Robert W. Duck

Approaching the physical-biological interface in rivers: a review of methods for ecological evaluation of flow regimes

New European legislation known as the Water Framework Directive (WFD) challenges catchment hydrologists and freshwater biologists to quantify the risk of damage to the organic communities of rivers that arises from anthropogenic distortion of the natural flow regime. Here, we take the first step towards this goal by collecting together relevant information from the two disciplines. An extensive biological literature is examined for insights into the ways in which the species and communities associated with rivers might change when the flow regime is altered. From the hydrological literature, the indicators of flow regime and flow regime change that are pertinent to ecology are described, and consideration is given to means of deriving flow regime data for ungauged river reaches. Attempts to combine hydrology and ecology in classifying rivers and in setting flow objectives to favour biota are then reviewed, together with integrated approaches to river management that aim to promote ecological quality. A significant scale disparity is noted between the disciplines, hydrology being studied at catchment, subcatchment and reach scales, and biology generally at local level. Nonetheless, both yield methods with potential applications in aspects of WFD implementation. The approach with most appeal for general risk assessment is based on the concept of hydrological alteration. This technique employs flow regime variables selected for their importance to aquatic and riparian ecology, and quantifies deviations from the natural values of these variables at reach scale. For WFD purposes, calibration of the scale of hydrological alteration in terms of risk to ecological status is desirable. In this, priority should be given to identification of the level of hydrological alteration that corresponds to the division between good and moderate ecological status.

Subaqueous shrinkage cracks and early sediment fabrics preserved in Pleistocene calcareous concretions

Septarian cracks within concretions are widely believed to have formed by dehydration of initially soft concretion interiors. Other workers believe that such cracks form as tensile fractures in response to stresses induced during burial by 50 m and more of overburden. Herein attention is drawn to the fabrics of Pleistocene, calcareous concretions which have been subjected to a maximum overburden thickness of 10–12 m. These bodies, up to 10.5 cm in maximum dimension, are common in the silty clays of the Late Devensian (18000–13500 years BP), glacimarine Errol Beds of east-central Scotland. Within the concretions occur open fractures; microscopic lenticular, macroscopic, margin-parallel lenticular and septarian-like (radial) forms. These are interpreted as a product of subaqueous shrinkage during very shallow burial. This mode of formation is supported by SEM studies of the clastic fabrics preserved in the concretions which are indicative of deposition from originally flocculated suspensions. Spherical cavities, 30–35 μm in diameter, are interpreted as former gas bubbles derived from the decay of proteinaceous organic matter. This is believed to have promoted localized Ca2+ precipitation, in the form of a soap of fatty acids, through a rise in pH. The shrinkage cracks are thought to have been induced by the density increase which accompanies the conversion of Ca soap to CaCO3. The former gas cavities exert some control on the position of microcracks. Moreover, the delicate open fabrics preserved confirm that the concretions are of early diagenetic origin and formed close to the sediment water interface.

Evaluation of the nutrient inputs to a coastal lagoon: the case of the Ria de Aveiro, Portugal

The Ria de Aveiro estuary-coastal lagoon system of northern Portugal is estimated to currently receive mean annual influxes of total nitrogen (N) and total phosphorus (P) of c. 6118 t y−1 and 779 t y−1, respectively, from its influent rivers. In low summer flows the mean N and P fluxes decrease to c. 10% of the annual average. The sewage contribution to the inland-derived N load on an annual basis is c. 5% but, during the summer low flow conditions, the sewage component increases to c. 65% of the total river loading. The sewage contribution to the inland-derived P load on an annual basis is c. 11% but, during the dry season, it is 1.2 times larger than the river-derived flux. The construction of a regional sewer system linked to a submarine outfall, due for completion in 2005, is expected to lead to a reduction in nutrient fluxes from inland to the lagoon of c. 15% for N and c. 26% for P relative to the present values. While this system will reduce the nutrient loading in the upper reaches of the lagoon, an increase in nutrients derived from the ocean is anticipated, due to the proximity of the outfall to the inlet.

