Geoffrey E. Petts

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.

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.

WATER ALLOCATION TO PROTECT RIVER ECOSYSTEMS

River regulation has attracted considerable attention over the past 20 years. The effects of (i) changes in the seasonal flow regime below dams and reservoirs and (ii) reduction in flow caused by water abstraction and diversion, upon lotic and riparian ecosystems have been demonstrated for rivers in a range of geographical regions. This paper presents an approach to determining ecologically acceptable flow regimes and volumes. The approach is founded on a set of fundamental scientific principles concerning longitudinal connectivity, vertical exchanges, floodplain flows, channel maintenance flows, minimum flows and optimum flows. The need for a policy for allocating water to protect river ecosystems in England and Wales is discussed and the method is illustrated by a case-study of a chalk stream that has been affected by groundwater abstraction. Sixty per cent of the available resource is shown to be required to sustain the river as a trout stream. Several judgemental decisions are needed in setting an ecologically acceptable flow regime and further research is required to improve our capability for modelling the roles of different flows and patterns of flows in sustaining river ecosystems.

River energy budgets with special reference to river bed processes

This paper uses detailed hydrometeorological data to evaluate the influence of channel bed processes on the river energy budget at an experimental site on the regulated River Blithe, Staffordshire, UK. Results from a pilot study are presented for eight days during July, September, October and November 1994. Total energy gains were dominated by net short-wave radiation (97·60%) with significant contributions from sensible heat exchange and friction (1·17 and 1·06%, respectively) and minor additions from condensation and bed conduction (0·16 and 0·01%, respectively). Net long-wave radiation, evaporation, conduction into the river bed, sensible heat transfer and the energy advected during evaporation accounted for 53·98, 23·56, 16·27, 5·25 and 0·94% of the total heat losses. On average, over 82% of the total energy transfers occurred at the air–water interface. Approximately 15% of the total energy exchanges occurred at the channel bed, but maximum daily heat exchanges accounted for up to 24% of the daily total energy transfer. The amount of short-wave radiation attenuated in the water column, and values measured at the channel bed varied considerably from those calculated using a standard coefficient. Values of bed conduction varied in response to different vertical thermal profiles in the channel bed, reflecting the variable influence of sedimentology and groundwater flux. Fluctuations in levels of periphyton and macrophyte cover were also shown to have a significant effect on energy fluxes at the channel bed.

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.

Lowland floodplain rivers: geomorphological perspectives

Modelling Channel Migration and Floodplain Sedimentation in Meandering Streams Estimation of the Fractal Dimension of Terrain from Lake Size Distributions Investigating Contemporary Rates of Floodplain Sedimentation Influence of Farm Management and Drainage on Leaching of Nitrate from Former Floodlands in a Lowland Clay Catchment Floodplain Assessment for Restoration and Conservation: Application of the Instream Flow Incremental Methodology to Assess Ecological Flow Requirements in a British Lowland River.

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.

Long-term consequences of upstream impoundment

Since 1970 a large number of environmental problems have been identified as resulting from the long-term effects of human impacts. Consideration of human activity within the environment as three orders of impact, provides a basic framework for the appreciation and evaluation of long-term problems. Consequent upon dam construction, major changes of flood magnitude and frequency and of the quantity and calibre of sediment loads (first-order impacts), will induce the readjustment of channel morphology and ecology (second-order impacts). However, the macrophytic and macro-invertebrate population, for example, are also adjusted to channel morphology—particularly channel shape and substrate composition—so that further readjustments of the macrophyte and macro-invertebrate populations may be effected by changes of channel form (third-order impacts).

Alternatives in regulated river management

Perspectives for Ecological Management of Regulated Rivers. WATER QUALITY PROBLEMS. Water Temperature, Dissolved Oxygen, and Turbidity Control in Reservoir Releases. Water Quality Modeling of Regulated Streams. Flushing Flows. CHANNEL MODIFICATION AND MANAGEMENT. Alternative Channelization Procedures. Channel Engineering and Erosion Control. The Use of Instream Habitat Improvement Methodology in Mitigating the Adverse Effects of River Regulation on Fisheries. Flood Plain Fisheries Management. Mitigation for Impacts to Riparian Vegetation on Western Montane Streams. ECOLOGICAL MODELING AND MANAGEMENT. Models for Predicting Benthic Macroinvertebrate Habitat Suitability Under Regulated Flows. The Application of a Classification and Prediction Technique Based on Macroinvertebrates to Assess the Effects of River Regulation. Instream Habitat Flow Modeling Techniques. Alternative Approaches in Predicting Trout Populations from Habitat in Streams. Index. Major Topic Areas Include: