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Hydropeaking has negative effects on aquatic biota, but the causal relationships have not been studied extensively, especially when hydropeaking occurs in combination with other environmental pressures. The available evidence comes mainly from case studies demonstrating river-specific effects of hydropeaking that result in modified microhabitat conditions and lead to declines in fish populations. We used multiple lines of evidence to attempt to strengthen the evidence base for models of ecological response to flow alteration from hydropeaking. First, we synthesized evidence of ecological responses from relevant studies published in the scientific literature. We found considerable evidence of the ecological effects of hydropeaking, but many causal pathways are poorly understood, and we found very little research on the interactive effects of hydropeaking and other pressures. As a 2nd line of evidence, we used results from analyses of large-scale data sets. These results demonstrated the extent to which hydropeaking occurs with other pressures, but did not elucidate individual or interactive effects further. Thus, the multiple lines of evidence complemented each other, but the main result was to identify knowledge gaps regarding hydropeaking and a consequent pressing need for novel approaches, new questions, and new ways of thinking that can fill them.
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Drawing together multiple lines of evidence
from assessment studies of hydropeaking
pressures in impacted rivers
Andreas H. Melcher
, Tor H. Bakken
, Thomas Friedrich
, Franz Greimel
, Nona Humer
Stefan Schmutz
, Bernhard Zeiringer
, and J. Angus Webb
Institute of Hydrobiology and Ecosystem Management, University of Natural Resources and Life Sciences (BOKU),
1180 Vienna, Austria
SINTEF Energy Research, 7465 Trondheim, Norway
Department of Infrastructure Engineering, The University of Melbourne, Victoria 3010, Australia
Abstract: Hydropeaking has negative effects on aquatic biota, but the causal relationships have not been studied
extensively, especially when hydropeaking occurs in combination with other environmental pressures. The avail-
able evidence comes mainly from case studies demonstrating river-specic effects of hydropeaking that result in
modied microhabitat conditions and lead to declines in sh populations. We used multiple lines of evidence to
attempt to strengthen the evidence base for models of ecological response to ow alteration from hydropeaking.
First, we synthesized evidence of ecological responses from relevant studies published in the scientic literature.
We found considerable evidence of the ecological effects of hydropeaking, but many causal pathways are poorly under-
stood, and we found very little research on the interactive effects of hydropeaking and other pressures. As a 2
of evidence, we used results from analyses of large-scale data sets. These results demonstrated the extent to which
hydropeaking occurs with other pressures, but did not elucidate individual or interactive effects further. Thus,
the multiple lines of evidence complemented each other, but the main result was to identify knowledge gaps regard-
ing hydropeaking and a consequent pressing need for novel approaches, new questions, and new ways of thinking that
can ll them.
Key words: Eco Evidence, evidence-based practice, systematic literature review, conceptual model diagrams, sh,
hydropeaking, hydroelectric power
Global demand for energy is rising, and interest in renew-
able sources of electricity, among which hydroelectric power
is prominent worldwide, is increasing (Wagner et al. 2015,
Zaret al. 2015). However, dams built for hydroelectric power
production are not environmentally benign and have strong
negative effects on sh and other aquatic fauna. In Europe,
complementary environmental risk and impact assessments
are essential to meet the major aims of the EU Water Frame-
work Directive (WFD) by 2020 (European Commission 2000,
Birk et al. 2012, Hering et al. 2015).
Flow variability is an intrinsic feature of river systems
and is essential for their ecological function (Poff et al. 1997,
Bunn and Arthington 2002). In general, ow uctuations
caused by hydropeaking are often much more severe than
those experienced in natural ow systems (e.g., Parasiewicz
et al. 1998, Saltveit et al. 2001, Scruton et al. 2003, 2008,
Smokorowski et al. 2011, Young et al. 2011, Nagrodski et al.
2012). Hydropeaking is the rapid rise and fall of discharge
levels when hydroelectric plants are switched on and off, typ-
ically in response to subdaily changes in demand for elec-
E-mail addresses:
*BRIDGES is a recurring feature of FWS intended to provide a forum for the interchange of ideas and information relevant to FWS readers, but beyond
the usual scope of a scientic paper. Articles in this series will bridge from aquatic ecology to other disciplines, e.g., political science, economics, education,
chemistry, or other biological sciences. Papers may be complementary or take alternative viewpoints. Authors with ideas for topics should contact BRIDGES Co-
Editors, Sally Entrekin ( and Allison Roy (
DOI: 10.1086/690295. Received 22 July 2016; Accepted 27 October 2016; Published online 8 December 2016.
