine sediment on habitat availability and faunal movement is dependent on both sedimentological characteristics and an individual's body size. The results illustrate the importance of both abiotic and biotic factors when evaluating the ecological effects of fine sediment deposition. [ABSTRACT FROM AUTHOR]

Freshwater mussels are among the world's most imperilled species, and much effort has been expended to understand their precipitous decline. The current paradigm is that relative river bed stability over decades is critical to maintaining mussel beds at a given river reach. Such information, however, is in stark contrast to the fundamental understanding of self‐formed rivers.Here, we examine the relationship between long‐term occurrence of aggregated freshwater mussel communities (mussel beds) and the dynamic rivers in which they live.We surveyed mussel assemblages in two streams with historical records to document their long‐term persistence, and we simulated bed mobility with a two‐dimensional flow and sediment transport model to estimate bedload transport during channel‐forming discharges.We found evidence of long‐term mussel bed persistence, in time and in kind, within these two streams and that significant bed mobility within mussel beds occurs every 1–2 years, as to be expected for self‐formed rivers with labile beds. Flow refugia, or bed immobility, thus cannot completely explain the persistence of these mussel beds.Our results suggest that mussels have adapted to proliferate in river channels that often experience significant bed mobility, but further studies are needed to understand the biophysical mechanisms enabling the long‐term persistence of mussel communities. [ABSTRACT FROM AUTHOR]

Dams fragment river systems worldwide, and Mediterranean-climate rivers, characterised by highly seasonal hydrographs and adapted biotas, are particularly impacted by flow regulation. Whereas the effects of flow regulation on hydrology, sediment transport and biodiversity have long been examined, responses at the food-web level remain understudied., Environmental variation is a key control of food-web structure. Thus, we predicted that flow regulation would impact food-chain length ( FCL) via changes in the flow variation regime, and we tested this prediction in a set of flow unregulated to completely regulated reaches in a Mediterranean river basin., In each reach, we characterised flow variation, together with two other putative controls of FCL (productivity and habitat size). We combined community data with carbon and nitrogen stable isotopes to estimate food-chain length, and Bayesian mixing models allowed estimates of dietary proportions of consumers., Flow variation was paramount in controlling FCL in the studied river network, and this same control largely explained the degree of omnivory among top predators. Thus, omnivory mechanisms were the main proximate structural mechanism allowing shifts in food-web structure and linking disturbance regimes to FCL., Our results suggest that flow regulation in Mediterranean rivers may impact food-web structure even when no significant changes in community composition are observed. If highly variable Mediterranean streams become increasingly affected by flow regulation, the resulting more stable conditions could enhance intraguild predation and thus lengthen riverine food chains. [ABSTRACT FROM AUTHOR]

1. Many plants disperse their seeds in waterbodies via hydrochoric transport. Despite a growing body of research into hydrochory, little is known about the fundamental seed traits that determine floatation ability or hydrochoric transport behaviour more generally. Seeds are transported in fluvial systems in one of three phases: surface transport, within the flow or incorporated in bedload. Seeds are often categorised as buoyant or non-buoyant based on density, with little consideration of the morphological traits that determine how long seeds spend in each transport phase. 2. We investigated the seed traits that best predict time spent floating under laboratory conditions, using sixty species of riparian plants from south-eastern Australia. We measured and calculated key physical attributes (length, width, volume, surface area, mass, density, sphericity, roundness and shape category) and categorised seeds according to their primary dispersal modes (water/wind, vertebrate, adhesion, ants and unassisted). 3. We used Cox's proportional hazards modelling to reveal that seed density and volume : surface area ratio were the physical traits that best predicted time spent floating. Volume : surface area ratio represents both seed size and shape, as it increases with size and sphericity. Coir results showed that denser, larger and/or more spherical seeds (i.e. higher volume : surface area ratio) were poor floaters. They are therefore more likely to be transported in subsurface flow or as bedload. 4. We combined our data with geomorphic models of sediment transport in flow to develop a biophysical process model of how hydrological forces and seed physical traits determine hydrochorous seed transport behaviour in rivers. The model describes how surface tension, buoyancy and flow velocity act on seed density and volume : surface area ratio to characterise time spent floating. We extrapolate from our data to conceptualise how these traits predict transitions between surface, subsurface and bedload transport. Hydrochoric seed transport behaviour and deposition are both threshold-driven and multidirectional. 5. Coir process model is likely to be applicable across a range of different seed types in a range of rivers. It substantially increases our understanding of hydrochoric processes in rivers, lakes and lotic wetlands and will help illuminate the links between seed traits, hydrochoric transport and patterns of riparian vegetation and species composition. [ABSTRACT FROM AUTHOR]

