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Renewable and Sustainable Energy Reviews 187 (2023) 113729
Available online 16 September 2023
1364-0321/© 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
100 key questions to guide hydropeaking research and policy
D.S. Hayes
a
,
*
, M.C. Bruno
b
, M. Alp
c
, I. Boavida
d
, R.J. Batalla
e
,
f
, M.D. Bejarano
g
, M. Noack
h
,
D. Vanzo
i
, R. Casas-Mulet
j
,
k
,
l
, D. Vericat
e
,
m
, M. Carolli
n
, D. Tonolla
o
, J.H. Halleraker
p
,
M.-P. Gosselin
q
, G. Chiogna
r
,
s
, G. Zolezzi
t
, T.E. Venus
u
,
**
a
University of Natural Resources and Life Sciences, Vienna, Department of Water, Atmosphere and Environment, Institute of Hydrobiology and Aquatic Ecosystem
Management, Vienna, Austria
b
Research and Innovation Centre, Fondazione Edmund Mach, San Michele all’Adige, Italy
c
RiverLy, INRAE, Villeurbanne, France
d
CERIS, Civil Engineering Research and Innovation for Sustainability, Instituto Superior T´
ecnico, University of Lisbon, Lisbon, Portugal
e
Fluvial Dynamics Research Group (RIUS), University of Lleida, Lleida, Spain
f
Catalan Institute for Water Research (ICRA), Girona, Spain
g
Natural Systems and Resources Department, Universidad Polit´
ecnica de Madrid, Madrid, Spain
h
Institute of Applied Research, Karlsruhe University of Applied Science, Karlsruhe, Germany
i
Laboratory of Hydraulics, Hydrology and Glaciology, ETH Zürich, Zürich, Switzerland
j
Chair of Hydraulic and Water Resources Engineering, Technical University of Munich, Munich, Germany
k
Aquatic Systems Biology Unit, School of Life Sciences, Technical University of Munich, Freising, Germany
l
Department of Infrastructure Engineering, The University of Melbourne, Melbourne, Victoria, Australia
m
Forest Sciences and Technology Centre of Catalonia, Solsona, Spain
n
Energy Systems, SINTEF Energy Research, Trondheim, Norway
o
Institute of Natural Resource Sciences, Zurich University of Applied Sciences, W¨
adenswil, Switzerland
p
Department of Civil and Environmental Engineering, Norwegian University of Science and Technology, Trondheim, Norway
q
Department of Aquatic Biodiversity, Norwegian Institute for Nature Research, Trondheim, Norway
r
Technical University of Munich, Chair of Hydrology and River Basin Management, Munich, Germany
s
University of Innsbruck, Innsbruck, Austria
t
Department of Civil, Environmental and Mechanical Engineering, University of Trento, Trento, Italy
u
Research Group of Bioeconomy Economics, University of Passau, Passau, Germany
ARTICLE INFO
Keywords:
Renewable energies
Sustainable development
Flow ramping
Load following
Water resources management
Science-policy interface
Funding
Applied science
Delphi method
Horizon scan
ABSTRACT
As the share of renewable energy grows worldwide, exible energy production from peak-operating hydropower
and the phenomenon of hydropeaking have received increasing attention. In this study, we collected open
research questions from 220 experts in river science, practice, and policy across the globe using an online survey
available in six languages related to hydropeaking. We used a systematic method of determining expert
consensus (Delphi method) to identify 100 high-priority questions related to the following thematic elds: (a)
hydrology, (b) physico-chemical properties of water, (c) river morphology and sediment dynamics, (d) ecology
and biology, (e) socio-economic topics, (f) energy markets, (g) policy and regulation, and (h) management and
mitigation measures. The consensus list of high-priority questions shall inform and guide researchers in focusing
their efforts to foster a better science-policy interface, thereby improving the sustainability of peak-operating
hydropower in a variety of settings. We nd that there is already a strong understanding of the ecological
impact of hydropeaking and efcient mitigation techniques to support sustainable hydropower. Yet, a disconnect
remains in its policy and management implementation.
1. Introduction
Hydropeaking has been receiving increased attention [1–4].
* Corresponding author.
** Corresponding author.
E-mail addresses: daniel.hayes@boku.ac.at (D.S. Hayes), Terese.Venus@uni-passau.de (T.E. Venus).
Contents lists available at ScienceDirect
Renewable and Sustainable Energy Reviews
journal homepage: www.elsevier.com/locate/rser
https://doi.org/10.1016/j.rser.2023.113729
Received 4 March 2023; Received in revised form 2 August 2023; Accepted 6 September 2023
Renewable and Sustainable Energy Reviews 187 (2023) 113729
2
Hydropeaking – rapid and frequent changes in river ow to optimize
hydropower operation – is a phenomenon observed globally, primarily
associated with large power-generating (storage) dams operated in
load-following mode (Fig. 1). Hydropeaking is widely discussed in the
context of climate change and the rise of renewables to integrate energy
production and demand in the power grid [5,6], and to increase exi-
bility in the energy system [7,8]. However, the ecological impacts of
hydropeaking, including reduction of species abundance [9] and
biomass [10,11], lowered primary production [12], and altered assem-
blages of river fauna and ora [13–15], are of great concern [16–18].
Despite research efforts, many knowledge gaps still need to be addressed
to encourage wide-scale implementation of mitigation measures, of
which only some examples exist to date [17,19–22].
The current freshwater biodiversity crisis demands that we solve
central knowledge gaps to expedite effective policy and management
efforts [25–27], particularly given a renewed commitment to hydro-
power as a green, sustainable, and low-carbon energy source [28–31].
So far, hydropeaking mitigation actions are primarily developed at
smaller (national) scales, such as in the Swiss or Italian alps [21,22,32].
To support the wide-scale establishment of targeted mitigation and
conservation frameworks in hydropeaked rivers, scientists must tackle
the most urgent knowledge gaps for policy and management decisions
[26,33]. As these high-priority questions related to hydropeaking have
yet to be dened [34], we identify 100 key questions for hydropeaking
research.
The 100 questions horizon scan exercise is a popular strategy to
identify and prioritize research needs. The 100 questions approach is a
process of identifying emerging issues or questions that, if answered,
have the potential to impact decision-making in the respective sector
[35–38]. Over the last 20 years, this approach has been successfully
conducted in many elds, including landscape restoration [39], forestry
[40], agriculture [41], urban stream ecology [42], microbial ecology
[43], hydrology [44], conservation physiology [45], sh migration [46],
recreational sheries [47], and smart (energy) consumption [48,49].
This integrative approach seeks to incorporate and dialogue with
various stakeholders, including practitioners, legislators, and re-
searchers, to rene and distill a set of questions until 100 high-priority
questions emerge [35–37].
This research targets three main types of actors: First, we address
policymakers and practitioners in public, private, and non-prot orga-
nizations as addressing their questions can meet their information
needs. Second, funders of research must better understand which broad
themes to prioritize. Third, researchers must know which questions
policymakers consider most important [36].
This study identied a list of policy-relevant and high-priority
questions in the hydropeaking research and management eld. We
created an online survey distributed globally to individuals and orga-
nizations in science, practice, and policy to solicit questions. The initial
list of questions was then distilled in a participatory follow-up expert
study [36,37], yielding the top 100 research questions for the eld of
hydropeaking presented in this work. This consensus list of high-priority
questions shall inform and guide researchers in focusing their efforts on
tackling policy and management needs [50], thereby improving the
sustainability of peak-operating hydropower production.
