Tributary connectivity, confluence aggradation and network biodiversity

In fluvial networks, some confluences are associated with tributary-driven aggradation where coarse sediment is stored, downstream sediment connectivity is interrupted and substantial hydraulic and morphological heterogeneity is generated. To the extent that biological diversity is supported by physical diversity, it has been proposed that the distribution and frequency of tributary-driven aggradation is important for the magnitude and spatial structure of river biodiversity. Relevant ideas are formulated within the Link Discontinuity Concept and the Network Dynamics Hypothesis, but many of the issues raised by these conceptual models have not been systematically evaluated. This paper first tests an automated method for predicting the likelihood of tributary-driven aggradation in three large drainage networks in the Rocky Mountain foothills, Canada. The method correctly identified approximately 75% of significant tributary confluences and 97% of insignificant confluences. The method is then used to evaluate two hypotheses of the Network Dynamics Hypothesis: that linear-shaped basins are more likely to show a longitudinal, downstream decline in tributary-driven aggradation; and that larger and more compact basins contain more confluences with a high probability of impact. The use of a predictive model that included a measure of tributary basin sediment delivery, rather than symmetry ratio alone, mediated the outcomes somewhat, but as anticipated, the number of significant confluences increased with basin size and basin shape was a strong control of the number and distribution of significant confluences. Doubling basin area led to a 1.9-fold increase in the number of significant confluences and compact basins contained approximately twice as many significant confluences per unit channel length as linear basins. In compact basins, significant confluences were more widely distributed, whereas in linear basins they were concentrated in proximal reaches. Interesting outstanding issues include the possibility of using spatially-distributed sediment routing models to predict tributary-driven confluence aggradation and the need to gather ecological data sufficient to properly test for increases in local and network-scale biodiversity associated with significant confluences and their network-scale controls

Chesil Beach grain-size report: a technical report on the impact of beach management works and evaluation of the Sedimetrics Digital Gravelometer software

Chesil Beach grain-size report: a technical report on the impact of beach management works and evaluation of the Sedimetrics Digital Gravelometer softwar

The effects of abiotic factors on plant health and biomechanics: a mesocosm study on potamogeton crispus

Interactions between flow and vegetation are widely investigated because vegetation is a primary factor controlling channel ecohydraulics, nearshore hydraulics and flood risk. Laboratory experiments are a critical tool in this research area and, to adequately represent the complexity of natural ecosystems, live plants, rather than artificial surrogates, are often used. In the present work, we expose a freshwater macrophyte (Potamogeton crispus) to a range of environmental conditions commonly found in ecohydraulic laboratories to investigate how these affect the level of plant health and associated variations in plant biomechanical properties. This is motivated by a need to understand how deterioration in live plants that are used in flume facilities affects their hydraulic performance and therefore the verisimilitude of the data they provide on flow interactions. Results show that short-medium term exposition to tap water or low irradiance levels is stressful for plants and can induce modifications in their biomechanics, with a potential effect on their hydrodynamic performance

Experimentation at the interface of fluvial geomorphology, stream ecology and hydraulic engineering and the development of an effective, interdisciplinary river science

One ‘2020 vision’ for fluvial geomorphology is that it sits alongside stream ecology and hydraulic engineering as a key element of an integrated, interdisciplinary river science. A challenge to this vision is that scientists from these three communities may approach problems from different perspectives with different questions and have different methodological outlooks. Refining interdisciplinary methodology is important in this context, but raises a number of issues for geomorphologists, ecologists and engineers alike. In particular, we believe that it is important that there is greater dialogue about the nature of mutually-valued questions and the adoption of mutually-acceptable methods. As a contribution to this dialogue we examine the benefits and challenges of using physical experimentation in flume laboratories to ask interdisciplinary questions. Working in this arena presents the same challenges that experimental geomorphologists and engineers are familiar with (scaling up results, technical difficulties, realism) and some new ones including recognizing the importance of biological processes, identifying hydraulically meaningful biological groups, accommodating the singular behaviour of individuals and species, understanding biological as well as physical stimuli, and the husbandry and welfare of live organisms. These issues are illustrated using two examples from flume experiments designed (1) to understand how the movement behaviours of aquatic insects through the near-bed flow field of gravelly river beds may allow them to survive flood events, and (2) how an understanding of the way in which fish behaviours and swimming capability are affected by flow conditions around artificial structures can lead to the design of effective fish passages. In each case, an interdisciplinary approach has been of substantial mutual benefit and led to greater insights than discipline-specific work would have produced. Looking forward to 2020, several key challenges for experimentalists working on the interface of fluvial geomorphology, stream ecology and hydraulic engineering are identified