Freshwater Science. 2017. 36(1):220230. © 2017 by The Society for Freshwater Science.
tricity. Hydropeaking causes rapid and large changes in the
subdaily ow regime of rivers (amplitude, rate, frequency,
and timing of ow uctuations) and is evident from hydro-
graph data (e.g., Greimel et al. 2016). Not all hydropower
plants cause hydropeaking, and among hydropeaking dams,
the level of hydrological effects vary depending on opera-
tional regime and mitigations used. In Austria, ~800 km
of rivers are affected by hydropeaking. Thus, hydropeaking
is not a local pressure, but affects long river stretches (e.g.,
Schmutz et al. 2015).
Fish are affected by hydrological impacts from hydro-
electric power facilities, including hydropeaking (Schmutz
et al. 2015). Ecological effects are severe, but we have little
detailed understanding of the causal mechanisms involved
(Harby and Noack 2013, Forseth and Harby 2014, Bruder
et al. 2016). Hydroelectric power is being marketed as a sus-
tainable form of electricity production, and we need to un-
derstand these mechanisms better so that environmental
effects of hydropeaking can be mitigated (e.g., Moog 1993,
Young et al. 2011).
Many natural environments are affected by multiple hu-
man pressures. Eighty percent of European rivers are af-
fected by altered water quality, hydrology, morphology,
or river connectivity. In 47% of these cases, rivers suffer
from >1 such stressor, and 12% suffer from all 4 stressors
(Schinegger et al. 2012). Human-induced stressors to rivers
can have serious consequences for aquatic life, e.g., sh
(Dudgeon et al. 2006, Pont et al. 2006, Birk et al. 2012, Eu-
ropean Union 2015), but not all of the potential effects are
well enough understood to guide decisions related to actions
that might alter human pressures on rivers.
Expert-knowledge-based conceptual models of poten-
tial effects of stressors can provide a starting point to guide
decision-making regarding how to manage rivers. Multiple-
lines-of-evidence studies can improve the scienticunder-
pinnings of such models. Results from case studies can be
combined with monitoring or experimental data to build con-
ceptual models that allow scientists to ask research ques-
tions regarding individual or interacting pressures. In Eu-
rope, such models are becoming increasingly important for
understanding the effects of single- and multistressor im-
pacts in aquatic environments (Feld et al. 2011, Marzin et al.
2014, Hering et al. 2015).
Scientists working within the context of several Euro-
pean projects (e.g.,, http://mars,, http:// have explored literature-based ev-
idence on effects of multiple pressures and hydropeaking
on sh to complement data analyses from eld and articial-
channel experiments. In this paper, we build on the methods
these investigators used to synthesize data from gray lit-
erature (i.e., unpublished reports), published peer-reviewed
studies, and data analyses. We chose to focus on the Euro-
pean context because the drive toward renewable energy
and hydropower production in Europe is clashing with the
WFD objective of achieving good ecological status in riv-
ers by 2020. The collated evidence is intended contribute
to the investigation of multiple stressor effects in European
waterways under the MARS project (Managing Aquatic
ecosystems and water Resources under multiple Stress; http://, and in particular, to the design of a diag-
nostic tool supporting management of multiple stressors
in aquatic systems under the WFD. In that context, and
within the focus of this BRIDGES cluster, we addressed the
utility of the multiple-lines-of-evidence approach. In par-
ticular, we assessed whether rapid evidence assessment im-
proved our understanding of the ecological effects of hydro-
peaking, including when it occurs in combination with other
We were guided by the Eco Evidence method (Norris
et al. 2012) to build a Driver- Pressure-State conceptual model
(DPS) based on evidence in the literature (EEA 2007, Feld
et al. 2011, Humer 2016). We also analyzed existing eld
data to illustrate how literature-based results might be sup-
plemented by de novo analyses.
We used the results of 3 published literature reviews on
the effects of hydropeaking (Zitek et al. 2006, Bakken et al.
