This thesis develops and tests a classification of ‘near-natural’ European single-thread rivers, which are free to adjust to fluvial processes. The research involves subdividing rivers along a continuum of geomorphological characteristics to assign river reaches to geomorphologically-meaningful classes according to their channel dimensions and forms, and floodplain characteristics. The classification was developed and tested through three research components. First, a preliminary classification was developed using information entirely derived from a new information system containing remotely-sensed imagery and digital terrain data: Google Earth. This research stage required the development of rules for identifying, extracting and standardising information from this source for a large sample of river reaches. 221 single-thread river reaches distributed across 75 European rivers were investigated. Analysis of the derived information resulted in the development of a classification comprising six classes of European single thread river. Second, the robustness of the classification was explored including assessments of (i) the degree to which the classes were interpretable in relation to the geomorphic features they displayed; (ii) the degree to which sub-divisions of the six classes could be identified and justified; (iii) the accuracy of some specific types of information extracted from Google Earth; and (iv) the degree to which the six classes corresponded to expected gradients in two controlling variables: stream power and bed sediment calibre. Thirdly, bar theory was applied to a sample of rivers representative of the six classes. Since bars are an important contributor to river channel form and dynamics, the correspondence of the bars in the six river classes to their expected distribution as indicated by bar theory, provided further confirmation of the robustness of the classification. The outputs of the research are (i) a fully-tested classification of European single-thread rivers; and (ii) a demonstration of how Google Earth can provide valuable information for research in fluvial geomorphology. Some additional future research stages are proposed that could turn the classification into an operational tool in the context of river assessment and management.

River flow is a major driver of community dynamics in riverine-floodplain ecosystems. Flow-induced disturbance can have large impacts on taxonomic groups at higher trophic levels such as birds. However, our understanding of river flow-avian relationships is constrained by a failure to focus on multiple species’ responses to hydrological variables across large geographical areas. The aims of this thesis are to combine a national-scale dataset of river bird surveys with river flow archives to: (i) understand how hydrological disturbance affects the distribution of river birds, and (ii) evaluate the potential impacts of climate change-induced shifts in river flow on such species’ distributions. Species have complex, but biologically interpretable, associations with hydrological variables. Variation in river flow acts as an environmental filter that influences community assembly processes. Specialist river birds are most vulnerable to climate-induced shifts in river flow and their distributions may shift in response to future changes in river habitat suitability. The success of relating hydrological variables to the distributions of river birds demonstrates that variability in river flow has consequences for ecological structure at high trophic levels and that climate-induced shifts in river flow may represent a previously unidentified mechanism by which climate change mediates range shifts in birds.

This study was designed to test the hypothesis that riverbank erosion processes altered with increasing distance from a river source. The River Swale, northern England, was monitored at nine sites throughout its 109-km length, from December 1995 – March 1998. Erosion pins, bank-edge surveying, and Photo-Electronic Erosion Pins (PEEPs) were used to determine rates and timings of erosion. The rates were compared against a range of environmental variables based on temperature, river stage, and precipitation at 14-day intervals for erosion pins and 15-minute intervals for PEEPs. This allowed processes of erosion to be inferred. Catchment erosion rates were modelled using quadratic equations, simulating a mid-basin peak of 3.58 m a\(^{-1}\). Rates of erosion were low upstream, 0.07 m a\(^{-1}\), and also downstream, 0.12 m a\(^{-1}\). Subaerial processes, especially frost action, dominated upstream. Fluvial entrainment was most influential mid-catchment. Mass failures were most efficient downstream, but were more frequent mid-catchment. Piping, sapping and cantilever failures did not follow the same trends and were modelled separately. The length of the erosion season increased downstream as the number of active processes increased.

This thesis examines the influence of large woody debris (LWD) on the physical habitat of British headwater streams. The distribution and density of LWD accumulations throughout catchments is considered using data from the River Habitat Survey (RHS) and more detailed catchment scale surveys. The effect of accumulations of LWD on stream hydraulics and physical habitat at a reach scale is examined using transect based measurements of depth and velocity over a range of discharges before and after LWD removal for two reaches, measurement of reach average hydraulic parameters for 25 reaches with differing levels of LWD and the application of the Aggregated Dead Zone (ADZ) and Physical Habitat Simulation (PHABSIM) models. The RHS and catchment scale surveys showed that LWD accumulations exhibit systematic patterns in abundance within catchments, smaller streams having a higher density of LWD and a greater number of LWD accumulations. The most hydraulically active type of LWD accumulations were found to reduce flow velocity by an average of 55%, increase channel roughness by 149% and increase depth by 165%. This impact was, however, found to vary with discharge. Application of the Aggregated Dead Zone (ADZ) model showed that LWD increases the volume of ADZ, which may indicate greater ecological refuge potential. Using the Physical Habitat Simulation (PHABSIM) model, it was found that LWD improved habitat quality and overall habitat diversity.

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The speciation and particle-water interactions of Ni were investigated in three estuaries with contrasting pollution levels, hydrodynamics and geochemical characteristics. Samples of water and sediments were collected seasonally from the Tamar, Tweed and Mersey Estuaries. The chemical speciation of dissolved Ni was determined by adsorptive cathodic stripping voltammetry (ACSV), and the sorptive behaviour of Ni was studied under simulated estuarine conditions using " Ni coupled to liquid scintillation counting. A relatively high particle reactivity was observed only in the Tamar Estuary, where ACSV-non labile Ni was removed in the low salinity region, followed by addition of ACSV-labile Ni in the mid-estuarine region. The largely conservative behaviour of dissolved Ni in the Tweed Estuary was attributed to the combination of rapid flushing and low suspended particles concentration. In the Mersey Estuary, dissolved Ni (and Co, Zn, Cd and Pb) often showed positive deviations from conservative behaviour, which were attributed to anthropogenic and/or geochemical inputs. The fraction of ACSV-non labile Ni behaved differently in the three estuaries, with a sharp decrease from 90 to 30% from freshwater to seawater in the Tamar, scatter between 30 and 70% in the Tweed, and relatively constant values of 50-70% in the Mersey. These differences are interpreted within the context of different hydrodynamics and geochemistry of the three estuaries. Speciation modelling showed that between 30 and 70% of dissolved Ni in the Tweed, and between 50 and 70% in the Mersey, was bound to a class of strong ligands (log K'NIL = 19.0 ± 0.4 and 18.7 ± 0.5, respectively), which were saturated by the ambient concentrations of Ni throughout the estuaries, suggesting that the ligands are highly specific towards Ni. Irradiation of riverwater samples with UV light (believed to remove dissolved organic matter) significantly enhanced Ni uptake onto estuarine suspended particulate matter, with an increase in the 63Ni distribution coefficients (Kds) of up to 10 fold. Conversely, the addition of synthetic humic acids to UV-treated riverwater decreased significantly the 63Ni Kds. Complexation of Ni by dissolved natural ligands also affected the progression of sorptive reactions, with a system response time (i.e. the time required to achieve 63% of the sorptive equilibrium) between 1.6 and 8.9 hours. The results from this study suggest that the low particle reactivity generally exhibited by Ni during estuarine mixing is due to its ability to complex strongly with dissolved natural ligands (truly dissolved organic or colloidal organic/inorganic), and therefore resist adsorption onto particles.

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