2. Methods
In this study, we identied 100 high-priority questions in the eld of
hydropeaking research, policy, and management using the Delphi
method for expert consensus. The Delphi method is a structured
communication approach used to gather and rene the opinions of a
group of experts on a specic topic [51,52]. It involves a series of rounds
in which the experts provide their opinions, and the results are analyzed
List of abbreviations
EU European Union
SDGs United Nations Sustainable Development Goals
WFD EU Water Framework Directive
Fig. 1. Global map of larger dams used for hydroelectricity production and the share of renewable electricity production per country. Dams include those from the Global Dam
Tracker (GDAT) database [23] with ‘hydroelectricity’ registered as the main purpose or additional use, ltered by a capacity of >10 MW and a head of >30 m. It can be
expected that many large power-generating (storage) dams are operated in peaking mode at least part of the time. A detailed overview of hydropeaking dam distribution,
however, is still missing. Renewables include electricity production from hydropower, solar, wind, biomass and waste, geothermal, wave, and tidal sources [24].
D.S. Hayes et al.
Renewable and Sustainable Energy Reviews 187 (2023) 113729
3
and summarized. The summary is sent back to the experts for review and
comments. This process is repeated until a consensus is reached or until
the experts’ opinions converge [51,52]. The Delphi method is often used
to make informed decisions and forecast future developments in elds
such as public policy [53], management [52], industry [54], and energy
consumption [49].
The implementation of the Delphi expert study was divided into
three steps (Fig. 2): (1) we conducted a global call to gather research
questions. The solicited questions were then (2) categorized, thema-
tized, and consolidated. Finally, (3) expert rating identied the top 100
questions.
In the rst step, we called for questions by inviting experts (i.e.,
policymakers, hydropower managers, researchers) from various key
disciplines or sectors (for example, government, non-governmental or-
ganizations (NGOs), industry, academia) and geographic locations (i.e.,
from all continents where hydropower is used; Fig. 1) to contribute their
key questions in the eld of hydropeaking [47]. We gathered the
questions through an anonymous online survey. The baseline question
was: “What are the unanswered research questions in the eld of
hydropeaking?” [43]. We encouraged participants to list as many as they
feel are relevant.
In addition to formulating questions, surveyors were also asked to
disclose information about their expertise (topic and years of experi-
ence), occupation, and country of work. The questionnaire was available
in six different languages (English, Spanish, French, German, Italian,
and Portuguese), following the suggestion of Cooke et al. [38].
The call to the online survey was distributed through means of
circulating emails, newsletters, professional societies, social media
(Twitter and LinkedIn), and key regional informants (for example, hy-
dropower managers). This global distribution was largely based on the
contacts and efforts of the Hydropeaking Research Network (HyPeak
[55]) and the further solicitation of survey participants to their col-
leagues and networks. The online survey ran from December 2021 to
February 2022.
In the second step, the questions were (i) translated into English (if
necessary), (ii) rened and rephrased (if necessary), and (iii) sorted into
sub-categories within eight major topics: (a) hydrology, (b) physico-
chemical properties of water, (c) river morphology and sediment dy-
namics, (d) ecology and biology, (e) socio-economic topics, (f) energy
markets, (g) policy and regulation, (h) management and mitigation
measures (Table 1). In addition to the survey outcomes, (iv) the
hydropeaking questions posed by Hayes et al. [17] and Alp et al. [55]
were integrated into the list. Finally, (v) any duplicate questions were
removed due to redundancy.
The third and nal step aimed to winnow and rene the questions by
conducting formal voting in the form of a Delphi study. We distributed
the nal list of questions to all survey participants who indicated their
willingness to contribute to such a follow-up expert study. Each expert
could decide on which and how many topical groups they wanted to join
[41]. The experts had to rank each question within a topical group
Fig. 2. Schematic owchart providing an overview of the step-wise implementation of the Delphi method for this study.
D.S. Hayes et al.
Renewable and Sustainable Energy Reviews 187 (2023) 113729
4
according to (i) the importance in knowledge gain for hydropeaking
management, (ii) how well it has already been studied (i.e., the question
should not have already been answered), and (iii) how feasible it is to
answer the respective question through a realistic research design of
spatial and temporal scope [36]. The ranking scale ranged from 1 to 10,
whereby 1 indicates the least and 10 the highest levels of importance,
already existing research, or study feasibility. Expert group members
were also invited to revise and rephrase questions where they felt rele-
vant or leave comments [37,56].
Essential questions are dened as those questions that, if answered,
would have the greatest impact on global hydropeaking research and
policy. For each question, we calculated the mean score of the expert’s
evaluation regarding the three evaluation categories mentioned above,
including the percentage of experts that evaluated the question. We then
combined the three values per question into one ranking index (1–30) by
summing up the means (the values regarding how well the respective
question has been studied were re-coded by inverting the order).
Furthermore, the percentages of expert participation were combined
(0–300). As selection criteria, we used the ranking index to sort the
questions in descending order, picking the top 100 but excluding ques-
tions with an expert participation score ≤150 across the three questions
(i.e., importance, how well studied, feasibility). In cases where questions
that the experts marked as redundant ended up in the 100 questions list,
these were combined into one question by expert focus groups. Then the
next question according to the ranking index order was added to have a
total number of 100 questions. This process was repeated as often as
needed.
The questions were tested against the following further criteria for
the identication of properly formulated scientic questions: (i) ques-
tions should have a factual answer that is not based on personal opinions
or beliefs, (ii) they should be specic rather than covering a general
topic area, (iii) they should not be answerable with “it all depends”, (iv)
unless they are questioning a specic statement, they should not be
answerable with a simple “yes” or “no” (for example, not “is the miti-
gation option X better than Y?“), (v) when related to impact and inter-
vention, they should include a subject, an intervention, and a
measurable outcome [36,41,56]. In cases where a question was removed
due to one of these criteria, the next question according to the ranking
index was selected and added to the nal list (as in the previous steps).
This stepwise approach to winnowing and rening gathered
questions through a participatory exercise eventually yielded what we
consider to be the top 100 research questions of relevance to hydro-
peaking research and policy.
3. Results
3.1. Round 1 – global call for gathering research questions
In the rst round of the Delphi study, the sample included 220 re-
spondents who submitted research questions (out of 2879 survey clicks).
Respondents had an average experience of 18.7 years in their eld of
work and 9.8 years in hydropeaking. The participants had their working
base in all continents where hydropower is used. Of the experts who
disclosed their primary working areas (n =212), the majority of par-
ticipants work in Europe (n =173), Asia (n =13), North America (n =
11), Africa (n =8), South America (n =5), and Australia and Oceania (n
=2). The seven most prevalent countries represented were Switzerland
(n =43), Italy (n =26), Austria (n =24), Germany (n =17), Spain (n =
16), Portugal (n =13), and France (n =11) (Figure S1).
Nearly half of the respondents had a background in research (n =
103), followed by government/authority (n =37), hydropower man-
agement (n =24), and NGOs (n =20). Other stakeholders included
individuals from the eld of consulting (n =13), energy provision (n =
10), sheries (n =9), and others (n =4) (Figure S1).
In total, 432 unique research questions associated with eight topical
areas could be identied (Table 1; Fig. 3). Of the 220 respondents, 48
indicated their willingness to contribute to the follow-up expert study to
rate the gathered questions in order to identify the most relevant ones.
3.2. Round 2 – expert rating to identify high-priority questions
In total, 29 experts contributed to the next round of rating the
questions (Table 1). The majority of these experts were researchers (n =
24). Some work in the government/authority sector (n =4) or hydro-
power management (n =1). The experts’ working locations represent all
ve continents mentioned above (up to three countries per expert), the
largest share work in Europe (Figure S2).
The experts were presented with the topical groups shown in Table 1.
They could join as many of these topics as they identied with, resulting
in 55 total expert responses (Figure S2).