2012, Schmutz et al. 2013). The reviews were undertaken
independently, and their authors focused on literature that
was available online, including review papers and reports
(in multiple languages). Schmutz et al. (2013) also used the
collection of hard-copy papers at the University of Natural
Resources and Life Sciences (BOKU University), Austria. The
authors searched journals systematically on Google Scholar
and ISI Web of Knowledge (Thomson Reuters, Philadelphia,
Pennsylvania) using combinations of the key words: sh,
benthic invertebrates,biota,hydropeaking,ow uc-
tuation,ecological status,river,andfreshwater. None
of the authors provided more detail on their search strategies
(e.g., specic combinations of key words, dates searched), lim-
iting repeatability. We extended the search results with snow-
ballsearches in which we examined references in relevant
papers, and we updated the collection of references based
upon our knowledge of recent literature and suggestions from
colleagues and reviewers.
We searched the initial collection of references for evi-
dence validating hydropeaking causeeffect relationships for
a number of biological indicators (e.g., sh, benthic inverte-
brates) relevant to the European context (i.e., similar species
or river types). We cross-tabulated the retained literature
results and potential causal relationships in an abiotic and
biotic state interaction matrix and synthesized them into a
DPS conceptual model. We stored information on study type
and location (e.g., waterbody type, ecoregion, biota, pressure
types, causes and effects, experimental design scale) in an
Eco Evidence Database (based on Zitek et al. 2006, Webb
et al. 2015) and uploaded all papers to Mendeley (and open-
Volume 36 March 2017 | 221
access hydropeaking group) so that they would be available
for further use by any interested researchers.
Second, the lack of direct evidence regarding the effect
of multiple stressors from the standardized review led us to
conduct analyses of data from a large-scale eld-sampling
data set (; Schinegger et al. 2016).
The EFI1database includes information on sh, environ-
mental variables, and various human pressures relevant to
the WFD (e.g., hydrology, morphology, connectivity, or water
quality). Data were compiled from 14 European countries,
3100 rivers, and 9330 sh sampling sites (Schinegger et al.
2012, 2013).
Below, we provide an overview of the review results, but
this presentation is not comprehensive, partly because of
limited space within this cluster. Instead, it serves to show
what the review achieved, and why it was necessary to in-
clude empirical data analysis.
Seventy-eight of 186 articles (from 16 countries) found
in the initial literature searches contained empirical evidence
of hydropeaking impacts on sh (Fig. 1). The most com-
mon countries from which information on hydropeaking
was found were: USA (45), Switzerland (21), Canada (19),
and Norway (17), followed by Austria (12) and France (11)
and 24 multiple-country studies. The literature review showed
that even partial hydropeaking operations (i.e., hydropeak-
ing in river sections above a sh sampling site, but that
has only minor effects on hydrology at the sampling site)
have signicant effects on river geomorphology and biota
(e.g., Smokorowski et al. 2011, Young et al. 2011, Nagrodski
et al. 2012, Harby and Noack 2013, Hauer et al. 2014). Fur-
ther, ow uctuation rates (e.g., ramping rate: the rate of
stage change) of >~15 cm/h affect sh assemblages in small-
and medium-sized rivers (Schmutz et al. 2015). Stranding
of organisms is one of the most obvious negative effects
of hydropeaking (e.g., Young et al. 2011, Nagrodski et al.
2012, Harby and Noack 2013, Hauer et al. 2014), although
less is known about the sublethal and long-term effects of
stranding (Nagrodski et al. 2012). A signicant relationship
between sh abundance and peak velocity was reported
(Young et al. 2011). Peak velocity causes ushing, leading
to sh depletion (Schmutz et al. 2015).
Only a few authors focused on the effect of hydropeak-
ing at the community, functional system, or food-chain level
(e.g., Lauters et al. 1996, Flodmark et al. 2002, Lagarrigue
et al. 2002, Robertson et al. 2004, Vehanen et al. 2005, Puffer
et al. 2015). In general, we found little evidence on the ef-
fects of hydropeaking for non-salmonids (e.g., Vehanen and
Lahti 2003, Bond et al. 2015).
Most studies showed that nighttime hydropeaking has
a greater impact on sh than equivalent ow variation dur-
ing the day (e.g., Sempeski and Gaudin 1995, Bradford 1997).
Moreover, although nocturnally active species may be less
Figure 1. Classication and number of hydropeaking studies from the standardized literature search (total 5186). Shaded
portions of the bars represent the 78 studies used to develop the DriverPressureState (DPS) conceptual model (Fig. 2) and the biotic
abiotic interaction matrix (Table 1). Published case study 5peer-reviewed observational eld study published in a journal or book,
unpublished case study 5observational eld study in a report, review 5review of published and unpublished literature in a scientic
journal, experimental study 5laboratory ume or eld study in which ow was manipulated, methodological study 5paper/report that de-
scribes and synthesizes techniques related to hydropeaking research, thesis 5MS or PhD thesis, policy article 5government document
related to managing hydropeaking.