Bed disturbance via foraging fish increases bedload transport during subsequent high flows and is controlled by fish size and species

Benthic foraging by fish can modify the nature and rates of fine sediment accrual and the structure and topography of coarse-grained fluvial substrates, with the potential to alter bed material characteristics, particle entrainment thresholds, and bedload transport fluxes. However, knowledge of what controls the nature, extent, and intensity of benthic foraging and the consequent influence of these controls on geomorphic impact remain rudimentary. An ex-situ experiment utilising Barbel Barbus barbus and Chub Leuciscus cephalus extended previous work by considering the role of fish size and species as controls of sediment disturbance by foraging and the implications for bed material characteristics and bedload transport. In a laboratory flume, changes in bed microtopography and structure were measured when a water-worked bed of 5.6-22.6. mm gravels was exposed to four size classes of Barbel (4-5″, 5-6″, 6-8″, 8-10″ in length) and a single size class of Chub (8-10″). In line with other studies that have investigated animal size as a control of zoogeomorphic agency, increasing the size of Barbel had a significant effect on measured disturbance and transport metrics. Specifically, the area of disturbed substrate, foraging depth, and the fish's impact on microtopographic roughness and imbrication all increased as a function of fish size. In a comparison of the foraging effects of like-sized Barbel and Chub, 8-10″ in length, Barbel foraged a larger area of the test bed and had a greater impact on microtopographic roughness and sediment structure. Relative to water-worked beds that were not foraged, bed conditioning by both species was associated with increased bedload transport during the subsequent application of high flows. However, the bedload flux after foraging by Barbel, which is a specialist benthivore, was 150% higher than that following foraging by Chub, which feed opportunistically from the bed, and the total transported mass of sediment was 98% greater. An interesting implication of these results, given the abundance and widespread distribution of foraging fish, is that numerous fish species belonging to a variety of functional groups may be acting as zoogeomorphic agents in rivers, directly affecting bed material conditions and sediment transport fluxes in proportion to their body size and feeding traits

Reduced bed material stability and increased bedload transport caused by foraging fish: a flume study with juvenile Barbel (Barbus barbus)

The plants and animals that inhabit river channels may act as zoogeomorphic agents affecting the nature and rates of sediment recruitment, transport and deposition. The impact of benthic-feeding fish, which disturb bed material sediments during their search for food, has received very little attention, even though benthic feeding species are widespread in rivers and may collectively expend significant amounts of energy foraging across the bed. An ex situ experiment was conducted to investigate the impact of a benthic feeding fish (Barbel Barbus barbus) on particle displacements, bed sediment structures, gravel entrainment and transport fluxes. In a laboratory flume changes in bed surface topography were measured and grain displacements examined when an imbricated, water-worked bed of 5.6 to 16 mm gravels was exposed to feeding juvenile Barbel (on average, 0.195 m in length). Grain entrainment rates and bedload fluxes were measured under a moderate transport regime for substrates that had been exposed to feeding fish and control substrates which had not. On average, approximately 37% of the substrate, by area, was modified by foraging fish during a four-hour treatment period, resulting in increased microtopographic roughness and reduced particle imbrication. Structural changes by fish corresponded with an average increase in bedload flux of 60% under entrainment flows, whilst on average the total number of grains transported during the entrainment phase was 82% higher from substrates that had been disturbed by Barbel. Together, these results indicate that by increasing surface microtopography and undoing the naturally stable structures produced by water working, foraging can increase the mobility of gravel-bed materials. An interesting implication of this result is that by increasing the quantity of available, transportable sediment and lowering entrainment thresholds, benthic feeding might affect bedload fluxes in gravel-bed rivers. The evidence presented here is sufficient to suggest that further investigation of this possibility is warrante

Foraging fish as zoogeomorphic agents: An assessment of fish impacts at patch, barform, and reach scales

