B. Belletti

A multi-scale hierarchical framework for developing understanding of river behaviour to support river management

This paper introduces this special issue of Aquatic Sciences. It outlines a multi-scale, hierarchical framework for developing process-based understanding of catchment to reach hydromorphology that can aid design and delivery of sustainable river management solutions. The framework was developed within the REFORM (REstoring rivers FOR effective catchment Management) project, funded by the European Union’s FP7 Programme. Specific aspects of this ‘REFORM framework’ and some applications are presented in other papers in this special issue. The REFORM framework is founded on previous hierarchical frameworks, sixteen examples of which are reviewed. However, the REFORM framework has some particular properties that reflect the European context for which it was developed. The framework delineates regional landscapes into nested spatial units at catchment, landscape unit, segment, reach, geomorphic unit and finer scales. Reaches, regardless of their ‘naturalness’, are assigned to a river type based on valley confinement, planform and bed material. Indicators are quantified at each spatial scale to feed three groups of assessments. First, contemporary indicators at reach and geomorphic unit scales investigate present processes, forms and human pressures within each reach. These feed assessments of present reach hydromorphological function/alteration, including whether the reach is functioning appropriately for its type; riparian corridor function and alteration; and hydromorphological adjustment. Second, indicators at catchment to segment scales investigate water and sediment production and delivery to reaches and how these are affected by human pressures now and in the past. These are used to construct an inventory of changes over space and time. Third, historical reach and geomorphic unit scale indicators are used to construct the trajectory of reach-scale changes. Contemporary reach-scale assessments, space–time inventory, and trajectory of changes are then combined to establish how river reaches of different type, subject to different human pressures, and located in different environmental contexts behave in response to changes at all considered spatial scales. These support forecasts of the likely responses of reaches to future scenarios (e.g., changes in climate, land cover, channel interventions).

A review of assessment methods for river hydromorphology

Numerous hydromorphological assessment methods have been developed in different countries during recent decades, with notable differences in their aims, scales, and approaches. Although these methods are increasingly applied to support river management, the strengths and limitations have been insufficiently investigated. This review of 121 methods analyses hydromorphological assessment methods dating from 1983 to 2013, identifying their main strengths, limitations, gaps, the potential to integrate different approaches, and the need for further improvements. For this purpose methods have been grouped into four categories: (1) physical habitat assessment; (2) riparian habitat assessment; (3) morphological assessment; (4) assessment of hydrological regime alteration. Seventeen categories of information covering general characteristics, recorded features and river processes encompassing over 90 features were recorded for each method reviewed, allowing a comparative analysis of the four assessment categories. The main gap in most methods is insufficient consideration of physical processes. Thus, an integrated hydromorphological analysis is recommended, where the morphological and hydrological components are the key parts to classify hydromorphological conditions. Additional physical and riparian habitat methods strengthen the link with ecological conditions.

Analysis of Post-flood Recruitment Patterns in Braided- Channel Rivers at Multiple Scales Based on an Image Series Collected by Unmanned Aerial Vehicles, Ultra-light Aerial Vehicles, and Satellites

This paper uses imagery captured by ultra-light and unmanned aerial vehicles (ULAVs) in combination with satellite remote sensing to explore vegetation development on braided reaches in the Southern and Eastern French Alps. Findings demonstrate that while asexual reproduction processes dominate, sexual reproduction through seed dissemination and wind transport play a significant role during periods of low flow following large floods. This progressive vegetation establishment leads to a general trend of river corridor narrowing. Key climatic conditions lead some years to be critical to initial vegetation development. Sensitivity to encroachment depends on local controls, such as rainfall conditions, distance to water table, local topography, seed availability, summer temperature, and dryness.

