Prof. Takashi Asaeda — Saitama University

(Grammatical editing by Greg Pasternack; bracketed text added by Greg Pasternack to aid translation.) I am wondering a little how we can cope with neighboring fields from now on, such as Ecohydrology (Journal of Ecohydrology), River Science (River Research and Applications), etc. Key to ecohydraulics and where ecohydraulics plays an important role are the topics of species–habitat relationships, biota movements, e-flows, and mitigation measures / river restoration — including infrastructure-related matters. Considering the background of ecohydraulics emerging from IAHR, ecohydraulics roles should be the intermediate field between hydraulics/fluid mechanics and biology/ecology. For instance, it is a little hard for people from other fields to come to addressing the problem of species–habitat relationships, particularly related to the physical processes and e-flows as an application as well as considering biota movement. The reason is that these functions are more or less based on hydraulics/fluid mechanics, which is not well understood by biologists [presumably because of their lack of formal education in physics and hydraulics in many curricula]. Thus, ecohydraulics can be of major relevance to these topics. An important large phenomenon treated in IAHR as a major topic is, I suppose, fluvial dynamics and resulting engineering. Recent increasing floods are making it more and more important. The way ecohydraulics comes into play here is its coverage of biota and riparian vegetation, particularly woody species. These are very important because in many regulated rivers the lack of natural disturbance means vegetation abundance is increasing all over the world. As a result, ecohydraulics and its coverage of vegetation–hydraulics linkages makes it very important in the management of flood protection, too. Looking at even larger scales, phenomena related to global warming will become more important, and thus we should add such topics to ecohydraulics.

Prof. Sung-Uk Choi — Yonsei University

Ecohydraulics is a study that attempts to understand ecosystem structure and functions using physical methods.

Prof. Peter Goodwin — University of Maryland Center for Environmental Science

During the Anthropocene, where climate and the environment are primarily influenced by human activity, the field of ecohydraulics studies the interface between these human activities (examples include infrastructure, climate, land use change, water supply, water quality, wildfire) and the changing dynamics in the aquatic environment (examples include floods, droughts, habitat, species behavior, population dynamics, food webs) and includes innovative strategies such as nature-based solutions and the removal or modification of historic infrastructure that has contributed to the demise of a healthy ecosystem.

Dr. Chris Katopodis — Katopodis Ecohydraulics Ltd

Ecohydraulics is the inter- and trans-disciplinary field broadly characterized by the ecohydraulic trilogy: (1) movements, abilities and passage of aquatic organisms; (2) e-flows, by which I mean environmental, ecological or instream flow regimes for aquatic flora and fauna; and (3) restoration of aquatic habitats and ecosystems. I outlined the ecohydraulic trilogy concept at ISE 2014 in Trondheim when I was asked to make some remarks on 20 years of Ecohydraulics. The concept was expanded at a Special Session I organized at ISE 2016 in Melbourne. It is also incorporated in the scope and topics for the Journal of Ecohydraulics. Below is a more detailed list of ecohydraulic topics.

The ecohydraulic trilogy — at the heart of the inter- and trans-disciplinary field of Ecohydraulics:

1. Movements, abilities and passage of aquatic organisms
   • Migratory behaviour, life-cycle requirements in fresh, brackish & saline waters
   • Upstream and downstream passage systems
   • Biomechanics (behaviour, swimming abilities, bioenergetics, modeling)
2. E-flow regimes (environmental, ecological or instream flows)
   • Hydraulic and habitat quantification & modeling involving aquatic flora and fauna
   • Ecohydrology, thermal & water quality aspects
   • Population dynamics and ecological modeling
3. Restoration of aquatic habitats and ecosystems
   • Morphodynamics, sediment transport, ice dynamics
   • Riparian vegetation dynamics and modeling
   • Habitat & ecosystem restoration, dam removal

