Angela M. Gurnell

Effects of deposited wood on biocomplexity of river corridors

Under natural conditions, most rivers are bordered by riparian woodland. Many studies have highlighted the ecological importance of these wooded zones, but the impact of riparian woodland dynamics on the complexity of the active tract (the area of bare sediment adjacent to the river) has been overlooked. This paper highlights the impact of downed trees and sprouting driftwood on the development of islands and associated ponds within the active tract of large rivers and the effects of these features on the abundance and diversity of plants and animals, and points to the benefits of riparian woodland for channel restoration.

Large woody debris and river geomorphological pattern: examples from S.E. France and S. England

Abstract The study of accumulations of dead wood within the fluvial environment has been mainly undertaken in mountain streams and rivers within the Northwestern United States, and particularly in hydrosystems which have experienced little riparian vegetation cutting or disturbance by man. Appraisals of the spatial variability in the physical character of accumulations of dead wood has mainly highlighted the volumes of large woody debris (LWD) accumulations and the local channel morphological properties induced by their presence. The spatial variability in the accumulation and processing of organic material forms one of the central concepts of the River Continuum Concept, which characterises the occurrence and processing of organic material, of which LWD is an important component, according to a longitudinal gradient along a rivers course. Some studies have extended the concept by illustrating the importance of the lateral dimension, particularly in large rivers with extensive floodplains, and by relating the occurrence of dead wood to fluvial morphodynamics. However, to date there has been no synthesis of the relationship between LWD and the geomorphic pattern of the river channel. Although the research literature shows that the routine clearance of wood from water courses is not an environmentally-sympathetic strategy, within Europe LWD accumulations are usually seen as a river management problem and are routinely cleared from river channels. This paper addresses these physical and applied aspects of the role of LWD. It presents an analysis based upon semi-natural hydrosystems in S.E. France and S. England. The forested corridors discussed are currently or have recently been maintained. They are essentially young and so produce relatively small amounts of woody debris in relatively small-sized individual pieces in comparison with the rivers studied in North America. Using observations from these example river corridors, the relationship between rivers of a particular size and geomorphic pattern and the dynamics of dead wood is described and evaluated. Major constrasts in the role of LWD are found between small, single thread rivers, and larger, piedmont, braided and wandering rivers. Some points of synthesis concerning the ecological, hydraulic and morphological impacts of dead wood are drawn from these examples, and are used as a basis for proposing some simple maintenance rules.

The hydrogeomorphological e•ects of beaver dam-building activity:

A characteristic of beaver ecology is their ability to build dams and, thus, to modify the landscape to increase its suitability for their occupation. This ability gives beaver great significance as a geomorphic agent. In order to review the hydrogeomorphological effects of beaver dam-building activity, this article places a context on the likely distribution and magnitude of beaver activity by considering the spatial and temporal variability of distributions of beaver and the habitat characteristics which might favour the establishment of substantial beaver populations. A description is then given of the nature and potential dimensions of instream structures built by beaver and the environmental conditions under which dam building has been observed to occur. The hydrogeomorphological impact of dam building is then appraised both locally and at the landscape scale, illustrating the very significant process modification caused by beaver. While the European beaver, Castor fiber, is the main focus of this review, it necessarily draws extensively on the much larger literature concerning the North American beaver (Castor canadensis).

Riparian plant invasions: Hydrogeomorphological control and ecological impacts

Biological invasions are a threat to ecosystems across all biogeographical realms. Riparian habitats are considered to be particularly prone to invasion by alien plant species and, because riparian vegetation plays a key role in both aquatic and terrestrial ecosystems, research in this field has increased. Most studies have focused on the biology and autecology of invasive species and biogeographical aspects of their spread. However, given that hydrogeomorphological processes greatly influence the structure of riparian plant communities, and that these communities in turn affect hydrology and fluvial geomorphology, scant attention has been paid to the interactions between invasions and these physical processes. Similarly, relatively little research has been undertaken on competitive interactions between alien and native riparian plant species. Further research in these fields is necessary at a variety of spatial and temporal scales before the dynamics of riparian invasions, and their impacts, can be properly understood.

The Tagliamento River: a model ecosystem of European importance

In NE Italy is a remarkable floodplain river that retains the dynamic nature and morphological complexity that must have characterized most Alpine rivers in the pristine stage. This river system, the Fiume Tagliamento, constitutes an invaluable resource not only as a reference site for the Alps, but as a model ecosystem for large European rivers. The Tagliamento has a number of attributes that have not been given due consideration in river ecology: (i) an immense corridor of more than 150 km2 that connects the land and the sea and two biomes, the Alps and the Mediterranean; (ii) unconstrained floodplain segments characterised by a dynamic mosaic of aquatic/terrestrial habitats; and (iii) a large number of vegetated islands (ca. 700). We believe it is critical to understand the functional roles of these endangered attributes in order to effectively engage in river conservation and management programmes. The Tagliamento River in Italy offers the rare opportunity to investigate natural processes at a scale that can be studied almost nowhere else in Europe.

Changing river channels

Partial table of contents: Changing River Channels: The Geographical Tradition (G. Petts). TEMPORAL AND SPATIAL DIMENSIONS. Changes of River Channels in Europe During the Holocene (L. Starkel). Channel Networks: Progress in the Study of Spatial and Temporal Variations of Drainage Density (V. Gardiner). Channel Cross-Sectional Change (C. Park). PROCESSES OF CHANGE. Bedload Transport and Changing Grain Size Distributions (B. Gomez). Catchment Sediment Budgets and Change (S. Trimble). INFORMATION FOR THE MANAGEMENT OF CHANGE. Information Flow for Channel Management (M. Clark). Information from Topographic Survey (S. Downward). Information from Channel Geometry-Discharge Relations (G. Wharton). MANAGEMENT FOR CHANGE. River Channel Restoration: Theory and Practice (A. Brookes). Towards a Sustainable Water Environment (J. Gardiner). Index.

