No description available.

No description available.

No description available.

Methane is oversaturated relative to the atmosphere in many rivers, yet its cycling and fate is poorly understood. While photosynthesis is the dominant source of autotrophic carbon to rivers, chemosynthesis and particularly methane oxidation could provide alternative sources of primary production where the riverbed is shaded or at depth beneath the sediment surface. I highlight geographically widespread methanotrophic carbon fixation within the gravel riverbeds of over 30 chalk rivers and in 15 of these, the potential for methane oxidation (methanotrophy) was also compared to photosynthesis and stable isotope analyses were used to trace methane into the wider food web. Detailed concurrent measurements of photosynthesis and methanotrophy in one large chalk river over a complete annual cycle, showed methanotrophy to be active to at least 15cm into the riverbed and to be strongly substrate limited. The seasonal trend in methanotrophic production reflected that of the riverine methane concentrations, and thus, the highest contribution to autotrophic production was in mid-summer. At the sediment surface, photosynthesis was limited by light for most of the year with heavy shading induced by dense beds of aquatic macrophytes and riparian vegetation. Across 15 rivers in midsummer, methane derived carbon was estimated to contribute 18% of production (methanotrophic plus photosynthetic) in well illuminated riverbeds and 51% in the shaded areas (median values). With warming conditions and associated increasing methanogenesis in fine sediments, methanotrophy is predicted to prevent increased methane emissions from rivers due to the strong kinetic response of methane oxidation. The gross carbon fixation efficiency of methane oxidation was calculated as 50% and was conserved across eight rivers with varying methane oxidation capacities and ambient methane concentrations. Methanotrophic production is widespread, efficient and most important when ambient methane concentration is high and light availability is low.

River restoration has developed over the last three decades in the context of a more holistic approach to river management. One of the most important issues facing river managers is the design of river channel dimensions. The successful design of cross-sectional dimensions requires an understanding of river channel stability and the sensitivity of rivers to change. A need for more work in this field was identified. This research investigates the variability of downstream channel geometry discharge as a basis for assessing river channel stability and sensitivity to change. A national database containing 124 semi-natural UK river sites is used to develop a linear regression model relating bankfull width with dominant discharge. The magnitude and direction of standardised residual values from the model are then investigated in terms of their geomorphological significance. Particular groups of residual values are found to be related to specific controlling factors. Extreme high magnitude positive residuals (>1.0) are dominated by baseflow dominated chalk rivers. Negative residuals (>-0.5 to -1.0) are found to have a bedrock control. Other controlling factors operating at a local scale, including bank materials, bed materials and vegetation cannot be identified as having an exclusive influence on residual values. The variability in channel geometry - discharge relationships is broadly indicative of river channel adjustment based results from the field study of a subset of 50 sites. Residuals closest to the regression line demonstrate active or inactive stability and negative and positive residuals show a tendency towards erosional and depositional processes respectively. A study of temporal changes in a subset of 16 rivers supports these findings whilst highlighting the importance of the mutual adjustment of channel parameters through time. To investigate river channel adjustment at a reach downstream within a single river, a more detailed catchment study forms the second part of the research. Three contrasting catchments were used to investigate the implications of changing downstream channel geometry and stability on river channel adjustment at a reach. The results reinforce the importance of assessing the mutual adjustment of width, depth, and gradient to identify the dominant form of adjustment at a reach. Drainage basin form was found to exert an important control on channel geometry adjustment both laterally and longitudinally downstream. The results support the findings from the national model which show that stability is a function of complex combination of controlling factors which are represented by the residual values. River channel stability can be viewed as a function of the flow regime related to local environmental factors. The variability of channel geometry - discharge relationships does appear to represent the direction of river channel adjustment, but stability at a reach must be evaluated in the context of the adjustment of downstream parameters related to both catchment and local scale controls.