Flume studies have demonstrated that foraging by fish can modify the structure and topography of gravel substrates, thereby increasing particle entrainment probabilities and the amount of sediment mobilized during subsequent experimental high flows. However, the zoogeomorphic impact of benthic foraging has not previously been investigated in the field. This paper reports field experiments that examined the nature and extent of disturbance of riverbed gravels by foraging fish, predominately Cyprinids, at patch, riffle, and reach scales and complementary ex situ experiments of the impacts on bed stability. At patch scale, benthic feeding fish displaced particle sizes ≤90 mm in diameter, increased bed surface microtopography and grain protrusion, and loosened surface structures. Although enhanced mobility was expected from these structural changes, foraging also caused localized coarsening of sediments, and the ex situ experiments recorded significantly reduced grain entrainment, bedload flux, and total transported mass from foraged patches. Foraging disturbed bed materials at all 12 riffles in the study reach and, on average, disturbed 26.1% of riffle area per 24 h feeding period. These findings demonstrate for the first time that foraging fish, which are widespread and feed perennially, can act as zoogeomorphic agents in rivers, affecting grain-size distributions and bed material structure, with potential implications for bed stability and bedload transport at reach and river scales. Whether fish increase or reduce bed mobility is probably dependent on a host of factors, including the net effects of both structural disturbance and biogenic particle sorting, as these affect entrainment stresses under subsequent competent flows

Temporal variability in lotic macroinvertebrate communities associated with invasive signal crayfish (Pacifastacus leniusculus) activity levels and substrate character

Invasive signal crayfish (Pacifastacus leniusculus) are considered to be the most prevalent non-native crayfish species in Europe. Where large populations become established they have significant and long-term effects on benthic macroinvertebrate communities. However, much less is known about how community effects associated with crayfish invasion change in the short-term as a function of varying activity levels during the summer months. We examined the macroinvertebrate community composition of two lowland UK rivers, one which supported a well-established non-native crayfish population (invaded) and one in which crayfish had not been recorded (control). Colonisation cylinders were deployed which recorded community composition over a 126-day time period. Results indicate that once the activity period commences, invasive crayfish consistently altered macroinvertebrate community structure regardless of substrate character. Invaded communities displayed reduced beta-diversity compared to control sites. However, effects on the macroinvertebrate assemblage varied over the period when crayfish were active probably reflecting the behavioural activity of crayfish (which intensifies with increasing water temperature and during the spawning season) and life histories of other macroinvertebrates. The results indicate that crayfish invasions modify macroinvertebrate community composition, but over shorter timescales, the effects vary associated with their activity levels

Discharge and suspended sediment time series as controls on fine sediment ingress into gravel river beds

Fine sediment availability and channel hydraulics are two of the primary controls on the ingress of fine sediment into gravel river beds. A novel dataset consisting of fine sediment ingress measurements coupled with high-resolution turbidity and discharge time series, was analysed to investigate relations between ingress, discharge and turbidity. Discharge and turbidity demonstrated a weak association with each other, and their relations with fine sediment ingress were relatively weak. An alternative, but widely applied ‘redundancy’ approach was investigated that focused on key metrics, or facets, of the discharge and turbidity time series and their association with fine sediment ingress. Principal component analysis was used to distil the most important facets driving variation in the discharge and turbidity datasets and these were then used as independent variables in regression models with sediment ingress as the dependent variable. These models accounted for a larger amount of the statistical variation in sediment ingress over time than discharge and turbidity time series. Facets of the turbidity time series were found to be the most effective explanatory variables. The results suggest that this approach could be valuable and justify its application and testing across a range of river types in different hydrological and sedimentary settings. Application of this method could improve our generic understanding of what controls ingress at larger spatial and temporal scales and therefore complements process-based approaches, which is vital for the development of fine sediment management strategies

Temporal effects of enhanced fine sediment loading on macroinvertebrate community structure and functional traits

Deposition of fine sediment that fills interstitial spaces in streambed substrates is widely acknowledged to have significant negative effects on macroinvertebrate communities, but the temporal consistency of clogging effects is less well known. In this study the effects of experimentally enhanced fine sediment content on aquatic invertebrates were examined over 126 days in two lowland UK streams. Taxonomic approaches indicated significant differences in macroinvertebrate community structure associated with sediment treatment (clean or sedimented substrates), although the effects were variable on some occasions. The degree of separation between clean and sedimented communities was strong within 7 of the 9 sampling periods with significant differences in community composition being evident. EPT taxa and taxon characterised as sensitive to fine sediment demonstrated strong responses to enhanced fine sediment loading. In marked contrast, faunal traits did not facilitate the detection of enhanced fine sediment loading. More widely, the study highlights the temporal dynamics of sedimentation effects upon macroinvertebrate communities and the need to consider faunal life histories when examining the effects of fine sediment loading pressures on lotic ecosystems