222 | Rapid evidence synthesis on hydropeaking A. H. Melcher et al.
likely to be stranded at night than during the day, this
difference can be reversed for salmonids at higher water
temperatures (e.g., Halleraker et al. 2003, Flodmark et al.
Rivers with intense hydropeaking operations, character-
ized by a high ramping rate, extreme water-level variation
including dewatering, high ow peak frequency (number
of peaks per year), and rapid changes (decreases) in the
ramping rate, showed the most negative effects on sh as-
semblages and their life stages, including spawning and
successful reproduction, especially when habitat was lost
or conditions were poor (e.g., Berland et al. 2004, Hauer
et al. 2013, Person et al. 2014, Schmutz et al. 2015, Casas-
Direct evidence of the interactive effects of other pres-
sures with hydropeaking was difcult to identify in the lit-
erature review. Authors of most of the hydropeaking eld
studies focused on a single river (e.g., Young et al. 2011,
Harby and Noack 2013). Single rivers are often affected
by multiple pressures, but the lack of replication across en-
vironmental gradients made disentangling the effects of
such stressors impossible within those studies. For exam-
ple, no investigators have used multiple systems and pres-
sures in a comparative framework to study stranding in the
context of hydropeaking (e.g., Young et al. 2011, Nagrodski
et al. 2012, Harby and Noack 2013).
A few authors included consideration of multiple stress-
ors in their discussion sections but did not provide empirical
data. These authors contended that hydropeaking, in com-
bination with river channel straightening and simplica-
tion (channelization), has severe negative effects (e.g., Moog
1993, Smokorowski et al. 2011, Bruno et al. 2013, Schmutz
et al. 2013, Kennedy et al. 2016). Channelization signi-
cantly increases loss of habitat and inundation frequency,
and hydropeaking increases scouring and substrate embed-
dedness (e.g., Hauer et al. 2013).
The EFI1data set contained evidence of many inde-
pendent, but co-occurring human pressures and impacts
on sh but did not enable us to assess their relative impor-
tance or interactive effects (Schinegger et al. 2012, 2013,
Trautwein et al. 2013). A maximum of 12 independent pres-
sure types was found in rivers affected by hydropeaking.
scale categories: hydrology (number of pressure types [n]54),
morphology (n53), water quality (n53), and river connec-
tivity (n52). In addition, sh sampling sites affected by
hydropeaking (n5632) were affected by a mean of 5.5 other
pressures types (Fig. 3A), whereas 8698 sites not affected
by hydropeaking experienced fewer additional pressures
(mean 53.5 pressure types). Sites partially affected by hy-
dropeaking (n5254) experienced an intermediate number
of additional pressure types (mean 54.9; Fig. 3A). This re-
sult reects the reality that hydroelectric power develop-
ment generally occurs in concert with other forms of hu-
man exploitation of river systems. Species richness of sensitive
shspecies unable to tolerate habitatdegradation (Segurado
et al. 2011) was lower at sites affected by hydropeaking
(Fig. 3B). Results were much more variable for sites affected
by partial hydropeaking (cf. error bars in Fig. 3B).
Acceptance that hydropeaking causes ecological dam-
age is growing (e.g., Harby and Noack 2013, Forseth and
Harby 2014, Bruder et al. 2016). Nevertheless, in the ab-
sence of strong evidence, few general principles exist for
how best to restore ow regimes while retaining the ben-
ets of hydroelectric power (Bruder et al. 2016). In envi-
ronmental management, identifying the most likely causes
of an observed environmental impact is important for plan-
ning and implementing remediation actions. Ecological re-
sponse models backed by rigorous and transparent evidence
assessment can be used to inform management of hydro-
peaking dams for both environmental and human outcomes.
Our literature review provided many examples of the nega-
tive effects of hydropeaking, but quantifying the response
of specic biological metrics (e.g., the number of intolerant
sh species) to specic changes in the river and habitat was
difcult, especially for different river types. This difculty is
compounded when one attempts to use the existing scien-
tic literature to assess the generality of results of local eld
studies. In addition, when investigators use reductionist ap-
proaches and study single human stressors, quantifying and
prioritizing the interactive effects of multiple co-occurring
humanpressuresisextremelydifcult. This difculty mo-
tivated our use of a large-scale data set as a 2
line of ev-
idence in our analysis. This approach enabled us to demon-
strate the prevalence of multiple stressors, but it still did not
enable us to achieve the primary goal of our study, which
was to better elucidate the individual and interactive effects
of hydropeaking.