Braided rivers exhibit extremely complex and dynamic morphologies as their multiple channels are constantly re-worked. The research reported in this thesis explored a number of properties of braided river form and dynamics and some controlling factors through three individual but complementary research elements. The first research element was concerned with some of the controls on the transition between single thread and multi-thread channel patterns. Twenty-seven different flume experiments were conducted, supported by fourteen replicates. In these experiments, channel confinement (maximum possible channel width) and formative discharge were varied in a 25 x 2.9 m flume of constant slope (1%) and bed material (D50 = 1mm) with sediment supply constrained to match sediment output. As the maximum potential channel width increased, the channel pattern changed from a single channel with alternate bars, to the formation of mid-channel bars, and finally to a multi-thread braided pattern. Bed elevation frequency distributions showed distinct changes in their median, standard deviation, skewness and kurtosis as channel width and discharge increased, indicating the consequences of confining braided channels and regulating discharge on their bed elevation and morphology. The second and third parts of the research use remotely sensed data sets to explored (i) the degree to which a real river shows similar characteristics to those generated in the flume experiments and (ii) the variety in braiding patterns that are found in association with different boundary conditions of slope, width, discharge, and riparian vegetation. For the second research element, a Lidar survey of a 36 km reach of the lower Tagliamento river, Italy, was investigated. Within this reach, the river shows only small variations in slope and bed material size and is subject to the same flood flows. Analysis focused on thirty-six 1 km sub-reaches and demonstrated clear associations among the median, standard deviation, kurtosis and skewness of the bed and also clear downstream trends. Measures of vegetation cover showed statistically-significant associations with the median, standard deviation, kurtosis and skewness of the bed, particularly when only the 32 truly braided reaches were analysed. The measures of vegetation cover also showed downstream trends that corresponded with the trends in bed morphology. Abstract 5 Overall, variations in bed morphology showed similar characteristics to those observed in the laboratory flume, but also they showed correspondence with riparian vegetation cover, indicating a topographic signature of vegetation on the bed morphology. The downstream trends appear to be associated with the changing vigour of the riparian vegetation and possibly variations in river baseflow characteristics associated with varying groundwater levels in the alluvial aquifer. The most mature patches of vegetation within the braid plain of the most downstream part of the 36 km reach appear to occur on remnants of braid plain isolated by river bed incision. The third and final research element considered the morphology of six European braided rivers of different slope, width, discharge and riparian vegetation type. Information extracted from Google Earth and other aerial imagery, and gauged river flow data supported an analysis of changes in braided river characteristics through time, and among the six European river sites. Four traditional planform indices were used to characterise the braiding pattern (Bi – braiding index, Ai and Ai2 –anastomosing indices; Si – main channel sinuosity) were combined with measures of stream power and its component variables (width, Q10, and slope). Robust data for bed material calibre was not available. Statistical analysis of the entire data set revealed a potential influence of riparian vegetation type on the relationship between unit stream power and braid channel width; and a trend of increasing Bi, Ai, Ai2, and Si with decreasing unit stream power. However, a larger and more complete data set is needed to confirm these general trends and to fully explore transitional rivers. This research has illustrated the morphological consequences of confining braided rivers and the dependence of the braiding pattern on stream power. It has also illustrated the role of vegetation in contributing to the morphological complexity of braided rivers and the potential role of riparian vegetation in constraining the relationship between stream power and braided river width.

This study investigates the bar morphology, sediment properties and amount of sediment yield in relation to channel dynamics of the Brahmaputra-Jamuna River over a decadal timescale (1987-1997) using digital satellite images and field observations. Two typical reaches were chosen for study, representing the upper widest reach (Bahadurabad Ghat Reach) and the lower narrowest reach (Jamuna Bridge Reach) of the Brahmaputra-Jamuna River. Erosion and accretion of channel banks appears to be the root of all the processes of braiding. Channel banks of both the study reaches are more severely affected by erosion than accretion and both banks are retreating each year. An increased amount of sediment load in excess of transport competence immediately downstream node of a flow convergence seems to initiate the process of development of a braid bar. The process of braiding and channel expansion appears to be interdependent which reveals 'chicken and egg' relationships between them. Bars are usually diamond or triangular-shaped in plan view and their long axes are oriented parallel to the channel. The bars of the Brahmaputra-Jamuna River are grouped into two types, island and attached according to their morphological characteristics, this classification provides increased functional capability with less ambiguity. Between these two types, island bars are prominent features relative to attached bars. Both forms of bars are characterised by three level successions of topographic features although they constantly change their position with few localities left to be permanently stable. Most of the bars are submerged during high flow and erosion tends to occur at the upstream end of a bar and deposition on its downstream, while during falling stage the upstream end and lateral margins of bars receive sediment deposits and the downstream faces occasional erosion. There are considerable mutual adjustments in bar erosion and deposition between the two forms of bars. During the decadal timescale both the study reaches are accreted by bar deposition and the Brahmaputra-Jamuna River is in a condition of active aggradation. Sediment size characteristics at both banks and bars are dominated by very fine sand to fine sand particles. Very little discernible variability of particle size parameters and mineralogical compositions between the banks and bars indicate channel bank material as the potential source of bar sediments. There is no evidence of downstream diminution of sediment particle size, indeed the study results reveal a slight trend of downstream coarsening. Estimation of reach-scale sediment balance using cross-sections and satellite images provides information of quality comparable to that of measured cross-sections or a sediment continuity approach, and demonstrates a preferred method for sediment balance estimations in the Brahmaputra-Jamuna River. These findings suggest that analysis of digital satellite images has an advantage over the traditional field-based studies while a very intensive field work program supplements ground truth information that fills in the drawbacks of satellite imagery. Combination of both methods and relevant computer analysis is useful as a means of mapping and quantifying spatial and temporal change of channel morphology, and as a means of measuring some of the variables which promote, sustain and control channel braiding over annual-decadal timescales.