3.3. One hundred key questions in hydropeaking
The step-wise implementation of the Delphi method identied the
top 100 questions in hydropeaking from 432 original questions
(Table 1). Fig. 3 provides a graphical representation of this process,
showing which original questions were selected, combined, split, or not
selected by the experts. We assigned questions to thematic sub-
categories for grouping irrespective of their association to one of the
eight topical categories.
The following sections present the nal 100 questions list organized
by category. Each category is prefaced with a brief introduction. The
order of questions does not reect a priority as they are sorted according
to theme.
3.3.1. Hydrology
From a hydrological perspective, hydropeaking is a phenomenon
that has been addressed by considering multiple spatial and temporal
scales [57,58]. Time series of river discharge have been analyzed at
single gauging stations [59], in a network of gauging stations belonging
to the same catchment [60–63], and also at larger regional scales [64,
65]. The focus of these studies was mainly the identication of changes
in the hydrological regime due to the construction and operation of
hydropower infrastructures, and the problem was addressed at temporal
scales ranging from minutes to years, showing how the temporal dy-
namics of hydropeaking ow regimes differ from natural ones [66,67].
Table 1
Identied hydropeaking topics and the total number of questions classied by
each topic before and after the rating approach, and the number of experts
involved in ranking questions in each topic.
Topic No. of
original
questions
No. of questions
included in the
nal list
No. of experts
involved in the
ranking
Hydrology 50 15 9
Physico-chemical
properties of water
19 4 6
River morphology
and sediment
dynamics
47 13 8
Ecology and biology 140 34 13
Socio-economic
topics
a
28 6 5
Energy markets
a
27 9
Policy and regulation 47 8 6
Management and
mitigation
measures
74 11 8
Total 432 100 29
b
/55
c
a
Socio-economic topics and energy market questions were ranked by the
same experts.
b
Number of experts involved in the ranking of questions.
c
Total count of expertise involvement, including certain experts who joined
multiple topics and were thus counted for each.
D.S. Hayes et al.
Renewable and Sustainable Energy Reviews 187 (2023) 113729
5
Catchment-scale hydrological models that aim to reproduce the ef-
fect of hydropeaking often use daily time steps and are, therefore, unable
to address sub-daily streamow variability, particularly when the
research question focuses on climate change projections and hence long
simulation times [68]. Also, coupling energy production and hydro-
power generation mechanisms with process-based models at multiple
spatial and temporal scales remains challenging. However, machine
learning methods could contribute to overcoming this limitation [69].
A hydrological approach to studying hydropeaking also requires
considering the effects of river stage uctuations on surface water-
groundwater interaction. In this case, several authors have acknowl-
edged the importance of investigations at the local scale [70] and for
river reaches [71,72]. When designing and implementing suitable
restoration measures, it is necessary to consider the typical
spatio-temporal interaction among the different hydrological processes.
Further, attenuation and ramping rates inuence morphological and
ecological impacts within the river system [73].
The following questions demonstrate the complexity of processes
linked to water storage and release effects for hydropower generation
and the importance these have on the hydrological cycle. Adequate
monitoring, modeling, and mitigation will require developing new tools
that embrace this multiscale aspect.
1. How does the temporal resolution of streamow (or river stage)
data affect assessments of hydropeaking hydrology?
2. What spatiotemporal variations of ow velocity, water depth,
and wetted width can be observed in hydropeaking rivers?
3. How can zero-ow events occurring in-between hydropeaks,
when hydropower stations are on hold due to low energy demand
or low electricity prices, be adequately measured?
4. How does base ow duration and magnitude between hydro-
peaks differ from natural ow uctuations?
5. How does peak ow duration and magnitude in rivers affected by
hydropeaking differ from natural ow uctuations?
6. Which ow quantiles can be used to standardize global assess-
ments of hydropeaking frequency?
7. How can improvements in remote/local sensing techniques,
modeling tools, and smart energy grids allow for more dynamic
(i.e., real-time) release strategies to minimize hydropeaking im-
pacts while answering energy demand?
8. How do hydropower cascades affect hydropeaking, including the
potential amplication of hydropeaking waves at different ow
conditions?
9. How does hydropeaking hydrology change over time in relation
to energy markets?
10. How does hydropeaking affect natural ice regimes?
11. How do environmental ows affect characteristics of hydro-
peaking hydrology, such as the rate of change, ow ratio, peak
amplitude, or between-peak ow magnitude?
12. How do characteristics of the hydropower station, such as
reservoir size and location or operational rules, inuence the
degree of hydrological alteration?
Fig. 3. Alluvial plot showing how the eight topics (on the left) are linked to thematic sub-categories (on the right, sorted alphabetically; all categories with ≤5
questions were added to “Other”). The line colors indicate if the respective original question (n =432) was selected, combined, split, or not selected for the nal 100
questions list.
D.S. Hayes et al.
Renewable and Sustainable Energy Reviews 187 (2023) 113729
6
13. What are the implications of non-stationary hydrological regimes
(for example, due to climate change or natural/anthropogenic
forcing mechanisms) on hydropeaking hydrology?
14. How do different morphological rehabilitation measures dampen
the hydrological effects of ow or river stage uctuations by
impacting ow retention of the hydropeaking wave?
15. How will climate change alter hydropeaked rivers, considering
both changes in the management of hydropower systems and the
hydrological cycle?
3.3.2. Physico-chemical properties of water
River damming creates lentic ecosystems that affect physical,
chemical, and biological processes and characteristics in the down-
stream reaches [74,75]. Accounting for the sub-daily alterations of
physical (for example, thermopeaking, temperature [76–78]) and
biochemical (for example, gas supersaturation, water quality [79])
processes and patterns related to hydropeaking adds challenge to their
further understanding. It may require multi-parametric and
high-frequency eld sampling, but also the modeling of biogeochemical
processing occurring in the upstream reservoirs and the downstream
sections [80] as well as changes in the interaction with the hyporheic
zone [81] and the aquifer [82].
Some of the most frequently studied physical alterations associated
with hydropeaking are the sharp and intermittent alterations of river
thermal regime associated with hydropeaking, so-called thermopeaking
[76–78]. The general role of damming and related hydropower opera-
tions on river biogeochemistry, including nutrient and carbon cycling,
has been studied [83,84]. However, specic studies and analyses of the
effects associated with hydropeaking are lacking, although in-
vestigations on how hydropeaking affects the dynamics of dissolved
gasses have been growing in recent years. Pulg et al. [79] provided
evidence of gas (nitrogen) oversaturation (“saturopeaking”), while
Calamita et al. [85] shed light on the hydropeaking effects on carbon
dioxide uxes (“carbopeaking”). The effect of river uctuations on ow
exchanges with the aquifer and solute transport has also been investi-
gated at multiple spatial and temporal scales [86,87].
Despite growing attention, the short and long-term consequences of
physico-chemical alterations on the downstream river and aquifer eco-
systems are still partially overlooked. Currently, the most remarkable
knowledge gaps, as indicated by the following questions, refer to the
understanding and quantication of biogeochemical alterations at the
temporal scales at which hydropeaking occurs.
16. How does hydropeaking affect the water quality of the down-
stream river sections when released from eutrophic reservoirs?
17. How does hydropeaking (and, if co-occurring, thermopeaking)
affect daily and seasonal dynamics of dissolved gasses (for
example, oxygen, carbon dioxide, methane)?
18. How does hydropeaking inuence the interdependent processes
of nutrient cycling and their downstream transport (nutrient
spiraling)?
19. To which extent are physical (hydraulic and thermal)
hydropeaking-driven alterations more (or less) relevant than
chemical (water quality) ones as environmental stressors?