The methods for evidence synthesis reported in our
paper were developed specically for this case study be-
cause no standard method was available. The steps de-
scribed above (literature synthesis supplemented by empir-
ical data) were an attempt to synthesize existing evidence on
the individual and interactive effects of hydropeaking rap-
idly, systematically, and transparently. Two lines of evi-
dence are less than what might normally be considered in
a multiple-lines-of-evidence study (Downes et al. 2002), but
the restriction was caused by the rapid nature of the evi-
dence synthesis undertaken. Our assessment also was re-
stricted to some degree by the fact that it was built on
3 existing reviews. Authors of those reviews did not spec-
ify their search methods or the criteria used to include or
Volume 36 March 2017 | 223
exclude studies from detailed consideration, thereby greatly
reducing the transparency of any conclusions reached. We
recommend that, at a minimum, search methods (dates,
databases, key words) and criteria for inclusion/exclusion
of studies should be reported along with the results.
We identied substantial amounts of evidence for the
individual effects of hydropeaking, but little information on
the direct pathways linking cause to effect, the interactive
effects of multiple pressures combined with hydropeaking,
or effects on nonsalmonids. Detailed categorization of the
evidence into an abioticbiotic state interaction matrix of
the evidence (Table 1, Fig. 2) can be used to identify impor-
tant information gaps currently preventing better-informed
decisions. These gaps include the interactive effects of other
pressures with hydropeaking. We conclude that the rapid
evidence synthesis done here was enough to identify the
existence of evidence (or a lack of evidence), but did not
achieve its primary goal because of: 1) the small amount of
evidence on interactive effects of hydropeaking, and 2) the
lack of a specic method for combining such data to dis-
entangle the effects of multiple pressures. Personnel work-
ing on the MARS project are developing a European data-
base on ecological effects of multiple stressors in European
rivers (Hering et al. 2015) and a method to synthesize ev-
idence on these issues that will be more rigorous than the
ad hoc approach reported here. The standard methods and
tools for synthesis of evidence in the literature from the
USA (Norton et al. 2008) and Australia (Norris et al. 2012,
Webb et al. 2015) also may be able to inform development
of a future standardized method (Webb et al. 2017).
Despite ongoing progress elucidating multiple stressors
in European rivers, Europe presents novel challenges for
synthesizing literature evidence. Peer-reviewed literature on
hydropeaking comes mainly from North America. Studies
from Europe are more difcult to access because they are
mainly published as government reports, often in European
languages other than English (German, French, Italian, or
Norwegian). English language bias and gray literature biases
Table 1. Interaction matrix and classication of 78 references based on the DriverPressureState (DPS) conceptual model, which con-
tained empirical data from the standardized review (see Fig. 2). These empirical data illustrate the specic causal linkages not shown in
Fig. 2. Numbers in the cells are the number of studies that contained empirical evidence on the combination of hydropeaking related
stressors (abiotic factors and state) and biological responses (biotic state). Many studies are counted more than once in the table be-
cause the authors studied multiple combinations. Citations from 2005 to 2015 are provided.
Abiotic factors
and state
Biotic state
Habitat behavior/
species composition
Inuencing factors
Fluctuation amplitude 27 26 27 21 24 17 11
Ramping rate 23 19 19 16 16 15 8
Frequency of peaking 11 13 13 8 12 8 5
Timing 22 29 20 19 21 18 9
Abiotic state
habitat loss
13 13 12 13 8 12 5
Sediment type 8 7 9 19 7 8 4
Turbidity 16 12 13 14 11 9 5
Temperature 18 12 9 14 7 10 5
Number with evidence
of interaction
138 131 122 124 106 97 52
Total 34 33 29 26 25 21 12
Examples (20042015) b, h, m, n,
o, q, r, v, w,
x, bb, cc,
dd, jj, kk
e, h, j, l, p,
v, x, y, aa,
cc, dd, ee,
gg, ii, jj, kk
b, f, q, s, y,
aa, cc, dd,
ff, gg, hh, jj,
kk, ww
b, d, v, x,
y, jj, kk, cc
a, b, c, f,
h, i, k, y, z,
aa, cc, dd,
ff, gg, jj, kk
b, g, l, u,
v, y, dd,
ee, jj, kk
f, h, y,
gg, jj, kk
Arnekleiv et al. 2006,
Bain 2007,
Bell et al. 2008,
Bond 2013,
Bond and Jones 2015,
Bond et al. 2015,
Casas-Mulet et al. 2014,
Clarke et al. 2008,
Fette et al. 2007, Flodmark et al.