The planktonic rotifers of Priest Pot were studied from January 1983 to September 1984. Routine weekly samples from January 1983 to March 1984 provided a uniquely detailed data set giving information on the vertical distribution of the rotifer community in relation to the environmental conditions. Seasonal abundance of the rotifers was also noted. Diel studies in 1983 investigated the vertical migration of the community and experimental work in 1984 produced new information on the causes of migration in planktonic rotifers. Sixteen species were found but six of these were very rare. The remaining ten were divided into perennial species (Keratella cochlearis, K. quadrata, Polyarthra vulgaris, Synchaeta kitina and S. pectinata), springautumn species (Brachionus calyciflorus, B. angularis and Filinia terminalis) and summer-autumn species (Anuraeopsis fissa and F. brachiata) according to their periods of occurrence and abundance. When the tarn was isothermal most of the rotifer species were homogeneously distributed in the water column. This was only altered when algal blooms occurred in spring and autumn, causing some species to aggregate at depths with high chlorophyll concentrations, and when the rotifers were hatching from resting eggs, producing large populations in one section of the water column before dispersal. When the tarn was stratified, from May to September, all the rotifers were confined to the epilimnion by the anoxic water of the bottom two metres. F. brachiata, B. calyciflorus, B. angularis and P. vulgaris were concentrated nearer the surface than the other species. K. cochlearis and K. quadrata were aggregated just above the oxycline and A. fissa often occurred in very low oxygen concentrations and always had maxima at the oxicanoxic boundary. These stratified distributions are linked to food availability and show clearly the oxygen and temperature requirements of the different species. Diel studies in 1983 showed that most of the rotifer species can perform vertical migrations. These were more often a reverse migration (up during the day and down at night) than the expected normal migration (down during the day and up at night) commonly found in studies of crustaceans. The movements were not only linked to the photoperiod but also showed some response to changing chlorophyll levels and to oxygen concentration in the water column. One species, A. fissa, did not show a response to either light or chlorophyll stimuli but its distribution was linked to movements of the oxic-anoxic boundary. Experimental work on the vertical migration of the rotifers produced further evidence that reverse migrations are more common in rotifers than the classical "normal" migration. Manipulation of the community using altered photoperiods showed light to be a controlling stimulus for the migrations. It is suggested that the migration is a result of a circadian rhythm entrained to the exogenous 24 hour cycle by the changing light intensity.

No description available.

The hydrogeochemistry of the southern Lincolnshire Limestone is investigated employing inductively coupled plasma-optical emission spectrometry (ICP-OES) as the primary analytical technique. Two simultaneous, multi-element instruments were used. The principles of the technique and comparisons of the hardware, operating conditions and routine sample preparation methods are outlined and realistic levels of precision and detection limits evaluated. The range of determinations in the groundwaters was extended by the development of novel sample treatment techniques including the determination of dissolved sulphide down to the 1 level using a gas-liquid separator, and the determination of rare earth elements in groundwater evaporation residues. The methods were applied to the analysis of groundwaters and rocks of the Lincolnshire Limestone in southern Lincolnshire. The geology, hydrogeology and hydrochemistry of the aquifer are reviewed and the sample collection methods described, stressing the precautions taken to avoid contamination. The Lincolnshire Limestone is an aquifer of considerable heterogeneity, argillaceous limestones are intercalated with pure oolites and it is confined by predominantly clastic formations. The major element chemistry of the groundwaters follow a down-dip trend between calcium-bicarbonate-sulphate waters near outcrop and saline, non-potable waters in the east. The groundwaters are sub-divided into zones based on the processes of calcite dissolution, ion exchange and the mixing of recharge and saline interstitial waters. Minor and trace element behaviour is controlled by a combination of oxidation-reduction reactions, adsorption on clay minerals and organic matter, mineral solubilities, complex formation andgroundwater pH. Ultimately the low levels of most trace metals are limited by their low abundance in the limestone. Temporal hydrochemical trends are identified, and the influence of man, with reference to the down-gradient migration of agrichemical pollutants (e.g. nitrates) is assessed. Fluctuations in the movement of the potable/saline water interface are noted and theories on the origin of the saline waters reviewed.