Question: What is the relative importance of the initial seed bank and subsequent seed dispersal for floristic composition of bank vegetation two years after creation of a newly‐cut reach of a river channel? Location: River Cole, West Midlands, United Kingdom. Methods: We took bank and bed sediment samples from a 0.5‐km reach of a new river channel cut into intact flood‐plain. After river diversion, seed samples deposited on artificial turf mats placed on the river banks and flood‐plain edge were taken in summer and winter 2002 and 2003. Seed rain samples from funnel traps were taken during summer 2002 and 2003. We undertook greenhouse germination trials to assess viable seed species within these samples. In summer 2004, we surveyed river bank vegetation. Agglomerative cluster analysis was used to investigate floristic similarity between seed bank, seed rain, seed deposition samples and final bank vegetation cover. DCA was used to explore contrasts between the samples and to assess whether these reflected interpretable environmental gradients. Results: Seed rain samples contained a small subset of species in the summer depositional samples. 38 species were found within the final vegetation, the seed bank, and at least one of the four sets of depositional samples; a further 30 species not present in the seed‐bank samples were present in at least one of the four sets of depositional samples and the final vegetation. Floristic composition of the vegetation was most similar to the depositional samples from winter 2002 and 2003 and summer 2003. DCA axis 1 reflected a time sequence from seed‐bank samples through depositional samples to the final vegetation. Conclusions: Newly cut river banks were colonized rapidly. Seed remobilization and hydrochorous transport from the upstream catchment are important for colonization. Species richness was highest in samples deposited during winter when high river flows can remobilize and transport viable seeds from upstream. This process would also have enhanced the species richness of seed production along the banks during the second summer (2003).
A conceptual model of the morphological development of the riparian margins of newly cut river channels is presented, suggesting early feedbacks between vegetation growth and bank form. To test the model, observations of long and cross profiles, bank sediment and seed deposition, and bank vegetation development were collected over the first 2 years of river flows through a reach of the River Cole, West Midlands, UK. The newly created channel had a sinuous planform and varying asymmetric trapezoidal cross section in sympathy with the planform. No imposed bedforms or bank reseeding were included in the design. Over the 2 years, development of bedforms was rapid, with bed sediment sorting and bank profile adjustment occurring more steadily and progressively. Six classes of bank profile were identified by the end of the study period, illustrating close associations with sediment aggradation, vegetation colonization, and growth patterns. Vegetation colonization of the banks was seeded predominantly from local sources during the summer and from hydrochory (transport by the river) during the winter. Colonizing vegetation on the riverbanks appeared to act as a significant propagule source by the second summer and as an increasingly important roughness element, trapping both propagules and sediment, within the second year and providing early feedback into bank evolution. As a result, the time required for riparian margin development in the conceptual model was found to be considerably longer than observed in the study river. In addition, the role of surface wash/bank failure in modifying the bank profile and transporting seeds onto the upper bank face during the first year of bank development was found to be important in initiating rapid bank vegetation colonization and surface stabilization. This set of processes had not been incorporated in the initial conceptual model. In relation to channel restoration, this research illustrates that in small temperate rivers of modest energy the provision of an initial, sinuous corridor is sufficient to induce rapid development of fluvial features and vegetation cover without the need to construct bed forms or to seed the banks.
Few habitat survey, classification or assessment methodologies have been developed specifically for urban or heavily engineered rivers, and yet these rivers need careful assessment if management options are to be considered and prioritised in an economically and environmentally effective way. This paper presents a refinement of a previously proposed urban river survey (URS), which is a modification of the Environment Agency's River Habitat Survey. It then describes indices and classifications that have been developed from URS data and that provide a link between the type of engineering applied to a stretch of urban river and its habitat characteristics. Finally, some simple decision trees and scenario‐modelling tools are presented that allow (i) newly surveyed urban stretches to be classified and (ii) some simple scenarios of channel management change to be assessed in order to prioritise management options for stretches of urban river across urban catchments.
Urban rivers are often engineered to increase flood conveyance and stabilize channel size and position. This paper analyses habitat surveys of 180 urban river stretches of differing engineering type from four river basins (river Tame, West Midlands, UK; tributaries of the lower river Thames, UK; river Botic, Prague, Czech Republic; river Emscher, North-Rhine Westphalia, Germany). Kruskal-Wallis tests identify significant differences in extent and/or frequency of flow types, bank and bed physical habitats, and vegetation characteristics associated with different styles of engineering. Principal Components Analysis identifies four key environmental gradients in the data set: sediment supply and retention; extent and diversity of in-channel vegetation and riparian trees; bed and bank sediment calibre; flow type energy and complexity. These gradients discriminate stretches of differing planform, cross section and reinforcement and are significantly correlated with indices of degree and type of bank and bed reinforcement, pollution and presence of alien nuisance plant species. The analytical results illustrate statistically significant associations between different styles and levels of engineering intervention and the number and nature of physical habitats present in urban rivers. The results provide a basis for filtering sites for potential remedial measures prior to site-specific surveys and modelling, for comparing sites and for tracking trajectories of change at sites that are subject to changes in channel engineering. They provide evidence that river condition and degree of engineering are not inversely related in a simple linear way, and that engineering of urban river channels, in the form of mixed, patchy reinforcement can contribute a great deal to habitat diversity where other controls on flow heterogeneity are more difficult to manipulate.
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