Garcia et al. 2011,
Golder Associates 2015,
Harby and Noack 2013,
Hauer et al. 2014,
Heggenes et al.
Irvine et al. 2009,
Jones and Stuart 2008,
Korman and Campana 2009,
Marty et al. 2009,
McMichael et al. 2005,
Murchie et al. 2008,
et al. 2012,
Person 2013,
Person et al. 2014, Puffer et al.
Sauterleute 2009, Schmutz et al.
Scruton et al. 2008, Smokorowski
et al.
Ugedal et al. 2008,
Vehanen et al. 2005,
Young et al. 2011,
Zitek et al. 2006.
224 | Rapid evidence synthesis on hydropeaking A. H. Melcher et al.
(e.g., Bauersfeld 1978, Baumann and Klaus 2003, Bakken et al.
2012, Baumann et al. 2012, Person 2013, Schmutz et al. 2013,
Golder Associates 2015) create problems related to access
to information. These issues may partly explain why we
uncovered comparatively little quantitative evidence on the
effects of hydropeaking, and essentially no evidence on the
interactive effects of other pressures with hydropeaking.
In light of these results, we moved beyond the assessment
Figure 2. A conceptual DriverPressureState (DPS) model summarizing the results from the standardized literature review.
Results are organized hierarchically to show how drivers and pressures link (via inuencing factors) to ecological responses on sh,
all of which can be assumed to be negative changes in the state variable listed. The arrows and numbers show the number studies
with evidence for that response.
Volume 36 March 2017 | 225
of literature and used empirical data analyses as a 2
of evidence. This additional evidence was still insufcient
to fulll the original goal of the evidence synthesis, but our
incremental approach highlights the advantage of being able
to consider additional lines (e.g., sources) of evidence when
an initial line of evidence is insufcient to reach a conclusion.
Further research and development could lead to an eco-
logical ontology to enhance the searching, sharing, and un-
derstanding of evidence (Ziegler et al. 2015). This ontology
might improve our ability to locate literature sources of ev-
idence constrained by language or publication type in Eu-
rope or elsewhere. Together with large-scale empirical data
sets, this improved ability would provide additional analyti-
cal strength when investigating large-scale patterns and eco-
logical responses to multiple environmental stressors. The
interactive effects of hydropeaking with other pressures have
deed elucidation for many years (Harby and Noack 2013),
and the evidence synthesis we presented does not advance
knowledge in this area. Future advances in our understand-
ing of this area will require novel approaches and new ways
of thinking. One possibility is a greater focus on process-
based studies, potentially in laboratory umes, that can
directly elucidate causal mechanisms. Another would be
research on cascading ecological consequences caused by mul-
tiple human pressures. These cascading effects could in-
clude changes of hydrology and morphology, continuum dis-
ruption, water quality, and climate change, all of which may
inuence the diversity and resilience of the biota in rivers
subjected to multiple impacts (e.g., Feld et al. 2011, Hering
et al. 2015, Nõges et al. 2015). Such research, evidence data-
bases, and causal analysis methods have the potential to
revolutionize evidence-based practice in environmental man-
agement and policy in Europe.
Author contributions: AHM wrote and structured the man-
uscript following many meetings with THB, TF, FG, NH, SS, BZ
and JAW. All authors provided input and revision to the submitted,
revised and earlier versions.
This work is part of the EnviPEAK project (Effects of rapid
and frequent ow changes) implemented at CEDREN (Centre for
Environmental Design of Renewable Energy) Norway (http:// and the MARS project funded
under the 7
EU Framework Programme, Theme 6 (Environment
including Climate Change), Contract Number 603378 (http:// The data were drawn from the EU research
project Improvement and Spatial extension of the European Fish
Index (EFI1), supported by the European Commission under the
Framework Programme (FP 6) contributing to the implemen-
tation of task Ecological Status Assessmentlling the gaps,Con-
tract Number 044096 ( We thank all ed-
itors, referees, Sue Norton, Sue Nichols, Michael Peat, and Tim
Cassidy for their helpful comments, support, and discussion.