Prof. Paul Kemp — University of Southampton

When posed the question What is Ecohydraulics?, my immediate, almost instinctive, response is to recite a definition. There have of course been lots of previous attempts to do this, including by John Nestler and others in 2008, Steven Rice and others in 2010, and Ian Maddock and others in 2013. Indeed, Vlad Nikora raised this question at a really interesting presentation given at a workshop held in Loughborough over a decade ago. Based on these discussions, Chris Katopodis and I provided a definition in the editorial of the first issue of the Journal of Ecohydraulics in 2016: “Ecohydraulics is the synthesis of ecology and hydraulics, and as a discipline exists at the interface of the two. This definition can be broadened more generally to include other disciplines that are related to aquatic biology (such as physiology, population dynamics, and evolution), engineering (e.g. fluid mechanics and hydrodynamics), and other physical sciences (e.g. geomorphology and hydrology).” Since that time, however, I have often questioned just how useful such a definition is. What I have found really impressive about the International Symposia on Ecohydraulics is the mix of people, disciplines, cultures, and topics presented and discussed. There is a recognition that the subject is innately interdisciplinary, and that the boundaries are somewhat fuzzy and dynamic, and may change over time. In a world where the global challenges we face are invariably complex, should we really worry too much about the significance of a definition? Perhaps we should “let it ride” a bit, and be prepared to go with the flow and see where we end up; it most certainly will be more exciting that way, and perhaps more productive!

Prof. Markus Noack — Hochschule Karlsruhe

From my perspective, ecohydraulics is a term that developed over time. In the beginning the focus was purely on the coupling of hydraulics with biota (the first interdisciplinary research and the first attempts to bridge the gaps between engineers and biologists/ecologists). Nowadays, the term “ecohydraulics” is used as a synonym including all interactions between abiotic factors and biota in fluvial systems, including many different sub-disciplines. To me, the term ‘ecohydraulics’ stands for linking different disciplines to achieve a common goal: conservation and sustainable development of vital river and riparian systems.

Dr. Valerie Ouellet — University of Birmingham

Ecohydraulics aims to unravel the links between stream/river ecology (e.g. fish physiology and habitat in my case) and the hydrodynamics or related abiotic factors (e.g. water temperature). To do so, we also need to look at other variables like the influence of stream geomorphology, local and regional climate, anthropic activities, riparian vegetation, etc. — so it is truly an interdisciplinary science!

Dr. Xiaodong Qu — China Institute of Water Resources and Hydropower Research

Ecohydraulics is still not a well-known discipline in China, although it has been used to reduce the effects of dam construction, control the movement of contaminants in water, and manipulate the habitat restoration of aquatic organisms for the purposes of ecosystem protection and restoration. During the past decades, ecohydraulics has gradually dominated and replaced environmental hydraulics. Almost all the ecohydraulics scientists in China originated from hydraulics or hydrology and had a very weak background in stream ecology. So, from my personal point of view, we need to expand the basic knowledge of stream ecology among ecohydraulics scientists, and — most importantly — we should attract young students with a background in ecology to study ecohydraulics. My working definition: the ecohydraulics studies the relationships between hydraulics and the aquatic ecosystem, which includes not only the ecosystem structures and functions, but also the environments supporting the process and succession of ecosystems.

Prof. Gregory Pasternack — University of California, Davis

I developed a definition for my pair of ecohydraulics annotated bibliographies published by Oxford Press in 2019, so I’ll quote from those: Ecohydraulics is the study of the mechanisms that explain hierarchically nested aquatic and riparian biotic phenomena. Mechanisms are sequential actions that can be physical, biological, or an interaction between the two. Biotic phenomena consist of individual, population, and community-level conditions, behaviors, and interactions. Hierarchical nesting means that phenomena are present across a wide range of spatial scales: from the smallest fluid-continuum scale to the scale of the entire Earth. Under this definitional framework, and given the widespread occurrence of water on Earth, ecohydraulics is the “proximal” science mediating the influence of “distal” landscape drivers (e.g., climate, geology, and topography).

Prof. Rafael O. Tinoco — University of Illinois at Urbana-Champaign

Ecohydraulics is a rapidly developing interdisciplinary field which examines the interactions and feedbacks between hydrodynamic, ecological, and geomorphic processes in natural and altered waters, in order to develop better approaches to monitor, manage, and restore the functions of aquatic ecosystems.