Thermal radiometry: a rapid means of determining surface water temperature variations in lakes and reservoirs

Abstract To obtain remotely sensed synoptic measurements of surface water temperatures, use has been made of a Heimann Radiometer (Model KT79). This instrument, sensitive to thermal infrared radiation in the 8–14 μm wavelength range of the electromagnetic spectrum is fitted with a telephoto lens and gives a restricted field of view at the water surface, enabling temperature estimation to ±0.2°C. For example, over a period of 1 h it was possible, from suitably selected vantage points, to record 475 point measurements of temperature over the entire surface of a reservoir some 1 km2 in area (Embalse de Rioseco, northern Spain). Repeated sets of observations permitted the recognition of upwelling and sinking water masses. The technique enabled the detection of influx of cold water from springs along a geological fault zone in the Embalse de Tanes. Additionally, the inception and the temporal and spatial evolution of an influent plunging plume was monitored using this equipment. Although thermal radiometry is restricted to the measurement of surface water temperatures, its speed of data acquisition over a wide area should enhance appreciation of the thermal behaviour of lakes and provide an important complement to existing techniques.

A high-resolution seismic stratigraphy from a Scottish sea loch and its implications for Loch Lomond Stadial deglaciation

A detailed high-resolution seismic stratigraphy, calibrated by core data and terrestrial geomorphological mapping, has been constructed for Loch Ainort, Isle of Skye. This study has provided a palaeoenvironmental history of the area as well as important corroborative evidence for the stepped deglaciation of the Loch Lomond Stadial ice-field on Skye. The Ainort Glacier reworked pre-Loch Lomond glacial deposits terminating in a grounded tidewater ice-front potentially 800 m beyond the previously extrapolated limit. The first stage of deglaciation was characterised by the formation of De Geer moraines indicative of a period of interrupted retreat. The second phase, by contrast, produced hummocky relief with sporadic linear moraines suggesting periods of uninterrupted retreat with occasional stillstands/readvances. Paraglacial reworking of terrestrial slopes resulted in the deposition of thick, subaqueous, debris flows which graded into fluvioglacial dominated sediments and ultimately modern fjordic deposits. The identification of an initial period of active retreat punctuated by numerous readvances correlates directly with the terrestrial record. However, the offshore stratigraphy suggests that although the second phase was dominated by uninterrupted retreat, occasional stillstands/ readvances did occur.

Long-term sediment yield in Crombie Reservoir catchment, Angus; and its regional significance within the Midland Valley of Scotland

Abstract The drawdown of Crombie Reservoir in November 2001 afforded the opportunity to examine the exposed sediments trapped since impoundment in 1868. Direct measurements of infill depth enabled an isopachyte map to be produced. Gravimetric conversion using measured bulk densities and a trap efficiency term indicated a long-term catchment sediment yield of 59.1 t km−2 year−1. Core stratigraphy analysis indicated that sediments were dark brown/black cohesive silty-muds with multiple sandy sub-units, representing a combination of discrete flood events and previous drawdown surfaces. Dating, constrained by mineral magnetic and 137Cs analysis, indicated that sedimentation rates have varied from 0.2 to 0.8 g cm−2 year−1, corresponding to a four-fold variation in catchment sediment yield (approximately 20–93 t km−2 year−1), most likely controlled by extensive conversion of moorland to woodland, and post-World War II agricultural expansion. The Crombie investigation is combined with other reservoir sedimentation surveys within the Midland Valley of Scotland. Area-specific sediment yields (t km−2 year−1) evidence a weak, though statistically significant (p > 0.05), positive correlation with catchment area (km2). The increase in area-specific yield with catchment area contradicts the decline, which is generally expected, and is taken to reflect the significance of channel erosion within water supply basins featuring mainly natural and semi-natural vegetation cover and low-intensity land management practices. With stable slopes channel erosion dominates and area-specific sediment yield increases downstream due to greater entrainment and transport potential. The high degree of scatter in the Midland Valley database reflects significant variations in the extent of land-use change and the local importance of agricultural improvements and afforestation practices.