3.3.3. River morphology and sediment dynamics
Hydropeaking operations signicantly impact the morpho-dynamic
processes of river systems [3]. The rapid oscillations of ow generated
by hydropeaking directly interfere with rivers’ natural ow and asso-
ciated sedimentary regimes, and, in turn, with their ecological func-
tioning [88–90]. The high instability of channel habitats is a main
limiting factor for freshwater ecosystem functionality because hydro-
peaking modies ow hydraulics, the sedimentary structure of the
riverbed, sediment transport, and habitat availability [91–93]. The
morphological and sedimentary dynamics of river systems are occa-
sionally affected by the joint effect of reservoir sedimentation and
hydropeaking, a combination that may exacerbate sediment decit and
associated effects such as riverbed incision and armoring [94].
Overall, sediments in rivers experience cycles of entrainment,
transport, and deposition. Floods are major natural disturbances that,
together with anthropogenic impacts, control or modify such cycles
[95]. Particle mobility depends on bed structure, and ultimately, they
are both strongly inuenced by the upstream sediment supply in the
system. Therefore, understanding the frequency and magnitude at
which water ow exceeds the sediment mobility threshold is funda-
mental to correctly characterize such processes [96].
Hydropeaking-affected reaches, in particular, where the ow is arti-
cially increased and the upstream supply of sediments has been cut off,
frequently experience processes of full or partial bed mobility driven by
the entrainment of sediments [97]. This may generate a sedimentary
imbalance that can affect various ecological processes (for example, sh
spawning, invertebrate refuge). The sediment decit may be mitigated
through the regular release of natural-like oods providing sediments
[98] or augmentation of key sediment fractions, improving habitat
availability and maintenance [99].
Although a few studies focused on the morphological impacts of
hydropeaking, the following questions demonstrate substantial knowl-
edge gaps in the eld of morphological and sedimentary processes at
various spatial and temporal scales.
20. How are sediment depletion (removal of ecologically valuable
gravel) and hydropeaking related to each other?
21. How does hydropeaking affect ne sediment dynamics and
related habitat properties?
22. How can the impacts of hydropeaking and the impacts of dams on
morphology and sedimentary processes be distinguished?
23. How does hydropeaking affect the riverbed composition in terms
of ne sediment content, sorting processes and particle size
distribution?
24. How does hydropeaking affect mobility thresholds and sediment
transport processes compared to those found in non-regulated
rivers?
25. How does hydropeaking affect riverbed stability and bed
armoring?
26. What is the role of tributaries in mitigating sediment decit in
hydropeaked rivers?
27. What are short- and long-term morphological consequences of
hydropeaking?
28. How does hydropeaking alter morpho-dynamics in different river
types?
29. What are the timescales of aquatic habitat (wetted area) persis-
tence during turbine shutdown events in hydropeaked rivers?
30. What are the key hydropeaking ow characteristics (for example,
magnitude, frequency, ramping rate) that lead to changes in river
morphology and sediment transport?
31. What are – from a morphological perspective – the spatial scales
(for example, patch, reach, segment) that are most affected by
hydropeaking?
32. What ows and sediments need to be released from dams to
maintain or restore the sediment dynamics in hydropeaked
rivers?
3.3.4. Ecology and biology
Water ow is a key driver of physical and ecological processes within
rivers [100]. Therefore, any change to the natural ow regime will affect
aquatic habitats, organism communities, and ecological processes in
river systems [101–103]. The rapid and articial ow uctuations
associated with hydropeaking operations affect riverine biota (fauna
and ora) directly and indirectly. Direct effects include organism
displacement, involuntary drift, and stranding, often leading to deteri-
oration and death [104–109]. Indirect effects are linked to changes in
river hydro-morphology with consequences for habitat quality and
D.S. Hayes et al.
Renewable and Sustainable Energy Reviews 187 (2023) 113729
7
availability and include alterations of biochemical processes and biotic
interactions [13,110].
The study of ecological and biological impacts of hydropeaking has
focused to a large extent on responses of certain life stages of sh and
macroinvertebrates [111–114]. Research has recently also been con-
ducted on riverine plants and ow-vegetation relationships [2,108,115].
In contrast, other life stages and organism groups, such as biolm and
microbial communities, craysh, and bivalves, have received little or no
attention [116] despite being important river ecosystem components.
The same goes for terrestrial biota that depend upon river ecosystems for
their life cycle (for example, birds). Also, hydropeaking effects on the
propagation and establishment of non-native species in aquatic and ri-
parian environments are hardly studied.
Moreover, hydropeaking effects on river connectivity, including in-
teractions with other related factors, in its different dimensions (i.e.,
longitudinal, lateral, vertical, temporal [117]) are largely unknown. The
shallow river margins and sediment bars are particularly affected by
hydropeaking as articial ow uctuations with oscillations between
dry and wetted conditions create ‘articial intertidal zones’ [118]. These
oscillations affect the groundwater table and riparian environments [71,
72], as well as the lateral instream habitat connectivity [119,120],
including links between aquatic and terrestrial environments. The
hyporheic zone, which largely relies on intact vertical connectivity, is
important for biochemical and biotic processes [121]. Vertical connec-
tivity in hydropeaked rivers can be affected directly, for example, by the
propagation of the hydropeaking wave into the shallow aquifer [122], or
indirectly, for example, by altered sediment dynamics and associated
clogging processes [91]. The impacts of these hydropeaking-driven
connectivity alterations on biota are largely unknown.
To achieve sustainable management of hydropeaking and the con-
servation of river ecosystems, we must improve understanding of how
the interrelations of hydropeaking, thermopeaking, and saturopeaking
impact ecological processes in rivers [79,123,124]. This includes iden-
tifying the time scales over which biotic communities can adapt to these
changes. Additionally, given the range of hydrological variables
impacted by hydropeaking, it is crucial to identify which variables are
primarily responsible for the negative effects on biological communities
[11,13,14]. This information is essential for exploring potential miti-
gation strategies through direct mitigation measures [17].
Finally, hydropeaking is not the only anthropogenic stressor that
rivers face, as they are also frequently affected by various other human-
driven impacts, such as channelization [11], eutrophication, pollution,
the spread of exotic species or other types of ow modication (for
example, water abstraction) [125]. Therefore, in order to ensure the
effective conservation and management of riverine ecosystems, it is
essential to consider hydropeaking in this multiple-stressor context and
examine how the combinations of stressors, as well as their seasonal and
geographic variations, will inuence the resilience and adaptability of
riverine communities [126], particularly in light of climate change.
33. How does hydropeaking affect riparian or gravel bar invertebrate
communities?
34. How does hydropeaking affect the nutritional quality of
periphyton?
35. What are the effects of hydropeaking on the structure and
biomass of algal and microbial communities in the biolm?
36. How does hydropeaking (and, if co-occurring, thermopeaking)
impact river biochemical processes (for example, microbial
metabolism, nutrient spiraling) in rivers and/or their receiving
water bodies (i.e., lakes, estuaries)?
37. Which role does the duration of baseow between hydropeaks
play in structuring biological communities?
38. To which extent do tolerance, acclimation, or habituation allow
aquatic species to live in regularly-occurring hydropeaking
conditions?
39. How do the ecological effects of very frequent, low-intensity ow
uctuations (‘hydrobrillation’) differ from those of regular, but
less frequent high-intensity hydropeaking?
40. To which extent do single high-ow events differ from reoccur-
ring hydropeaks in determining habitat dynamics and biotic
community composition?
41. To which extent do the effects of irregular (seasonal) hydro-
peaking differ from regularly (year-round) occurring hydro-
peaking in structuring habitat dynamics and biotic communities?