Regional variability of aquatic pattern in braided reaches (example of the French Rhône basin)

We analysed, at a regional scale, the braiding pattern and the structure of the water channel habitats of 53 braided reaches located in the Rhône hydrographical district (France), in relation to three control factors: (i) spatial location along the network (i.e. slope, altitude), (ii) hydrological conditions and (iii) morphological conditions. This research is based on aerial orthophotos belonging to the French National Geographic Institute (IGN) taken between 2002 and 2006. We defined a regional typology of braided reaches based on the distribution of the variables listed above, and identified five hydro-geographical types of braided reaches. Following this regional classification, we compared the pattern of braided reaches by testing several parameters, which describe the physical characters of water channel habitats (such as braided index, channel sinuosity, or aquatic habitat diversity). We found that discharge appears to be less relevant than sediment supply or groundwater level in structuring the braided geometry and the channel habitat pattern in natural braided rivers at a regional scale. We discussed the importance of local factors, such as the position of the groundwater table and the width of the active channel, for explaining the intensity of the braided pattern. There are numerous cases where the discharge is very low but the braided index high. These findings support management and conservation recommendations of braided rivers for implementing the European Water Framework Directive (2000).

New tools for the hydromorphological assessment and monitoring of European streams

Hydromorphological stream assessment has significantly expanded over the last years, but a need has emerged from recent reviews for more comprehensive, process-based methods that consider the character and dynamics of the river with greater accuracy. With this as a focus, a series of hydromorphological tools have been developed and/or further extended in Europe within the context of the REFORM (REstoring rivers FOR effective catchment Management) project. The aim of this paper is to present the set of REFORM hydromorphological assessment methods and, based on some examples of their application, to illustrate and discuss their synergic use, specific features, limitations and strengths. This assessment and monitoring includes three tools: the Morphological Quality Index (MQI), the Morphological Quality Index for monitoring (MQIm), and the Geomorphic Units survey and classification System (GUS). These tools constitute the assessment phase of an overall multi-scale, process-based hydromorphological framework developed in REFORM. The MQI is aimed at an assessment, classification and monitoring of the current morphological state; the MQIm aims at monitoring the tendency of morphological conditions (enhancement or deterioration); the GUS provides a characterization, classification and monitoring of geomorphic units. A series of examples are used to illustrate the potential range of application, including: (i) an assessment of morphological conditions; (ii) an assessment of the morphological effects of restoration projects; (iii) an evaluation of the geomorphic impacts of interventions for risk mitigation; and (iv) an integrated use of MQI and GUS to assess and characterise morphological conditions. Finally, some of the main features, strengths and peculiarities of the three hydromorphological tools are discussed with the support of examples of their application.

Indicators of river system hydromorphological character and dynamics: understanding current conditions and guiding sustainable river management

A set of multi-scale, process-based hydromorphological indicators of river character and dynamics has been developed to support river management and restoration activities. Indicators are selected to represent key hydromorphological processes at each spatial scale, i.e., catchment, landscape unit, river segment, river reach. Their evaluation allows identification of the cascade of these processes through the spatial units and the historical changes in their propagation as a consequence of natural or human induced hydromorphological changes. The approach is deliberately open-ended so that it can be adapted to local environmental conditions and management, and it can make the most effective use of available data sets. The indicators support assessments of the current condition of the river and its catchment; past changes within the catchment and their impacts on river reaches. Therefore, they represent a sound foundation for assessing the way the catchment to reach scale units and the geomorphic units within reaches may respond to future natural changes or human interventions. The procedure is illustrated using the example of the river Frome (UK).

Diagnosing problems induced by past gravel mining and other disturbances in Southern European rivers: the Magra River, Italy

The multi-scale hierarchical framework developed within the REFORM project, for the study of the functioning of river reaches and their catchments, was applied to the Magra River catchment (Northern Tuscany, Italy). The Magra River is a quite dynamic gravel-bed river that has undergone severe channel adjustments over the last century (i.e. incision and narrowing). The REFORM framework was then applied in order to (1) explore the locations and causes of these adjustments, and (2) assess how different river reaches responded to specific human activities (i.e. land use changes, dams, gravel mining). The work aims at providing information to aid sustainable river management and restoration. In accordance with the framework, initially, all the relevant spatial units (i.e. from the catchment to the reach scale) were delineated and then characterised in further detail. Then, a summary of the trajectories of change following human impact was provided. Combined with an overview of the main human pressures still influencing the river system, this allowed us to define the current conditions at the river reach scale. Finally, the knowledge acquired in the previous parts was assembled to build a better understanding of current river conditions and controls on past changes, as well as to enable the formulation of some future scenarios. The latter analysis was focussed on a couple of reaches located in a sensitive and relatively low impacted segment, in order to better highlight the interpretative power of the hierarchical framework. The framework was applied by combining the knowledge derived from previous studies and the computation of additional and relevant indicators. This study demonstrated the helpfulness of the hierarchical framework in diagnosing problems and supporting management strategies for a medium-sized, gravel-bed river catchment.

