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Lateral connectivity between rivers and terrestrial landscapes is critical for both river and landscape health. Due to widespread anthropogenic degradation of riverscapes, river management is aiming to connect rivers to floodplains, riparian zones, and wetlands, putting a spotlight on lateral connectivity. However, there is currently no consensus on how to conceptualize and study lateral connectivity in rivers across disciplines. Here, we review lateral connectivity between riverscapes and terrestrial landscapes. We focus on the natural sciences, considering hydrology, geomorphology, ecology and biogeochemistry, but also consider social connectivity and the management and restoration of lateral connectivity. We emphasize the importance of considering the bidirectional nature of lateral connectivity, operating both into and out of river channels and the balance between these directions. The resulting “lateral connectivity balance” provides a framework to understand natural spatial and temporal variability in connectivity. Anthropogenic impacts have swung the balance of lateral connectivity, enhancing the transport of materials into and through river networks while suppressing fluxes from rivers to adjacent landscapes. We conclude that further research at the interfaces between the aquatic and terrestrial components of riverscapes is critical to advance our conceptual understanding of river and catchment systems. We propose that such research should be framed within the paradigm of “rebalancing” lateral connectivity, explicitly recognizing the natural bidirectionality of laterally connecting processes, the significance of the hydrologic, geomorphic, and biologic functions they support, and the value to society of the ecosystem services and climate change resilience they provide.

River channels, riparian and floodplain forms and dynamics are all influenced strongly by biological processes. However, the influence of macroinvertebrates on entrainment and transport of river sediments remains poorly understood. We use an energy-based approach to explore the capacity of benthic animals to move surficial, gravel-bed particles in field and laboratory settings and use the results to assess the relative significance of biological and physical benthic processes. Our results showed that in 11 British gravel-bed rivers, the maximum energy content (i.e., calorific content) of macroinvertebrate communities generally matched the flow energy associated with median discharges and, at multiple sites, exceeded that of the 10-year return interval flood. A series of laboratory experiments used to estimate the minimum energy expended by signal crayfish (Pacifastacus leniusculus) when performing geomorphic work established that crayfish move gravel particles at energy levels below that expected of the flow, complicating direct comparisons of the capacity for macroinvertebrates and fluvial flows to influence bed mobility. Our findings suggest that the influence of macroinvertebrate communities in either promoting or suppressing, the mobilisation of the bed may be large compared to equivalent values of fluvial energy. Based on these findings, we conclude that in the gravel-bed rivers studied, the macroinvertebrate community's potential to perform geomorphic work matches or exceeds the stream power during most of the year. Although our study examined biological and fluvial energy systems separately, it is important to recognise that in nature, these systems are highly interactive. It follows that utilising the energy framework presented in this paper could lead to rapid advances in both fluvial biogeomorphology and river management and restoration.

Boulders are globally widespread and influence processes across many landscapes including hillslopes, coasts, rivers and extra-terrestrial settings. Boulders are described as particles, sufficiently large, that they have a disproportionate effect on the surrounding landscape. Moving beyond this conceptual definition, however, requires a somewhat arbitrary decision of how to define a minimum boulder size. The implications of boulder definition on study findings are rarely considered. We investigate the suitability of five boulder definitions, two based on fixed sizes: (1) 0.26 m and (2) 1 m, and three definitions which vary based on system characteristics: (3) grain mobility, (4) grain protrusion and (5) surface grain size distribution (> 84th percentile, D84). We consider the impact of definition on calculated boulder metrics, and, for the >1m and >D84 definitions, their association with channel and catchment characteristics across 20 boulder-bed streams in northern Sweden. We also surveyed river managers responsible for restoring these rivers, to gain a practitioner insight on boulder size definition. We found that boulder definition matters; for metrics relating to the number or density of boulders, the >D84 and >1m size definitions were negatively correlated. Surveys indicated the importance of communicating boulder definition. We conclude that, whilst the best choice of boulder size definition will vary based on the questions of interest and techniques employed, evaluating the implications of the chosen boulder size definition and communicating the reasoning behind boulder definition choice is crucial.

River management is founded on predictable self-organisation between river form and catchment controls in alluvial rivers. However, a substantial proportion of rivers are not fully alluvial. In previously glaciated landscapes, boulder-rich glacial till influences river channel form and process. Increasing interest in nature- and process-based river restoration requires knowledge of pre-disturbance natural processes, which does not exist for semi- and non-alluvial rivers in Fennoscandia. We aimed to determine the role of Pleistocene glaciation and subsequent deglaciation versus Holocene fluvial processes in controlling channel form of boulder-bed rivers in Fennoscandia. We quantified morphological characteristics of northern Swedish boulder-bed rivers, in which channel morphology was minimally impacted by humans, and used the degree of alluvial signatures to infer fluvial and legacy glacial controls. We conducted surveys of reach-scale channel geometry, boulder and wood distributions and catchment characteristics for 20 reference reaches (drainage area: 11–114 km2). Reaches ranged in slope from 1% to 8% and were extremely diverse in channel geometry. Rivers showed little self-organisation at the reach scale; no association exists between channel width and channel slope or bed sediment size. Boulders were rarely clustered into bedforms (e.g., step-pools) typical of boulder-bed mountain rivers. Drainage area was positively correlated with channel capacity but not channel width, slope or sediment size. Channel boulder density was best predicted by surveys of terrestrial boulders. Consequently, channel geometry, boulder size and the distribution of boulders were primarily controlled by legacy glacial conditioning rather than current fluvial processes, with some alluvial adjustment of smaller particles within the boulder template. Therefore, restoration of semi-alluvial rivers should take into account local sediment and geomorphic conditions rather than use management principles built for fully alluvial rivers.

The importance of two-way interactions between animals and the physical hydraulic and sedimentological environment are increasingly recognized (e.g., zoogeomorphology). Caddisflies (Trichoptera) are a group of aquatic insects known for their bioconstructions, particularly cases built from fine sediment and silk. Caddisfly cases differ in size, shape and density from the incorporated sediment, and case construction may therefore affect the mobility of these sediments in rivers. However, although communities of caddisfly often use substantial quantities of sediment in case construction, the effect of these bioconstructions on sediment transport in rivers is unknown. We use a flume experiment to compare the bed shear stress required to transport (a) empty caddisfly cases and (b) individual sediment particles, following disaggregation from the case. The cases of three species were considered; two that construct different styles of tubular case (Potamophlax latipennis and Sericostoma personatum) and one that builds a domed case (Agapetus fuscipes). P. latipennis and S. personatum cases were easier to entrain than the sediment grains incorporated into them, whilst A. fuscipes cases were not. Despite their low mass, A. fuscipes cases required the most shear stress to transport them because their domed shape impeded rolling. These findings are important to understand how differences in case design between species, reflect different adaptation strategies to the turbulent hydraulic river habitat. Furthermore, the results suggest that un-attached tubular caddisfly cases may be preferentially transported over other particles on the river bed and thus, where caddisfly occur in high abundance, they may increase fluvial entrainment of sand.

River gravel mobility is an important control on river behaviour, morphology, and ecosystem processes. Gravel stability is dependent on abiotic flow and sediment properties, alongside less widely acknowledged biotic processes. The quagga mussel (Dreissena bugensis), a highly invasive bivalve, frequently occurs at high population densities in rivers and lakes. Quagga mussels attach to benthic sediments using byssal threads, which might affect sediment stability and thereby broader river geomorphology. We aimed to (1) characterize controls of quagga mussel sediment attachment by conducting a field survey in an invaded river, (2) investigate resultant changes in the critical shear stress needed to entrain fluvial bed materials via an ex situ flume experiment, for measured average (135 m−2) and potential future (270 m−2) mussel densities, and (3) model how this may affect sediment transport rates. From field surveys, mean quagga mussel density was 122 m−2, attaching to an average of 591 g m−2 of mineral bed sediments. Across the survey reach, mussels attached to all grain sizes available, with attachment driven by grain availability, rather than active selection of particular grain sizes. In the ex situ flume experiment, densities of 135 mussels m−2 did not significantly increase the critical shear stress of fluvial bed materials, but a density of 270 mussels m−2 significantly increased critical shear stress by 40%. Estimates of the proportion of time these critical stresses are exceeded at the field site indicated high densities of quagga mussels may reduce the occurrence of a geomorphically active flood event from Q30 to Q2. These results present feasible invasion scenarios, as quagga mussels frequently reach benthic densities orders of magnitude greater than observed here. This study indicates that substantial alterations to bedload sediment transport may occur following quagga mussel invasion. Future geomorphic modelling should include biology to better understand rates and processes of landscape development.

Landscapes and ecosystems are the result of two-way interactions between hydro-geomorphic and biological processes. Many animals, particularly those that build structures or transport sediment, are important biogeomorphic agents. Glossosomatidae caddisfly larvae (Insecta, Trichoptera) are globally widespread and abundant inhabitants of gravel-bed rivers. Glossosomatidae build mobile cases from sand that they transport over the river bed. However, there is limited understanding on how Glossosomatidae bioconstructions may influence sand distribution in rivers or how their zoogeomorphic behaviours are influenced by hydraulics or characteristics of the river bed. First, we conducted surveys to quantify the magnitude of sand incorporated into Glossosomatidae (Agapetus fuscipes) cases within a UK river. Second, we studied A. fuscipes movement behaviour and quantified the direction and magnitude of sediment reworking, in a flume, under differing flow velocity and gravel size treatments. We found that 99 % of A. fuscipes larvae transported sediment vertically upwards. This resulted in an average conveyance per larvae of 0.06 g sand upwards by 25 mm (maximum of 50 mm). In gravel beds with a coarse surface layer, this resulted in displacement of sand from sheltered interstices onto the surface of exposed gravel particles. In the flume, this behaviour was maintained even at high flows, sufficient to entrain empty cases from these locations. Whilst the mass of sediment moved by individual larvae is small, dense populations of Glossosomatidae larvae may have important consequences for the vertical distribution of sand in rivers. At our field site, A. fuscipes case density averaged 2192 cases m- 2, equivalent to 1.4 t km-1. This finding is important because in gravel-bed rivers frequented by Glossosomatidae larvae, sediment transport is typically limited by the availability of entrainable fine grain sediment at the surface. We discuss the implications of this sediment movement for river bed sedimentary structure, the transport of sand and gravel, and the possible role of Glossosomatidae larvae as ecosystem engineers. 