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Figure 3. Mean (±95% condence interval) number of multi-
ple human pressures on hydropeaked European rivers (A) and
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mum number found at any site in the database) associated with
hydropeaking (data source: Schinegger et al. 2016; n59330 sh
sampling sites). Hydropeaking is dened as absent (none), partial
(river upstream is directly affected by hydropeaking, but not at the
site itself), and full (hydropeaking hydrological effects are observed
directly at the site) (Schinegger et al. 2012).
226 | Rapid evidence synthesis on hydropeaking A. H. Melcher et al.
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230 | Rapid evidence synthesis on hydropeaking A. H. Melcher et al.
... Non-natural sub-daily flow fluctuations like hydropeaking operations (hereafter also referred to as flow ramping) may have significant short and long-term impacts on river ecology (Meile et al., 2011;Harby and Noack, 2013) due to flushing, flow depletion and/or stranding of biota (Melcher et al., 2017). Flow ramping is considered most problematic when HPP have outlets into rivers, without effective mitigation measures (Greimel et al., 2018;Person et al., 2014), and can severely affect riverine and riparian species (Bejarano et al., 2018), although impacts are highly dependent on river morphology (Hauer et al., 2014;Schmutz et al., 2015). ...
... This could be done without significant riverine ecological impacts nor need for costly measures (Person et al., 2014). It is well-documented that that intensive non-natural river flow fluctuations below HP turbines has severe ecological impacts Melcher et al., 2017;Moreira et al., 2019;Ramon et al., 2021). Our study supplements the picture from previous studies of hydropeaking in Norway (Wandurraga, 2019) and Nordic rivers (Ashraf et al., 2018). ...
... regarding up ramping in ecologically critical seasons should be part of this regular evaluation of mitigation success. Several reviews Melcher et al., 2017) have indicated that flushing of macroinvertebrates (Schülting et al., 2019) and fish larvae's (Auer et al., 2017) may be severe from rapid up-ramping, and this up ramping wave may in contrast to down ramping increase longitudinally in steep rivers (Hauer et al., 2017). Together with other significant multiple stressors in rivers downstream dams and HP tailrace, such as e.g. ...
Full-text available
Hydropower production is a key electricity generation technology in many parts of the world which can play a significant role in the transition towards a green and clean energy system. Hydropower can mobilize flexible energy on demand (hydropeaking) to balance out intermittent electricity from wind and photovoltaics. Adoption of hydropower as a peaking power source could lead to increased frequency of flow ramping in rivers downstream hydropower tailraces, which is one of the main stressors for riverine biota in alpine rivers. Both planned and accidental shutdowns of hydropower turbines need ecological mitigation. Our survey revealed that >3000 km of rivers downstream ca 800 hydropower plants in Norway may be ecologically impacted by non-natural flow fluctuations, and few have appropriate mitigation thresholds. A considerable eco-friendly peaking service may come from the Norwegian hydropower portfolio of over 19 GW installed capacity, with outlets into fjords, reservoirs or other large water bodies which normally dampen the ecological impacts of flow ramping. Intensive flow ramping occurs with irregular intervals from most types of hydropower. Although the highest frequency of stops were revealed in hydropower turbines not impacting river flow from storage hydropower, a significant number of turbine flow stops lasting over half a day in most types and categories of diversion hydropower. We suggest that further emerging ecosystem-based mitigations need to be adapted in hydropower licenses. This includes operational thresholds for both up and down ramping, constructional measures like by-pass valves, retention basins and increased base-flow or flow cap to ensure sustainability for hydropower operations. Our data reveal some of the most intensive hydropeaking operations from hydropower impacting longer rivers. Hence, our data underpins the potential for restoring downstream modified flow by ecosystem based measures related to both up and down ramping events in many regulated rivers.
... Depending on the general characteristics of the site, habitat changes may affect biodiversity to variable degrees (e.g. Cushman 1985;Baumann and Klaus 2003;Melcher et al. 2017). In the absence of satisfactory alternative means of flexible nearly carbon-free energy production, in case of significant impacts of hydropeaking on biodiversity, plant managers and other stakeholders need to harmonize their thinking on mitigation, so as to reconcile the two issues locally and on a larger scale (Murchie et al. 2008;Person 2013;Bruder et al. 2016;Tonolla et al. 2016;Boavida et al. 2020). ...
... Such measures have been proposed in studies with specific objectives (e.g. reducing fish stranding) with very few aiming at mitigation at the community or ecosystem level (Melcher et al. 2017). ...