42. What are the most sensitive biological metrics to assess the
ecological effects of hydropeaking on the environment?
43. How does hydropeaking affect the riparian habitat and which
metrics can we use to measure the impacts?
44. How does hydropeaking affect crustaceans, such as native and
invasive craysh?
45. How does hydropeaking affect bivalves?
46. How does the temperature of the water released during hydro-
peaking affect riverine ora and fauna in different seasons?
47. How does hydro- and associated thermopeaking affect different
life cycle stages of aquatic organisms and their populations?
48. What are the thresholds above and below which thermopeaking
causes measurable harm for different life stages of aquatic
organisms?
49. How does the interaction of thermopeaking and climate change-
related thermal impacts affect different life cycle stages of aquatic
organisms?
50. How does hydropeaking affect functional diversity of
macroinvertebrates?
51. Through which life-cycle stages does hydropeaking have the
greatest impact on macroinvertebrate population structure and
dynamics?
52. How does hydropeaking affect aquatic-terrestrial functional links
of invertebrates?
53. Which are the ecological effects of hydropeaking on different
time-scales and how do they interact?
54. How does hydropeaking affect sh emergence from the gravel
substrate?
55. How does hydropeaking alter riverine lateral connectivity and
affect functioning shoreline habitats?
56. How do sh relocate (laterally and longitudinally) during
hydropeaking and to what spatial extent do hydropeaking effects
continue to inuence their movement?
57. What are the broader ecological effects of implementing the
‘emergence window’ approach proposed as a mitigation option
(Hayes et al., 2019, Sust. 11(6), 1547) to safeguard sh
populations?
58. How does hydropeaking affect riparian and aquatic birds, such as
gravel nest builders or waterfowl?
59. Does hydropeaking facilitate invasion of non-native species or-
ganisms and, if so, by which mechanisms?
60. Does hydropeaking facilitate out-competition of native species by
non-native ones? If so, by which mechanisms?
61. What are the combined ecological effects of cascading peak-
operating hydropower plants?
62. What are the combined effects of general (for example, chan-
nelization, pollution) and hydropeaking-specic (for example,
saturopeaking, thermopeaking) stressors on populations of
aquatic biota in hydropeaked rivers?
63. What is the role of river and tributary connectivity in determining
the ecological condition of hydropeaked rivers?
64. How will climate-driven changes in the hydrological regime
affect ecosystems already impacted by hydropeaking?
65. What role does long-lasting habitat degradation (for example,
from a decadal perspective) play in determining ecological
community structure of hydropeaked rivers?
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66. To what extent does the impact of hydropeaking differ between
scales and river types, for example, pertaining to different ow
regimes (glacier-melt vs. non-glaciated regimes; temperate,
tropical vs. semi-arid), river morphologies and sedimentary
structures (straight vs. braided rivers; armored vs. loose (mobile)
bed rivers), groundwater inuence (for example, hyporheic vs.
surface-dominant ows), or biocoenotic regions such as sh re-
gions (headwaters vs. lowland rivers)?
3.3.5. Socio-economic topics
A common framework for categorizing socio-economic effects on the
environment is the concept of ecosystem services, which describes the
values of healthy and functioning ecosystems for humans [127]. In
particular, hydropeaking may lead to socio-economic effects in rivers on
provisioning services (for example, fewer raw materials and less water
available and, in turn, effects on livelihoods) and cultural services (for
example, recreational activities in rivers such as angling and rafting,
education, beauty, and landscape) [128,129]. In contrast to their eco-
nomic impacts on energy markets and hydropower operators, the eco-
nomic questions here focus on societal externalities, individual’s
livelihoods, and distributional issues.
On a broader level, many of the public’s perceptions and concerns
about hydropower in general are also valid for hydropeaking. These
include concerns related to increased hazards (for example, soil erosion,
ooding, landslides), destruction of changing landscapes, impacts on
livelihoods, and unequal distribution of economic benets [130,131].
Given the potential impact on recreational and livelihood activities,
public involvement and consultation may be relevant to
decision-making processes about hydropeaking mitigation.
There have been a few previous studies, which have investigated the
impact of hydropeaking on specic recreational activities such as rafting
and kayaking [132,133], proposed methods to evaluate human safety
[134], and estimated the economic value of hydropeaking externalities
[135]. However, studies on other socio-economic dimensions are scarce.
Thus, open research questions focus on the role of stakeholder engage-
ment and institutions in decision-making about hydropeaking, public
awareness and perception of hydropeaking impacts, measurements of
hydropeaking impacts on cultural ecosystem services and relevant in-
dicators, and nally, the integration of social components in the man-
agement of environmental ows.
67. What respective roles do different stakeholders and institutions
play in shaping decision-making about hydropeaking?
68. What risks to the public are associated with hydropeaking?
69. What are the public perceptions of hydropeaking and associated
(for example, thermo-, saturo-, carbopeaking) impacts and how
can they better be communicated?
70. Given the existing hydropeaking indicators for ecological im-
pacts, what are appropriate indicators for measuring the socio-
economic impacts of hydropeaking (for example, other human
water uses both consumptive and in-stream)?
71. To what extent does hydropeaking lead to cultural ecosystem
services loss?
72. How can environmental and social components be integrated in
the management of environmental ows in hydropeaked rivers?
3.3.6. Energy markets
As electricity generation from renewable energy sources constantly
grows, storage hydropower systems have gained increasing attention,
particularly given their potential to expand electricity storage capacities
[136,137]. Storage hydropower provides the needed exibility to the
power system, and pump-storage facilities even allow certain sources of
green energy to be balanced with other green energy sources [138].
Thus, hydropeaking events are projected to increase to balance power in
a grid that sees intermittent energy sources being further developed
[139].
In Europe, for example, the liberalization of the electricity markets
led to closer integration of previously separated national power systems.
Thus, the energy prices used to control storage hydropower operations
are no longer exclusively linked to national supply and demand. Instead,
spot and intraday prices are connected to supply and demand on a
continental scale [138]. The uctuations caused by variable renewable
energy sources [7] directly inuence price uctuations at the electricity
exchanges and, subsequently, peaking operations [138] as storage hy-
dropower operators can benet from short-term price volatility. This
mechanism is summed up by the merit order effect, describing the
contribution of (the cheapest currently operating) power installations on
the electricity clearing price and volume.
To date, hydropower production constitutes a valuable source of
exible energy production to regional and supra-national grids,
balancing the imperative uctuations of other intermittent energy
sources [6,7]. The detailed extent to which hydropower exibility
contributes to the reliability and resiliency of the power grid varies ac-
cording to the composition of the energy production portfolio in
different countries or regions. For example, hydropower exibility is
projected to greatly contribute to energy production in the European
Nordic countries [139].
Hydropeaking mitigation measures will affect economic revenue and
energy markets by impacting peaking operations [140]. The extent of
economic effects on energy markets depends on the measure(s),
including the extent of operational restrictions, volume and investment
of peak retention basins or diversion hydropower, or morphological
improvements [141]. The energy system may entail losses of exible
power generation capacities and volume, effects on carbon emissions in
the utility system, or require additional investments in alternative
exibility options due to operational constraints [141]. However, there
is a need to better understand the relationship between peaking
hydropower-related services (for example, grid stability, exibility),
economic prots, and environmental costs of hydropeaking, including
economic costs related to hydropeaking mitigation measures [130,142,
143]. The following questions address hydropeaking’s current
economical-environmental status at different scales.
73. To what degree does the grid stability and the production exi-
bility of different countries rely on hydropeaking?
74. As electricity markets are changing, what are the implications for
hydropeaking in both developed and developing countries?