Invertebrates are important sediment engineers, making up for their small body size with abundance and behavioral diversity. However, despite the recognized importance of invertebrates as sediment engineers in terrestrial and marine environments, zoogeomorphology in rivers has primarily considered larger taxa, such as fish and beaver. This article reviews the zoogeomorphic effects of invertebrates in freshwater habitats, with a focus on rivers. To better synthesize current zoogeomorphic research and to help guide future studies we build a conceptual model considering biotic (behavior, abundance, body size, life history, and species invasions) and abiotic (geophysical energy and sediment grain size) controls on the direction and magnitude of zoogeomorphology. We also incorporate invertebrate engineers into conceptual sediment entrainment models, to understand their geomorphic role in the context of hydraulic power and sediment size. We structure our review around invertebrate behavior as a key control on whether invertebrates have a sediment destabilizing or stabilizing impact. Invertebrate zoogeomorphic behavior are diverse; the majority of research concerns bioturbation, a result of locomotion, foraging, and burrowing behaviors by many taxa. Similarly, burrowing into bedrock by a caddisfly and non-biting midge larvae promotes bioerosion. Attachment to the substrate, (e.g., silk nets by caddisfly larvae or byssal threads by some mussels) can stabilize sediment, providing bioprotection. Bioconstructions (e.g., caddisfly cases and mussel shells) may have either stabilizing or destabilizing effects depending on their density and abiotic context. Interactions between lotic invertebrates and fluvial processes are complex and understudied, requiring further research across a greater range of taxa, behaviors, and spatiotemporal scales.

Excess fine sediment, comprising particles <2 mm in diameter, is a major cause of ecological degradation in rivers. The erosion of fine sediment from terrestrial or aquatic sources, its delivery to the river, and its storage and transport in the fluvial environment are controlled by a complex interplay of physical, biological, and anthropogenic factors. While the physical controls exerted on fine sediment dynamics are relatively well‐documented, the role of biological processes and their interactions with hydraulic and physicochemical phenomena has been largely overlooked. The activities of biota, from primary producers to predators, exert strong controls on fine sediment deposition, infiltration, and resuspension. For example, extracellular polymeric substances associated with biofilms increase deposition and decrease resuspension. In lower energy rivers, aquatic macrophyte growth and senescence are intimately linked to sediment retention and loss, whereas riparian trees are dominant ecosystem engineers in high energy systems. Fish and invertebrates also have profound effects on fine sediment dynamics through activities that drive both particle deposition and erosion depending on species composition and abiotic conditions. The functional traits of species present will determine not only these biotic effects but also the responses of river ecosystems to excess fine sediment. We discuss which traits are involved and put them into context with spatial processes that occur throughout the river network. While strides towards better understanding of the impacts of excess fine sediment have been made, further progress to identify the most effective management approaches is urgently required through close communication between authorities and scientists.

Caddisfly (Trichoptera) larvae are an abundant and widespread aquatic insect group characterized by the construction of silk structures, including nets and cases. Case-building caddisfly have the potential to modify the sorting and mobility of sand and fine gravel via: (1) case construction, resulting in altered sediment properties; (2) transporting sediment incorporated into cases over the river bed; and (3) changing the structure of river beds via burrowing activity. To investigate these mechanisms, it is necessary to understand the mass, size distribution and spatial variability of sediment use by case-building caddisfly larvae. We quantified the mineral sediment used by individuals and communities of case-building caddisfly in 27 samples, from three sites on a gravel-bed stream. The mass and size distribution of sediment in individual cases varied between taxa (mass = 0.001-0.83 g, D-50 = 0.17-4 mm). The mean mass of sediment used by the caddisfly community was 38 g m(-2) and varied locally. Sediment use was predominantly coarse sand (D-50 = 1 mm). 64% of sediment use was attributable to Agapetus fuscipes (Glossosomatidae). Due to within-species variability in case mass, the abundance of most taxa, including A. fuscipes, was only weakly associated with the mass of sediment used by this species, at the river scale. Whilst the caddisfly community used a small percentage of the total sediment available (average 2.99% of the 1-1.4 mm size fraction), A. fuscipes used more fine sediment in their cases at sites where it was more available. Despite variability in local habitat, all sites supported diverse case-building caddisfly communities utilizing mineral sediment. Consequently, geomorphological effects of case-building caddisfly are potentially widespread. The results provide novel insights into the specific grain sizes and quantities of fine sediment used by caddisfly larvae, which represents an important step towards understanding their zoogeomorphic activities.