Climate change is altering temperatures and precipitation patterns all over the world. Melting glaciers increase surface run-off, thereby increasing the transport of suspended solids through streams. The increased load of suspended solids affects turbidity, which decreases the availability of photosynthetically active radiation for primary producers., We analysed how glacial loading of clays influences the light : nutrient ratio and photosynthetic parameters (measured with a pulse amplitude modulated fluorometer) and the carbon : phosphorus ( C : P) elemental ratio of periphytic primary producers. A field study was conducted in two canopy-free streams that receive water from the glaciers of Mount Tronador ( Patagonia, Argentina), one with high glacial load and the other with clear water. In addition, we conducted an in situ colonisation experiment with three different light treatments., We observed that periphytic biomass (chlorophyll a and carbon content) in the streams and in the experiment varied directly with turbidity. Moreover, photosynthetic parameters varied similarly because of an increase in the efficiency of electron transfer per open reactive centre in the more turbid stream and a chronic photoinhibition of photosystem II in the clearer stream., Periphytic C : P also varied with turbidity as we observed a decrease in C : P with an increase in light in both streams and in the experiment., Our main conclusion is that an increase in glacial melting with its associated increase in glacial clay load should protect primary producers against high irradiances (photosynthetic active radiation + ultraviolet radiation) in canopy-free streams. [ABSTRACT FROM AUTHOR]

1. Although macrophytes play a key role in the structure and functioning of lowland rivers, most of the basic plant, hydrodynamic and sediment-water interactions have only been described qualitatively. We therefore studied quantitatively, the seasonal dynamics of matter deposition and mobilisation inside and outside (free path) a representative patch of arrowhead, Sagittaria sagittifolia, in the lowland River Spree, NE Germany, in August 2006. Our in situ study combined resuspension experiments, a hydrodynamically calibrated erosion chamber and concurrent measurements of the prevailing flow characteristics and bed load. 2. Increasing entrainment rates ( E) of particles ( ESPM) and total P ( ETP), with increments of shear velocity ( U*) from 0.53 to 2.42 cm s−1, were significantly higher inside the plant patch than outside. Indeed, ESPM and ETP at the lowest U* were 8- and 12-fold higher inside than outside the patch, reflecting the resuspension potential of the upper nutrient-enriched layer and the extent of pulsed P inputs even at small increases in U*. 3. Vertical distribution of velocity ( u) revealed a flow pattern of a mixing layer inside the S. sagittifolia patch, and that of a boundary layer in the free path. The highest gradient of u in the mixing layer was located in the water column at about 0.5 m depth, whereas the highest gradient of u for the boundary layer was found near the riverbed. The maximum of U* (1.65 cm s−1) was only 4 mm above the sediment. 4. A plant mosaic provides a low-energetic environment promoting extensive particle trapping and the accumulation of a fine-grained, nutrient-enriched sediment, and forming a large resuspension potential. Consequently, after plant decay and the concomitant increase of U* this material is preferentially entrained at higher rates. Hence, the key role of submerged macrophytes in lowland rivers is more directly related to modifying the dynamic equilibria between vegetation trapping and resuspension, than to the retention of nutrients, particularly P, and the reduction of P loads downstream to other waters. [ABSTRACT FROM AUTHOR]

1. We propose a model of plant strategies in temperate fluvial hydrosystems that considers the hydraulic and geomorphic features that control plant recruitment, establishment and growth in river floodplains. 2. The model describes first how the disturbance gradient and the grain-size of the river bed load affect the relative proportion of erosion and deposition processes, and how the frequency of flood disturbance affects the intensity of such processes. 3. Secondly, the model predicts plant strategies according to direct and indirect effects of floods (disturbances through erosion versus deposition processes, and associated nutrient excess or limitation). 4. The relevance of the model as a prediction tool is discussed. Some proposals are made to validate the model, and traits are proposed that should be considered in future research for improving the predicting value of the model. [ABSTRACT FROM AUTHOR]

1. Restoration of shallow turbid lakes to promote growth of submersed aquatic vegetation (SAV) requires knowledge of the environmental factors affecting SAV growth and persistence, and a means to predict the success of SAV reestablishment under different management scenarios to improve these environmental conditions. We used a dynamic ecological modelling approach relating SAV responses to changes in physical and chemical conditions, with information on water level, flow and transparency being provided by hydrodynamic and sediment transport models. 2. The potential persistence of Vallisneria americana was similar under simulated environmental conditions in 1946 and in 1954, as was the potential persistence of Potamogeton pectinatus, indicating that the disappearance of V. americana from Peoria Lake (U.S.A.) previously attributed to an extended spring flood in 1954, may have been related to the combined effects of changes in water level, flow and water transparency as well as possibly other factors. 3. Recent environmental conditions (for 2000) proved not to be conducive for the colonization and persistence potential of V. americana, but would allow colonization by P. pectinatus. The construction of a hypothetical levee along the eastern descending line of the navigation channel in Upper Peoria Lake, which was expected to reduce fetch- and navigation-related turbidity, did not improve the situation for V. americana and overall deteriorated the situation for P. pectinatus. Thus, such a hydraulic alteration, generally considered as beneficial for SAV restoration, may not always be successful. 4. The results of the simulations indicated that the environmental conditions for potential persistence in Peoria Lake were generally less favourable for V. americana than for P. pectinatus. Measures suggested to restore SAV communities in such a lake should aim at reducing concentrations of total suspended solids at the point of inflow by a factor of three to four and limiting fetch- and navigation-related resuspension. [ABSTRACT FROM AUTHOR]

