Eco-Hydromorphology – an emerging framework in river science

Abstract

Emerging perspectives in river science now consider fluvial ecosystems in the context of highly altered and evolving hydrosystems. In anthropogenic landscapes, human activities influence all major river processes critical to ecosystem function. Direct physical alterations (channelization, impoundment, flow abstraction) modify interactions between the ecological, flow, sediment and temperature regimes that are critical to maintenance of riverine habitat. Indirectly, the pervasive impact of climate change is now disrupting the magnitude and timing of flow variability, sediment transport and thermal response, components that previously provided dependable habitat characteristics for river biota. The prevalence of these pressures means that many rivers now require mitigation management to sustain their function and biodiversity. Progressive management approaches of river systems seek to correct disruptions to their processes, so that they can assimilate disturbance and adapt to shifting system drivers (e.g. climate change). Collectively, this broad approach can be synthesised within an eco hydromorphology framework and aligns with a broader conservation strategy of managing for resilience. The approach assumes that protection or restoration of critical processes may allow the river systems to respond to future perturbation through natural ecological and physical adjustments.

In this context, the response of ecologically important components (i.e. riverine vegetation, fish community and stream temperature dynamics) to altered hydromorphology was investigated to better understand its biological impact and inform appropriate mitigation strategies. The findings support conceptual advances in plant-hydromorphology interactions, factors shaping the temperature regime of rivers, and the implications of co-occurring pressures such as thermal disruption and habitat degradation for biological community structure (i.e. fish) in a changing climate. In sum, it progresses river science operating at the bio–physical interface, an expanding area of investigation that is increasingly dependent upon collaborations among those with expertise in ecology, hydrology and fluvial geomorphology. Correspondingly, the authors research demonstrates that some plant species possess adaptive traits that allow them to not only succeed in physically altered rivers, but also to direct channel forming processes. These engineer species affect channel adjustments by regulating fluvial processes and deserve greater attention in the broader context of river management. By integrating hydromorphological descriptors with water quality indicators, explanatory causes of river vegetation composition are also better captured here than in many existing bio-assessment metrics. The findings are pertinent to the implementation of regulatory instruments such as the Water Framework Directive (WFD), which aim to improve the ecological state of river systems by amelioration of pressures or direct interventions in the form of river restoration. Similarly, the complex relationship between riparian vegetation and hydromorphological components in influencing the physio-thermal habitat of rivers biota is further revealed. At present, many management strategies do not adequately integrate the interacting ecological, hydromorphological and thermal dimensions of rivers to protect ecosystem function and mitigate future disturbance. In a series of studies, potential limitations imposed by hydromorphological alterations for commonly proposed temperature management strategies are made clearer. Results suggest that the relative effectiveness of riparian tree cover as a temperature management tool can be significantly diminished by hydromorphological alterations. Holistic approaches that address both riparian and hydromorphological deficits seem more likely to replicate the natural temperature and habitat variability required by sensitive species, such as salmonids. The historical legacy of human activities in rivers together with future societal responses to climate disturbance will require careful scrutiny when devising climate change mitigation strategies that seek to protect river ecosystems.

Date of Award	Sept 2020
Original language	English
Awarding Institution	University of Brighton
Supervisor	Neil Crooks (Supervisor)