Bayesian sediment fingerprinting provides a robust tool for environmental forensic geoscience applications

Abstract Sediment fingerprinting is an approach for the quantitative determination of sediment provenance (both spatial sources and types of sediment supply) over a range of temporal and spatial scales. Though widely adopted, studies often vary in their attention to the underlying assumptions and in their treatment of modelling uncertainty. A Bayesian approach to the multivariate problem of ‘unmixing’ sediment sources is reported, showing the significance of source group variability and source group sampling density to the accuracy of model output. The model produces results as median source group contributory coefficients (and associated 95% quantiles). The model was applied to environmental data obtained from selected soil erosion studies reported within the peer-reviewed literature. Good correspondence (r2=0.89) between reported mean source group contributory coefficients and median values were found when recalculated using the Bayesian analysis. However, confidence levels are highly variable, ranging from 2% to 97%. The robustness of any unmixing solution depends on factors such as the number of samples, the number of source groups and the variance of source group properties. It is concluded that ‘forensic-style’ investigations must recognize these uncertainties and be appropriately resourced to achieve tolerable accuracy and precision. The discussion considers additional confounding factors such as non-conservative tracer behaviour and enrichment/depletion during the sediment delivery process.

Soil erosion near Barry, Angus

Abstract At the time of heavy rainfall in September, 1985, spectacular gully and associated rill erosion occurred in a field, newly sown with winter barley along downslope cultivation lines, near Barry, Angus. Rainfall intensities exceeding 3 mm hr‐1 were recorded during two separate storms in a three‐day period. An estimated 88 t of material was removed equivalent to a yield of approximately 14.7 t ha‐1 This is believed to be the first quantified account of water erosion of arable land in the Dundee‐Arbroath area of the Angus coastal lowlands.

Sediment yields in lowland Scotland derived from reservoir surveys

The sediments on the floors of nine reservoirs in the Midland Valley of Scotland have been examined to establish rates of sedimentation and catchment sediment yields. Information from Cullaloe, Drumain, Harperleas and Lambieletham Reservoirs (Fife), is compared with data from Glenfarg and Glenquey Reservoirs (Tayside) and earlier studies on Hopes, N Esk (Lothian) and Kelly (Strathclyde) Reservoirs. The earth-fill dams were constructed between 1850 and 1935 below catchments ranging from 1·53 to 23·5 km 2 in area. They are situated on sedimentary, volcanic or intrusive igneous rocks mainly of Devonian or Carboniferous age. In most areas bedrock is obscured by mantles of drift. Six of the catchments are dominated by moorland terrain with local afforestation. Mixed arable farming and woodland characterise the remainder. The higher basins receive on average over 1400 mm of precipitation per annum, while in the lower basins some 800 mm falls. Measurements of thickness variations of the deposits have enabled the construction of isopachyte maps from which the volumes of entrapped materials have been derived. The computed catchment sediment yields vary from 2·l t km −2 yr −1 (Lambieletham) to 52·0 t km −2 yr −1 (Glenfarg). Sediments incorporate both mineral and organic matter, of which combustible material commonly forms 15–20% by weight. Siliceous tests of diatoms are also an important component of the deposits. The sediment yields computed are about an order of magnitude greater than those reported from rivers draining similar sizes of catchment within the Forth basin. There is a direct relationship between mean annual inflow to the reservoirs and sediment yield.

Impact of frontal systems on estuarine sediment and pollutant dynamics

In this paper, a brief description of frontal systems, their modes of occurrence and impact on the estuarine environment, is presented. Previous studies of estuarine fronts have largely focused on the water surface and within the water column. New observations in the Tay Estuary, Scotland have shown that the presence of fronts within the water column may be marked, not only by surface foam bands, but also by abrupt (i.e. non-gradational) changes in the underlying bedform morphology and/or sediment facies, as detected using side-scan sonar. This preliminary evidence suggests that fronts may exert a control, not only on the surface and intra-water column sediment and pollutant partitioning, but also on the distribution and persistence of bedload transport pathways.