1. The term ‘catastrophic drift’ is used to describe the large-scale displacement of invertebrates that occurs during periods of increased river discharge. However, the physical processes that lead to animals entering the water column at such times remain poorly understood. Specifically, the hypothesis that the movement of bed sediments during floods triggers a large increase in drift has lacked a rigorous field test. 2. Using a portable flume, the hydraulic conditions and rates of bedload transport associated with small, frequent floods were created in situ within a reach of a gravel bed river. Experiments focussed on the patches of fine sediment which are the dominant source of bed material transported during small floods. The flume produced near bed velocities of up to 2 m s−1 over the patches, increasing shear stress, initiating sediment transport and causing invertebrates to enter the drift. 3. The total number of individuals lost from the bed, as well as the taxonomic composition of the drift, were influenced strongly by shear stress and bedload. The rate of loss from the bed was low at shear stresses <9 dynes cm−2 (0–4 individuals min−1 from the 0.5 m2 flume bed area). Once shear stress exceeded 9 dynes cm−2, the threshold that resulted in consistent bedload transport from the patches, the rate of loss of animals increased to a maximum of 56 individuals min−1. When bedload transport rates were at their highest, the taxonomic composition of the drift was more similar to the benthos than it was to the drift observed when bed material was stable. 4. Absolute rates of bedload transport created by the manipulations were extremely low (<7 g m−1 s−1) and typical of those measured during small, frequent floods. Events of this magnitude do not break up the armour layer across the reach as a whole and so exposed patches of fine sediment are the principal source of bedload material. Consequently, discharge events not considered as disturbances in geomorphic terms may initiate frequent episodes of so-called ‘catastrophic drift’ from patches of stream bed. [ABSTRACT FROM AUTHOR]

1. Large wood forms an important component of woodland river ecosystems. The relationship between large wood and the physical characteristics of river systems varies greatly with changes in the tree species of the marginal woodland, the climatic and hydrological regime, the fluvial geomorphological setting and the river and woodland management context. 2. Research on large wood and fluvial processes over the last 25 years has focussed on three main themes: the effects of wood on flow hydraulics; on the transfer of mineral and organic sediment; and on the geomorphology of river channels. 3. Analogies between wood and mineral sediment transfer processes (supply, mobility and river characteristics that affect retention) are found useful as a framework for synthesising current knowledge on large wood in rivers. 4. An important property of wood is its size when scaled to the size of the river channel. ′Small′ channels are defined as those whose width is less than the majority of wood pieces (e.g. width < median wood piece length). `Medium' channels have widths greater than the size of most wood pieces (e.g. width < upper quartile wood piece length), and `Large' channels are wider than the length of all of the wood pieces delivered to them. 5. A conceptual framework defined here for evaluating the storage and dynamics of wood in rivers ranks the relative importance of hydrological characteristics (flow regime, sediment transport regime), wood characteristics (piece size, buoyancy, morphological complexity) and geomorphological characteristics (channel width, geomorphological style) in `Small', `Medium' and `Large' rivers. 6. Wood pieces are large in comparison with river size in `small' rivers, therefore they tend to remain close to where they are delivered to the river and provide important structures in the stream, controlling rather than responding to the hydrological and sediment transfer characteristics of the river. 7. For `Medium' rivers, the... [ABSTRACT FROM AUTHOR]

1. Rivers are subject to thresholds of several types that define significant changes in processes and morphology and delimit distinctive riverine landscapes and habitats. Thresholds are set by the conditions that govern river channel process and form, amongst which the most important are the flow regime, the quantity and calibre of sediment delivered to the channel, and the topographic setting (which determines the gradient of the channel). These factors determine the sediment transport regime and the character of alluvial deposits along the channel. 2. Changes occur systematically along the drainage system as flow, gradient and sediment character change, so a characteristic sequence of morphological and habitat types – hence of riverine landscapes – can be described from uplands to distal channels. The sequence is closely associated with stream competence to move sediment and with bank stability. 3. The paper proposes a first order classification of river channel and landscape types based on these factors. The riverine landscape is affected seasonally by flow thresholds, and further seasonal thresholds in northern rivers are conditioned by the ice regime. 4. It is important to understand geomorphic thresholds in rivers not only for the way they determine morphology and habitat, but because human activity can precipitate threshold crossings which change these features significantly, through either planned or inadvertent actions. Hence, human actions frequently dictate the character of the riverine landscape. [ABSTRACT FROM AUTHOR]