75. How can hydropower plant turbine operations be optimized to
safeguard river ecology while maximizing revenue for the
operator?
76. How can current models that link energy demand and production
planning be improved?
77. How do hydropeaking mitigation measures affect the exibility
of peak-operating power plants?
78. How would reduced hydropeaking affect energy production and
prot for hydropower companies?
79. How much exibility loss through hydropeaking mitigation is
manageable for electricity markets?
80. How can other renewable technologies be used to support exible
energy generation and mitigate hydropeaking effects (for
example, demand side management)?
81. What is the relationship between the increase in volatile renew-
able inputs to the grid and hydropeaking?
3.3.7. Policy and regulation
Policymakers should support ecological hydropeaking practices in
light of the UN Decade on Ecological Restoration (2021–2030). A key
challenge for decision-makers is balancing increasing renewable energy
production, supporting exibility and grid security, and preserving
ecosystem services [144]. In recent years, guidelines [32,145], recom-
mendations [146,147], and evaluation approaches [148] for hydro-
peaking mitigation have received increasing attention. Although this
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9
trend can be considered positive for freshwater ecosystems, few docu-
ments are legally binding [149,150]. Some main policy approaches for
increasing sustainable hydropeaking include legal requirements,
ecosystem-based policy frameworks, and incentives (for example, the
EU taxonomy [151] or economic support of measures).
While a few countries have implemented legal requirements to
mitigate hydropeaking [144,152], many frameworks lack concrete
hydropeaking thresholds, including the EU Water Framework Directive
(2000/60/EC). Rather, the Water Framework Directive provides a
hybrid approach with multiple levels of control, one level of coordina-
tion (the river basin), and a common goal to reach the “good” ecological
status or potential. Further, the biodiversity strategy for 2030 and
REPowerEU, as part of the European Green Deal [153], including the
proposed new nature restoration law [154], will likely strengthen the
commitment to restoring the EU’s degraded ecosystems. The non-EU
country Switzerland has established some of the most specic legal
regulations regarding hydropeaking mitigation and thresholds (Swiss
Water Protection Act and Water Protection Ordinance). However, partly
diverging interests according to the Swiss legislation will need to be
fullled simultaneously (i.e., ecological impact mitigation according to
the Water Protection Act and the Water Protection Ordinance versus
increased hydropower production according to the Energy Strategy
2050). In other countries, hydropeaking mitigation is achieved indi-
rectly through, for example, the Fisheries Act or the Impact Assessment
Act in Canada [16]. Regardless of the legal framework, hydropeaking
mitigation decisions are often made on a case-by-case basis with various
environmental regulations and guidelines at different geopolitical levels
(for example, international, national, provincial, or local) [152]. For
example, operational hydropeaking rules are already included in >450
hydropower licenses in Norway [155], but compliance to reduce
ecological harm should be further improved through more dened
thresholds [156]. A river-specic approach is pivotal for appropriately
considering the local conditions (for example, climate, hydrology, river
morphology, species) of the hydropeaked watercourses [148,152] and
targeting the specic ow-alteration source in case of multiple hydro-
power plants in the basin [62]. A key uncertainty is how policy could
integrate hydropeaking mitigation into environmental ow assessments
more holistically [19,114,120,157].
On the other hand, incentives such as support schemes, feed-in-
tariffs, and green power labels can promote sustainable hydropower
and hydropeaking mitigation [130]. Sweden and Switzerland, for
example, have established a funding mechanism to compensate hydro-
power companies for production losses or other costs due to mitigation
measures. In Switzerland, measures are nanced via a tax of 0.1
cents/kWh on consumers’ electricity bills following the Swiss Energy
Act. In the USA, the Clean Water Act and the Endangered Species Act can
support restoration approaches [152]. The pressure pays principle is
quite common in Europe, so hydropower owners must pay all mitigation
measures themselves (for example, Norway). Mitigation may be done
with the support of public agencies, for example, the Water Agency
Rhˆ
one Mediterranean and Corsica in France, which covers associated
costs. Funding for hydropeaking mitigation may also be conducted by
the support of eco-labels that promote environmental measures, such as
‘Bra Milj¨
oval’ in Sweden [158].
Implementing a hydropeaking mitigation strategy into policy and
regulation programs requires a clear adaptive ecosystem-based man-
agement approach to determine, monitor, and adapt mitigation mea-
sures, if necessary [145]. Integrated policies and good governance are
needed to balance the environmental (for example, biodiversity) and
socio-economic needs (for example, energy production). Furthermore,
such an approach can foster iterative learning processes to re-evaluate
and implement inputs (for example, more effective measures from
research) and outputs (for example, monitoring of implemented miti-
gation measures) into policy and management actions, regulations, and
guidelines, thereby allowing policies to evolve with scientic knowledge
and experience from practice [144].
Key questions needing exploration regarding policy and regulation
actions include:
82. How can goals for the energy transition be harmonized with the
protection of habitats and biodiversity?
83. How can the hydro-exibility need for energy and grid security
be distinguished from the price-optimization (income) of hydro-
power operators?
84. How can hydropeaking mitigation be more consistently inte-
grated into environmental ows policy?
85. How does hydropeaking life cycle assessment perform compared
to alternative technologies such as battery storage, hydrogen, and
pressurized air?
86. How can hydropeaking assessment be standardized while still
considering local conditions of the watercourse (for example,
river morphology, species diversity)?
87. How can hydrological and hydraulic metrics (for example,
ramping rates, ow ratio, water stage, peak frequency, and
duration) and thresholds be used to update policies, legislations,
and guidelines?
88. What is the role of adaptive management in hydropeaking
regulation?
89. How can policy and regulations best implement state-of-the-art
research results and thus facilitate the learning process for
effective hydropeaking mitigation?
3.3.8. Management and mitigation measures
It is essential to have science-based frameworks and protocols to
minimize the environmental impact of exible energy production
through hydropeaking and identify relevant mitigation measures [159,
160]. Hydropeaking mitigation measures can be grouped into two broad
categories: (i) direct and (ii) indirect measures [17,18,146]. The rst
group aims to modify the peak hydrograph directly by releasing envi-
ronmental ows, modifying operational practices or building construc-
tional features (for example, retention basins, by-pass valves), leading,
for example, to lower peak amplitudes or reducing ramping rates. The
second group seeks to mitigate adverse hydropeaking effects by adapt-
ing the river morphology to improve hydraulic habitat conditions or
provide ow-refugia (shelter) for aquatic organisms [17,18,146,161].
Alternative technologies for providing exible electricity supply other
than hydropeaking operations exist and include, for example,
pump-storage facilities [162,163], energy storage vehicles [164],
inatable balloons in reservoirs, water pressure chambers, and various
types of accumulation batteries [17]. Hydropeaking operations without
impacting rivers, for example, by diverting peak ows into lakes or
fjords, is also common in some countries [155].
Although hydropeaking is a phenomenon observed worldwide and
various measures to mitigate it have been proposed in the literature [17,
18,113,152,159], comprehensive implementation of these measures is
still lacking (but see Refs. [20–22] for some case studies). Mitigation
measures are often disregarded due to their cost, technical complexity,
liability concerns, or potential impact on production and exibility
(resulting primarily from operational restrictions). Hydropeaking seems
to be less mitigated than other impacts related to hydropower (for
example, river continuity for sh) [146,165].
To ensure sustainable hydropeaking operations, it is essential to
implement best practice policies (chapter 3.3.7) that combine different
hydropeaking mitigation strategies and adopt integrated governance,
including legal requirements and incentives that support mitigation and
evidence-based adaptive management. For example, the EU taxonomy
of sustainable nance [151] is a valuable policy support emphasizing
the need for ecologically efcient mitigation of rapid ow changes
(including those from hydropeaking). This taxonomy also applies to
hydropower projects beyond Europe if the investor is based in the Eu-
ropean Union. This fact could increase the application of sustainable,
ecosystem-based management and mitigation actions globally [145,
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151]. Hydropeaking mitigation strategy should include (i) a
pre-mitigation assessment and characterization of the impacts and
pressures, (ii) a scenario assessment of the potential effects and
acceptability of different mitigation measures (feasibility study), and
(iii) a post-mitigation monitoring of the measure effectiveness [32,148,
159].
While there have been notable advancements in understanding the
ecological effects of hydropeaking based on experimental and case
studies (see Moreira et al. [152] and references therein), resulting in
targeted recommendations for species- and life-stage-specic mitigation
measures [113], examples of sustainable hydropeaking into rivers
remain scarce. The issues described above are touched upon in the
following questions.
90. What are the most effective ecological measures to mitigate im-
pacts in hydropeaked rivers?
91. How can knowledge and understanding of hydropeaking impacts
and mitigation be communicated to decision-makers?
92. What could be the role of hybrid energy systems (for example,
pumped-storage hydropower combined with solar), targeted
peaking operations and other technologies (for example, battery
storage) in hydropeaking mitigation strategies in the expected
future?
93. To what extent can increased pump-storage compensate for more
operational ow ramping restrictions?
94. What are the best practices to manage sediment regime in
hydropeaked rivers?
95. Under which circumstances should operational mitigation mea-
sures be prioritized over constructional ones or vice versa?
96. What are the most effective nature-based mitigation measures
(for example habitat structures, bedforms, natural ponds) for
hydropeaking?
97. What are the economic and system-relevant impacts of applying
the life stage-specic mitigation approach (for example, ‘emer-
gence window’ in Hayes et al., 2019, Sust. 11(6), 1547)?
98. What are the key bottlenecks for faster implementation of rele-
vant hydropeaking mitigation?
99. How should different mitigation measures be prioritized based on
cost-benet assessments?
100. How can we increase the stimulus to apply mitigation measures?
4. Discussion
Flexible hydropower production to balance intermittent electricity
(for example, wind and solar) is a key foundation in the low-carbon
energy transition and, therefore, constitutes a central aspect in
achieving multiple Sustainable Development Goals, such as SDG 7
(‘affordable and clean energy’) and SDG 13 (‘climate action’). However,
hydropeaking is also a controversial topic [166], considering that rapid
sub-daily ow uctuations due to turbine operations constitute one of
the most signicant hydro-ecological impacts on river ecosystems
downstream from dams [1,4,113], standing in contradiction to the
freshwater biodiversity targets of SDG 15 (‘life on land’). Therefore,
understanding hydropeaking drivers and their impacts, is critical to
determine adequate responses, such as best practice mitigation solu-
tions, protection measures [17,147], and policies.
This study aimed to identify emerging issues in the hydropeaking
research and management eld, resulting in a list of 100 high-priority
questions. These questions, if answered, would have a signicant
impact on global hydropeaking research and policy by impacting
decision-making in the respective sector towards a more holistic and
sustainable hydropower management.
4.1. Synthesis of emerging research needs
Hydropeaking has received considerable attention in the literature
due to its potential impacts on aquatic ecosystems [2,16,113]. However,
the research on hydropeaking has been polarized towards some aspects
(for example, stranding of salmonids [152]) while neglecting others,
leaving a row of gaps in our knowledge of hydropeaking. Here, we
present an ensemble look at the 100 high-priority questions stemming
from the Delphi expert study and discuss the broad research needs and
interdisciplinary research activities that should be developed in the
future.
This study highlights that the ecological effects of hydropeaking on
multiple organism groups, including algae and microbial communities,
crustaceans, bivalves, and birds, remain largely unexplored. Similarly,
the effects of hydropeaking on specic life cycle stages, functional di-
versity, aquatic-terrestrial links, and specic habitat types, as well as on
many key physical processes such as sediment mobility, depletion, and
transport, or changes in river substrate structure at multiple temporal
and spatial scales, are yet poorly understood. The results also pinpoint
the importance of further investigating the socio-economic impacts and
energy markets of hydropeaking, as well as implementing mitigation
measures at a larger scale and accompanying these through continuous
monitoring schemes.
The identied questions underscore the need to increase the
knowledge of hydropeaking processes by accounting for the high di-
versity of biogeographical and hydrological settings of hydropeaked
river reaches and the spatial arrangements of hydropower schemes
across single and multiple river catchments and scales, including
cascade hydropower plants, complex hydraulic schemes, and inter-basin
water transfers.
Hydropeaking patterns and impacts are likely subject to change due
to ongoing climate trends and socio-economic developments, including
a global hydropower boom, intensied water management, sprawling
urbanization, and agricultural land use expansion. These drivers often
result in further alterations in water ows and sediment transport,
deforestation, the input of pollutants and excess nutrients to freshwater
systems, and encourage the introduction of invasive species, among
others [167]. In this regard, it is imperative to consider the hydro-
peaking processes in the context of the biosphere changes mentioned
above to develop sustainable solutions for the future.
The distribution of nal questions across different categories
(Table 1) may not accurately reect the research effort required to
address them. For example, the four questions that emerged in the topic
‘physico-chemical properties of water’ may demand substantial effort to
gather environmental data, which are often already available in other
environmental elds, but are new regarding hydropeaking studies. Data
acquisition and processing will play a key role in addressing most
questions but might require new study designs and protocols for novel
parameters and a higher spatiotemporal resolution than previously
available in hydropeaking studies. Rapid advances in remote and
proximal sensing techniques and low-cost environmental sensors [168]
can potentially boost research activities in this direction [169].
Many questions can be addressed through computer modeling or
‘digital twin’ approaches. A digital twin of Earth is dened as “an
information system that exposes users to a digital replication of the
state and temporal evolution of the Earth system constrained by
available observations and the laws of physics” [170]. Traditionally,
digital replications of the law of physics for river systems have been
based on hydrological and hydraulic models, which provide approxi-
mate solutions of mathematical equations that express conservation
laws for mass, energy, or momentum. However, anthropogenic effects
on water systems [171], as well as biological feedbacks [172,173], are
crucial for replicating the behavior of these systems in reality. These
effects may even be dominant in comparison to physical processes.
Socio-economic driving forces determine water management de-
cisions, for example, those related to diversion, storage, and release of
water, and in turn, hydrogeomorphic processes may affect social and
economic dynamics [174]. Therefore, new ‘digital twin’ approaches
[170] are needed to describe the complex dynamics of river systems
D.S. Hayes et al.
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11
and their linkages with decision-making processes, which are not
controlled by the laws of physics. In this regard, articial intelligence
can be used to develop a new generation of socio-hydrological and
eco-hydraulic models that consider economic, social (behavioral), and
other datasets [175]. An example would be integrating
socio-economic drivers with time series data of river and turbine
ows, energy markets, and hydro-meteorological conditions, to name
a few [176]. Developing such innovative approaches requires a better
understanding of physical mechanisms, machine learning algorithms,
and their coupling, which can benet quantitative modeling of water
management decision-making.
4.2. A call towards mitigation
By identifying 100 high-priority questions, this study reveals the
unknown in the eld of hydropeaking research and management. The
quest for increased understanding is fundamental to science. We deem it
essential that researchers tackle these identied questions to foster even
better evidence-based decision support for maintaining socio-
ecologically sustainable river functioning [160]. Despite the variety of
open questions, it is important to note that there is already a deep un-
derstanding of hydropeaking impacts and processes that alter riverine
ecosystems [16–18], and there is no doubt that mitigation and restora-
tion efforts targeting hydropeaked rivers must be intensied in the
future to meet the UN Sustainable Development Goals.