A reference river system for the Alps: the ‘Fiume Tagliamento’

A major deterrent to a full understanding of the ecological ramifications of river regulation at the catchment scale is a lack of fundamental knowledge of structural and functional attributes of morphologically intact river systems. For example, both the River Continuum and the Serial Discontinuity Concepts, in their original formulations, had the implicit assumption of a stable, single-thread channel from headwaters to the sea. The Fiume Tagliamento traverses a course of 172 km from its headwaters in the Italian Alps to the Adriatic Sea. No high dams impede the rivers passage as it flows through the characteristic sequence of constrained, braided, and meandering reaches. The Tagliamento, the only large morphologically intact Alpine river remaining in Europe, provides insight into the natural dynamics and complexity that must have characterized Alpine rivers in the pristine state. The Tagliamento has a flashy pluvio-nival regime (mean Q=109 m3 s-1, with flood flows up to 4000 m3 s-1). Thousands of newly-uprooted trees were strewn across the active bed and floodplain along the rivers course following a major flood in the autumn of 1996. The active floodplain is up to 2 km wide and contains a riparian vegetation mosaic encompassing a range of successional stages. Up to 11 individual channels per cross section occur in the braided middle reaches. Islands are a prominent feature of the riverine landscape and island dynamics are postulated to play a key role in determining pattern and process across scales. Future studies will examine the roles of island dynamics and large woody debris in structuring biodiversity patterns of aquatic biota and successional trajectories of riparian vegetation. The high levels of spatiotemporal heterogeneity exhibited by the Fiume Tagliamento provide a valuable perspective for regulated river ecologists and those engaged in conservation and restoration.

A conceptual model of vegetation dynamics on gravel bars of a large Alpine river

The concepts of large river systems have been advanced with limited empirical knowledge of natural systems. In particular, virually all large Alpine European rivers were ‘trained’ during the 19th century. Without first hand knowledge of natural systems we lack baseline data to assess human impacts and to address restoration and conservation strategies. An exception is the River Tagliamento which rises in the limestone Alps of northern Italy and flows for 172 km to the Adriatic Sea. Following a very high flood, we observed the first stages of succession within the rivers active zone. This article presents a conceptual model of vegetation dynamics on gravel bars based upon those observations. Thousands of trees and other large woody debris (LWD) lay scattered across the active zone after the flood. The larger pieces of debris had a marked influence on the deposition of sediments and other debris, and were sites of colonization by pioneer plants. They represent the first stage in the development of vegetated islands that have the potential to increase in size during subsequent floods. Islands are also eroded, particularly by lateral channel erosion, and the materials may be reincorporated into new islands downstream. The island vegetation is dominated by five Salix species and Populus nigra. The dynamics of vegetated islands results from the interaction between the fluvial regime and the dominant woody species, the Salicaceae. These plants act as ‘autogenic ecosystem engineers’, because the plant structures themselves alter the environmental conditions through trapping sediment and organic debris. These processes may help to maintain an island-braided channel system that supports a high habitat diversity. Management of the river to regulate flow or to reduce the supply of LWD is likely to result in a loss of the habitat heterogeneity produced by island dynamics.

Channel change on the River Dee meanders, 1946–1992, from the analysis of air photographs

Gurnell et al. (1994) identified spatial and temporal trends in the planform of the River Dee on the Welsh–English border over a 115 year period (1887–1992) by overlaying information primarily derived from historical maps within a GIS. This paper focuses on channel change within the same river reach over the last 50 years based on the analysis of information from six sets of air photographs within a GIS. The shorter time period of study has resulted in greater difficulty in identifying channel changes which exceed the errors incurred in information extraction from the photographs and registration to a common map base. However, consistency in the spatial and temporal trends identified from different sources and for different stretches within the study reach support interpretation of channel change. Air photographs contain more information that is relevant to the interpretation of processes of channel change than can be obtained from maps. For example, information on bank cover and the pattern of sediment deposition are explored. However, information obtained from air photographs requires careful handling if errors in data extraction and registration are to be minimized. The analysis of channel planform information from air photographs presented in this paper confirms some of the tentative conclusions previously drawn from the analysis of information derived from maps of the same river stretch. Additional information on downstream changes in the spatial extent and pattern of sediment deposition and on temporal trends in bank vegetation cover, that has been extracted from air photographs, provides further details of possible adjustments to flow regulation, indicates the importance of vegetation cover to bank sedimentation processes and illustrates a downstream change in the mode and distribution of sedimentation across the fluvial–tidal transition.

An approach to hydrograph classification

The stream hydrograph is an integration of spatial and temporal variations in water input, storage and transfer processes within a catchment. For glacier basins in particular, inferences concerning catchment-scale processes have been developed from the varying form and magnitude of the diurnal hydrograph in the proglacial river. To date, however, such classifications of proglacial diurnal hydrographs have developed in a relatively subjective manner. This paper develops an objective approach to the classification of diurnal discharge hydrograph ‘shape’ and ‘magnitude’ using a combination of principal components analysis and cluster analysis applied to proglacial discharge time-series and to diurnal bulk flow indices. The procedure is applied to discharge time-series from two different glacier basins and four separate ablation seasons representing a gradient of increasing hydrological perturbation as a result of (i) variable water inputs generated by rainstorm activity and (ii) variable location and response of hydrological stores through a systematic decrease in catchment glacierized area. The potential of the technique for application in non-glacial hydrological contexts is discussed. Copyright