What is physical habitat and why is it vital in river/stream habitat management?
Generally speaking, habitats (or, more accurately, ecosystems) are a combination of all of their abiotic and biotic components, including all of the organisms present. For practical purposes it is often useful (perhaps necessary) to break them down into key components so that different impacts upon them can be evaluated and addressed. Drawing hard divisions between components is never satisfactory, but in freshwater habitats/ecosystems, a distinction is typically made between physical, hydrological, chemical and biological components.
The physical habitat component in naturally functioning river/stream systems is the product of interactions between flows of water, the erosion, transport and deposition of sediment and the development of riparian and aquatic vegetation. These interactions generate a dynamic and complex mosaic of physical habitats that supports the organisms inhabiting rivers, streams and their riparian margins. The magnitude and variability of water flows (the flow regime), the particle sizes and quantities of the sediments that the flows transport and the plant species with which they interact reflect the character of the catchment and also the local physical setting of any particular river reach. Human actions interrupt these natural water-sediment-plant interactions, often degrading the mosaic of physical habitats, the rate at which the habitats form and change, and individual habitat properties. Such changes need to be observed and understood because they have fundamental consequences for the survival and health of river ecosystems.
The physical habitat of rivers and streams tends to receive less public attention than the chemical habitat: currently, the negative impacts of pollution are motivational for citizens. However, a dynamic physical habitat mosaic is not just a set of physical patches, it also helps to improve water quality, e.g. by managing silt dynamics and inducing hydraulic effects that help to oxygenate the water. The aesthetic attraction of many rivers masks public perception of the long history of physical interventions, not just in channels but across the floodplain, not just in sizeable rivers but in the smallest headwaters. The UK has a long history of pervasive ‘land drainage’ and flood defence. Urbanisation and water supply/waste disposal activities have also had profound impacts on physical habitats. Damaged physical habitats are known to exacerbate the impacts of pollution and modified flow regime.
Where does flow regime fit in?
Owing to the critical role played by the flow regime in shaping and providing physical habitat mosaics in river/stream systems, the term ‘hydromorphology’ has been defined by the EU Water Framework Directive. Whilst this is useful for emphasising the intertwined nature of physical habitat and flow regime, it does lead to confusion when it comes to evaluating different types of human impact on physical habitat. Whilst physical habitat assessment methods may look at small-scale hydraulic patterns of flow in the channel (e.g. turbulent, smooth) and require an understanding of flow levels at the time of survey (to help interpret survey results), they do not evaluate critical impacts on the flow regime from activities such as river regulation, abstraction, land drainage and water diversion. These hydrological impacts have a profound effect on the physical habitat mosaic but are evaluated by other means, involving fixed gauging stations, modelling of natural flow regimes and assessment of deviations from the natural regime. The consequences for physical habitat are not very apparent from physical habitat assessment methods and need to be factored into data interpretation.
Deciding how best to assess physical habitat
Evaluating levels of human impact is a fundamental need but we also need to gain insights into how the river/stream system functions naturally and in response to remedial measures. Field survey methods are required, allied to data management and processing, to record, track changes, and generate reliable assessments of physical habitat and key controlling processes. River/stream surveying can be time-consuming and expensive, as can the essential associated training, quality control, data storage, processing and access. A vital consideration for method selection is therefore ‘fitness for purpose’. Strategically, it is also important to have available survey techniques capable of addressing issues across the whole catchment channel network, including the headwater stream system which constitutes 70% of the network by length in England.
River systems are a continuum but function at a number of (nested) spatial scales, as do their catchments. Survey/assessment methods have been designed with different purposes in mind, relating to the spatial scale at which information is required and the degree of quantification possible or needed to detect change and produce convincing evidence. Methods vary in terms of the length of channel covered in a single survey, the ease with which they can cover large numbers of sites (for catchment-wide appraisal), and their ability to quantify impact/change at individual sites. Selecting a method for a given purpose is essentially a trade-off between the speed, cost and resolution of the method and the scale of deployment needed.
The difference between assessment methods and data collection approaches
Increasing use is being made of fixed-point photography, drones and other aerial and remote sensing techniques in river surveillance. These will become increasingly important over time, not least to gain a holistic visual impression of river/stream systems and observable impacts upon them. They are not, however, a direct alternative to the physical habitat/geomorphological assessment methods outlined above, since they do not of themselves provide an assessment per se. A more direct comparison of these approaches would be with foot survey – in both cases an assessment system is needed to translate what is observed to useable information (unless only photographs or videos are needed to convey meaning).
Key methods and their uses
The suitability of different methods for different purposes is outlined in more detail in the tabular evaluation of the short list, but a brief account of key uses of short-listed methods is given below. In addition, Figure 1 illustrates how the trade-off between method resolution and scale of method deployment influences suitability for application at different scales. It is important to note that the use of ‘boxes’ in Figure 1 does not imply rigidity and the designers/ operators of survey systems often know best about deployment potential. The figure does not depict the potential for using a high-resolution method with a representative sampling programme to provide a catchment-scale evaluation (this is more suited to ‘state of the environment’ reporting). The ‘Method Profiles’ document helps portray method flexibility in a more nuanced way. A strategic monitoring plan, based on local need assessment, is a valuable first step in any survey-based investigation or project.
Rapid survey methods can quickly cover large areas of a catchment to provide either broad overviews (e.g. Naturalness assessment) or to capture specific impacts that fundamentally affect river habitat types and dynamics (e.g. River Obstacles, Mudspotter, Fluvial Audit). However, if the range of physical habitats and local human interventions are to be captured, more detailed surveys are required which necessarily are more time-consuming to implement. The River Habitat Survey (RHS, cRHS) is applied to a 500m length of river channel and provides a broad overview of flow patterns, sediments, physical habitats, vegetation structure and human pressures and interventions. Its key strength lies in quantifying habitat features and change at scales of 500m and above, but its deployment at catchment scale is time-consuming and costly (unless a representative sampling approach is taken). MoRPh (Cit Sci and Pro) provides the highest spatial resolution of physical habitat, designed to track changes associated with specific small-scale interventions on physical habitats (down to 10m channel length). This method is the only viable option for deployment on very small-scale restoration projects.
The degree to which the short list methods are suitable for application by non-specialist surveyors varies, but there are surveys designed for application by citizen scientists at all spatial scales (Naturalness assessment, River Obstacles, MudSpotter, cRHS, MoRPh CitSci). They support the design of integrated monitoring programmes to capture and develop understanding of the functioning of the physical habitat environment. Furthermore, their compatibility with existing databases and formal national reporting processes and policy instruments should not be forgotten, since this provides opportunities for citizen science information to contribute to strategic momentum for protection and restoration. RHS plays into strategic Environment Agency reporting processes on physical river/stream habitat. Naturalness assessment and RHS play into reporting on Defra indicator B6 and Natural England reporting processes under England’s biodiversity strategy. MoRPh Pro plays into Biodiversity Net Gain decision-making.