Many adverse ecological effects are already well-dened in the sci-
entic literature (see chapters 1 and 3.3.4).There is also a portfolio of
mitigation measures (see chapter 3.3.8), which has largely remained the
same in the last four decades [16]. Nonetheless, good-practice examples
for sustainable hydropower projects are still rare [17]. Compared to
other anthropogenic impacts, such as pollution and river fragmentation,
hydropeaking and its complex hydro-morphological impacts have only
recently been included in environmental legislation and management
practices – and this only in a limited number of countries [144,152,160].
The general lack of sustainable hydropeaking case studies might be
partly due to site-specic conditions often determining mitigation ap-
proaches [148,152]. Other reasons for the scarce implementation of
measures may be the lack of ecosystem-based governance [144] and the
low public awareness of human impacts on river ecosystems and the
value of riverine biodiversity, including ecosystem services. Innovative
management frameworks [159] and guidelines for consistent prioriti-
zation approaches are needed to ensure a common understanding of
which measures to choose [146], particularly since peak-operating hy-
dropower is, to date, a key source of exible, renewable energy of
mountainous regions – at least until technological advancements create
suitable, environmentally friendly alternatives to hydropeaking.
We see an urgent need for developing conceptual and practical
management approaches and cost-benet tools for predicting the po-
tential effects of mitigation measures [140,148] and their social
acceptability across the globe. This should be achieved by implementing
evidence-based approaches grounded in existing science. These mea-
sures could be continuously updated with new insights, for example, by
integrating the answers to the 100 questions or conducting
post-measure, long-term monitoring.
4.3. Limitations
Although we intended to reach an audience as broad as possible, we
acknowledge that the input received from participants had certain
limitations in terms of geography, background, and domains of interest –
an issue also inherent to other exercises of the type [43,44,47]. Despite
making the global questionnaire available in six widely-spoken lan-
guages [38] and widely distributing it through various channels
(resulting in ca. 2900 clicks), most of the respondents from both Delphi
rounds were based in Europe and came from academia (Figures S1-S2).
Also, the proportion of original and nal questions across topics
revealed a bias towards ecology and biology as well as management
(Fig. 3). The data showed a strong positive correlation between the
number of participants in the global survey, the initial questions, the
experts involved in the ranking, and the nal list of questions, respec-
tively, for each of the eight topics (Figure S3). These limitations reect
the current situation in the eld, as most published hydropeaking
research originates from Europe (Figure S4) and focuses particularly on
sh and macroinvertebrate impacts, and partly management [2,152].
International and interdisciplinary efforts, such as those of the
HyPeak network [55], may aid in bridging the gaps described above by
encouraging global stakeholder exchange. Besides fostering an integra-
tive and interdisciplinary culture, such an expansion to a wider inter-
national effort at the science-policy interface will be particularly needed
in light of the ongoing hydropower plant construction boom [31], which
urgently needs cross-cutting research projects and management out-
comes [55].
5. Conclusions
This work presents the outcomes of a multi-round Delphi expert
study to identify policy-relevant, high-priority questions in hydro-
peaking research and management. The nal list of 100 questions is a
distillation of the original submission consisting of over 400 questions.
The presented 100 questions target research objectives that are both
achievable and answerable, covering a broad range of topics. The
identied 100 high-priority questions, for example, underscore the need
to explore diverse organism groups, life cycle stages, and habitat types,
as well as the effects on sediment dynamics, energy markets, and miti-
gation measures. Additionally, considering hydropeaking in the context
of climate trends, urbanization, and invasive species is crucial for
identifying sustainable solutions. Advancements in remote and proximal
sensing and AI-driven socio-hydrological modeling hold promise in
addressing these challenges. Integrating multiple disciplines and data-
sets will be vital to develop holistic and innovative approaches to
manage the impacts of hydropeaking effectively. Therefore, the nal list
of high-priority questions can guide research efforts to provide decision-
makers with credible, science-based evidence to improve the sustainable
management of peak-operating hydropower facilities.
Credit author statement
Daniel S. Hayes: Conceptualization, Methodology, Validation,
Formal analysis, Writing – original draft, Writing – review & editing,
Visualization, Project administration. Maria Cristina Bruno: Methodol-
ogy, Resources, Writing – original draft, Writing – review & editing.
Maria Alp: Methodology, Resources, Writing – original draft, Writing –
review & editing. Isabel Boavida: Methodology, Resources, Writing –
original draft, Writing – review & editing. Ramon J. Batalla: Method-
ology, Resources, Writing – original draft, Writing – review & editing.
Maria Dolores Bejarano: Methodology, Resources, Writing – original
draft, Writing – review & editing. Markus Noack: Methodology, Re-
sources, Writing – original draft, Writing – review & editing. Davide
Vanzo: Methodology, Resources, Writing – original draft, Writing – re-
view & editing. Roser Casas-Mulet: Methodology, Resources, Writing –
original draft, Writing – review & editing, Visualization. Damian Veri-
cat: Methodology, Resources, Writing – original draft, Writing – review
& editing. Mauro Carolli: Methodology, Writing – original draft, Writing
– review & editing, Visualization. Diego Tonolla: Methodology, Writing
– original draft, Writing – review & editing. Jo H. Halleraker: Method-
ology, Writing – original draft, Writing – review & editing. Marie-Pierre
Gosselin: Methodology, Writing – original draft, Writing – review &
editing. Gabriele Chiogna: Methodology, Writing – original draft,
Writing – review & editing. Guido Zolezzi: Methodology, Writing –
original draft, Writing – review & editing. Terese E. Venus: Conceptu-
alization, Methodology, Formal analysis, Writing – original draft,
Writing – review & editing, Project administration.
D.S. Hayes et al.
Renewable and Sustainable Energy Reviews 187 (2023) 113729
12
Funding
This research was funded in whole, or in part, by the Austrian Sci-
ence Fund (FWF) [P 34061-B] and the European Union’s Horizon 2020
research and innovation programme [101022905]. Part of this work
beneted from the nancial support of the Morph-Hab [PID2019-
104979RBI00/AEI/10.13039/501100011033] and the MorphPeak
[CGL2016-78874-R/AEI/10.13039/501100011033] projects funded by
the Research State Agency (AEI), Spanish Ministry of Economy and
Competitiveness, Science and Innovation, and the European Regional
Development Fund Scheme. The authors acknowledge the support of the
Catalan Government through the Consolidated Research Group “Fluvial
Dynamics Research Group” – RIUS [2021SGR-01114] and the CERCA
Program. DVe is funded through the Serra Húnter Programme of the
Catalan Government. GC acknowledges the support provided by the
Deutsche Forschungsgemeinschaft (DFG) [CH 981/4–1]. MPG ac-
knowledges the support provided by the Norwegian Institute for Nature
Research (NINA) intern funds. The funding sources were not involved in
the study design, collection, analysis and interpretation of data, the
writing of the manuscript, and the decision to submit the article for
publication.
Declaration of competing interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
Data availability
The data (i.e., the questions) are included in the article.
Acknowledgments
This work is a product of the combined efforts of the interdisciplinary
network on hydropeaking research (HyPeak), founded in 2020. We wish
to thank the 220 respondents who contributed to the global survey on
soliciting research questions and the experts who took their time to rank
the list of original questions – without you, this study would have been
impossible! Thanks to Klejch Fly Fishing & Outdoor, Vienna, for free
survey giveaways. Thanks also to Justine Carey for helping create the
global map and to three anonymous reviewers who provided construc-
tive comments to improve this article.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.rser.2023.113729.
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