Hydromorphology Research Programme
What is Hydromorphology?
Hydromorphology in rivers involves studying the physical structure and processes of rivers, including water flow and the form of the riverbed, banks and floodplain, which act together to shape river habitat. The processes involved are known as fluvial geomorphology, which is the movement of water and sediment, and these processes interact with the biological components of river ecosystems, including riparian and instream vegetation. Together, these processes and interactions stabilise sediment, regulate erosion and influence the formation of landforms, such as channel meanders, channel bars, pools and riffles.
The three-way interaction between flowing water, sediment movement and vegetation establishment has shaped the Owenbrin River (Co. Mayo), its habitats and the landscape around it.
The Aims of the Hydromorphology Research Programme (HRP)
The HRP aims to link river management and restoration approaches to the important concepts in hydromorphology that underpin healthy river ecosystems. The programme does this through applied studies that investigate the interactions and feedbacks between river processes and their biological communities, as well as how these interactions can be altered by human activities.
The evidence base generated by this research helps us to understand the effectiveness of river management and restoration measures by demonstrating their benefit to stream biota and the extent to which they enhance the hydromorphological processes that sustain habitat.
Natural Rivers
The EU Water Framework Directive (WFD) has resulted in a focus on hydromorphology and on the physical habitat conditions that should exist in healthy functioning rivers. Identifying rivers with near natural conditions provides a baseline for what an undisturbed river should look like and how it should function with minimal human impact. Natural or near natural rivers are typically classified as having High Ecological Status under WFD assessment. This provides a target for restoration of degraded rivers, ensuring that interventions aim for outcomes that focus on restoring the natural hydrogeomorphic processes (e.g., flow regime, sediment transport, riparian vegetation establishment, channel migration) that create and maintain healthy river forms and functions.
Depending on a variety of factors, rivers show a remarkable diversity of natural forms, and this shapes their associated biological communities. Assigning a typology to a river helps us to understand the specific characteristics of different river systems, which is essential for developing appropriate and effective management strategies. River typology involves classifying rivers based on different criteria, primarily their morphological characteristics, such as shape (e.g., straight, braided or meandering); their hydrological characteristics (e.g., flow variability); and their bed material (e.g., boulder, gravel and sand). This classification is crucial for WFD assessments because it helps to define type-specific ecological conditions for accurate water quality monitoring and management.
Hence, not all restoration measures are suitable for all rivers. Habitat features occurring in a high-energy mountain stream are likely to be inappropriate for a low-energy, lowland meandering river. River typologies allow restoration practitioners to select measures that reflect those that promote development of characteristics that would naturally occur.
Cascade: straight, high-energy rivers with a 'disorganised' bed dominated by random boulders.
Wandering: typically rivers with a gravelly riverbed and exhibiting a single-channel planform that intermittently splits into multiple channels.
Lowland meandering: low-energy rivers characterised by high levels of sinuosity, fine sediment deposition and floodplain connectivity.
Examples of various river typologies found in Ireland.
The Role of Hydromorphology in Habitat Creation
The combination of hydrological and morphological processes creates and maintains diverse physical habitats that are crucial for the life stages of aquatic organisms. For example, meandering rivers are typically found in the middle to lower course of rivers, where lateral erosion creates wide bends, or meanders, in the river. This action results in the formation of pools on outside eroding bends, and some of this eroded material is then deposited downstream on inside bends to form point bars, or deposited instream to generate riffles.
At times of high flow, the adjacent floodplains may be covered in water when the river overtops its banks. Fine sediment is left behind as flood waters recede, and this contributes to floodplain development and provides opportunities for plants to establish in these fertile deposits. In turn, trees or boughs that fall into the channel through windthrow or natural mortality can create large woody structures that provide complex habitat for stream biota.
Example of the landforms and associated habitats that are shaped by hydromorphological processes in a lowland meandering river.
Hydromorphological Pressures
Human activities that impact or alter the hydromorphological processes in waterbodies may also alter the WFD Ecological Status of waters. These activities are known as hydromorphology pressures and include water abstraction; flow regulation; channelisation and realignment of rivers; construction of artificial barriers, such as weirs; and removal of natural features, such as channel sediments and vegetation.
Hydromorphology pressures that alter the natural functioning of a waterbody, such as its flow regime and sediment transport, can impede its ability to create and maintain a variety of features, such as pools and riffles in rivers. These changes in hydromorphological processes result in the loss of river habitat that aquatic organisms require to survive and thrive.
Channelisation/arterial drainage has resulted in simplified channels that lack diversity of key habitat features.
Artificial barriers limit sediment transport and connectivity for stream biota moving upstream and downstream.
Major dams reduce water flow quantity and natural variability, and they prevent movement of stream biota and sediments.
Hydromorphological pressures affecting rivers in Ireland.
Integrating Hydromorphology into River Management
Within river landscapes governed by human activities (e.g., intensive land use, altered drainage networks, channelisation, barriers, etc.), river restoration strategies will need to emphasise actions that promote functioning river processes and allow space for adaptation to less predictable disturbances.