Riparian vegetation as indicator of channel adjustments and environmental conditions: the case of the Panaro River (Northern Italy)

Disturbances affecting flow and sediment transport regimes (e.g. dams, diversions, gravel mining, weirs, bank reinforcements, climate changes) can promote riverbed degradation and channel narrowing, and thus influence vegetation dynamics and composition. This study investigates the relationships and feedbacks between channel adjustments and riparian vegetation dynamics by combining an analysis of morphological channel changes with a wider phytosociology analysis of existing vegetation within the river corridor. These relationships were illustrated by using the case study of the Panaro River (located in the Northern Apennines, Italy), being a representative case of a deeply incised and narrowed river. More specifically we analyzed: (1) the relations between landforms and distributional patterns of vegetation types and characteristic plant species (index species): these provided information about the hydrogeomorphic condition of fluvial landforms and about channel adjustments; (2) the distance of riparian vegetation conditions from expected conditions as a consequence of human impact, based on the fact that each species and vegetation type has a given tolerance for specific disturbance regimes or stresses. Although some expected relations between landforms and vegetation types were found, we recorded significant deviations from the typical correlation pattern existing between morphology and vegetation, and this could be used to infer the intensity and the typology of human disturbances. In particular, the index species can indicate present ecological conditions and on past channel evolution. With this knowledge it might be possible to develop botanical recovery models in the future and, even more importantly, enable the recognition of the differences between temporal and spatial diversity.

Fuzzy cognitive mapping for predicting hydromorphological responses to multiple pressures in rivers.

Summary 1. Different pressures often co-occur in rivers and act simultaneously on important processes and variables. This complicates the diagnosis of hydromorphological alterations and hampers the design of effective restoration measures. 2. Here, we present a conceptual meta-analysis that aims at identifying the most relevant hydromorphological processes and variables controlling ecological degradation and restoration. For that purpose, we used fuzzy cognitive mapping based on conceptual schemes that were created according to 675 scientific peer-reviewed river hydromorphology studies. 3. A model generated from this approach predicts responses that are consistent with common understanding of the direct interactions between hydromorphological pressures, processes and variables. However, it also leads to new knowledge beyond traditional hydromorphological models by dealing with the complex interactions of hydromorphology, vegetation, water chemistry and thermal regime. 4. Water flow dynamics appeared as the most important of all hydromorphological processes affected by simultaneously interacting pressures. Relevant processes such as vegetation encroachment and sediment entrainment are closely linked to water flow. 5. Synthesis and applications. Our results demonstrate the relevance of natural flow regime rehabilitation for river management. Hence, we suggest focusing primarily on rehabilitating the natural flow regime before carrying out extensive habitat restoration works. This challenging target in river rehabilitation could strongly increase the success of additional habitat restoration.

Development and application of a multi-scale process-based framework for the hydromorphological assessment of European rivers

Many current river assessment methods emphasise the river ‘reach’ scale (a fixed length of river of the order of a few hundred meters) and provide a wealth of useful information that characterises the river corridor at the time of survey. However, they also have several limitations when they are used for understanding physical processes and causes of river alteration. A multi-scale, process-based framework is needed, which incorporates reach scale information into a larger spatial and temporal assessment of the controls on reach dynamics, and a process-based interpretation of the contemporary status of reaches, their historical dynamics and their likely future trajectories of adjustment. This paper reports on the early development and application of a multi-scale framework that is applicable to European rivers and is aimed at improving understanding of hydromorphological and ecological processes and their interactions. This ongoing research is part of the EU-funded project REFORM (REstoring rivers FOR effective catchment Management) which has the overall aim to provide a framework for improving the success of hydromorphological restoration measures in a cost-effective manner, targeting the ecological status or potential of rivers.