... We used results from several literature reviews on the ecologic impacts of hydropeaking to develop our methodological framework (Cushman 1985, Young et al. 2011Bruder et al. 2016;Melcher et al. 2017;Greimel et al. 2018;Moreira et al. 2018;Hayes et al. 2019;Boavida et al. 2020). Mitigation measures were assessed following the Swiss Federal Office for the Environment (Tonolla et al. 2017) and a first review (of the gray literature: Cornu 2015) on hydropeaking mitigation worldwide, updated by a literature search on Scopus and Google Scholar with the search-terms "hydropeak à ," "mitigation," "cost," "economics," "efficiency" and "effectiveness." ...
Downstream of hydroelectric plants, hydropeaking can cause frequent flow variations, resulting in habitat modifications (e.g. hydraulics, reach morphology, temperature, water quality), which can impact organisms (stranding, dewatering, forced drift, growth disturbances) and ultimately may have negative and lasting impacts on biological communities, reducing resilience. Nevertheless, the severity of habitat disturbances vary depending on other existing pressures and local site conditions, which need to be taken into account to achieve effective hydropeaking mitigation. Preserving hydropower flexibility is also a priority to ensure the stability of electric systems without recourse to more polluting alternatives. Given these apparently opposing objectives, we propose a consensual technico-economic framework to guarantee the feasibility and effectiveness of site-specific hydropeaking mitigation, based on our experience as a hydropower operator and a literature review. While existing tools (such as habitat models) can be used to predict expected local effects of proposed mitigation and compare scenarios, predicting biological community responses is not currently possible (lack of in-situ evaluations of mitigation efficacy). These uncertainties and complex socio-ecosystems necessitate a forward-looking global approach that accounts for climate change, multi-purpose water use and electric system requirements, combined with site-specific analyses of the relative importance of hydropeaking impacts with respect to other pressures.
... Transferring mesocosm results to the river Experiments provide the opportunity to quantify the effect of individual factors under controlled conditions and can, therefore, directly elucidate causal mechanisms (Melcher et al., 2017). However, even in semi-natural mesocosms, using small-sized fish (here: TL < 20 mm) may pose difficulties retrieving all stocked fish and determining stranded or non-stranded fish. ...
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Rapid water level decreases due to hydropeaking are known to negatively affect riverine biota, mainly due to the stranding of organisms in the river bank area that becomes regularly dewatered. Even though studies of the last decades have focused on salmonid fish, also cyprinids may be affected. However, limited knowledge is available of this fish family. Therefore, we conducted mesocosm experiments under semi-natural conditions, simulating single hydropeaking events at two different lateral bank slopes (2% and 5%) with varying down-ramping rates (0.7–3.0 cm min−1) during day and night. As a response parameter, we quantified stranding rates of different larval stages (III-IV and V) of common nase (Chondrostoma nasus L.). The experiments revealed that lower sloped banks exhibited distinctly higher stranding rates than steeper ones. Daytime revealed a similar pattern, with more fish becoming stranded at night than during the day, and this was consistent for all down-ramping rates. The data also indicate increased stranding with higher down-ramping rates, particularly at low sloped riverbanks, and interaction effects between the tested parameters. Overall, this study, for the first time, quantifies the consequences of flow down-ramping on nase larvae, also revealing differences between larval stages. The gained information will, therefore, advance the ongoing discussion on hydropeaking mitigation by providing a deeper understanding of the effects of artificial sub-daily flow fluctuations on the early life stages of cyprinid fish. Our results can inform management and policy to sharpen existing mitigation concepts and fine-tune hydropower operations to reduce negative effects on riverine ecosystems.
... The loss of nesting areas results in increased competition among conspecifics for remaining sites and may result in the use of sub-optimal sites, potentially leading to early abandonment and ultimately the failure of broods (Kroon et al., 2000). Similarly, in small to moderate sized streams, water releases and extraction for irrigation, as well as releases of water for the environment, also has the potential to dramatically alter river height over short periods of time which can have short-and long-term effects on fish populations, including on their reproductive output (Melcher et al., 2017;Smokorowski, 2022). ...
Murray cod Maccullochella peelii (Mitchell) have a key ecological role in ensuring the health of Australia's largest inland waterway, but many aspects surrounding its reproductive strategies in the wild are unknown. From 2015 to 2019 within the Northern Murray-Darling Basin, Australia, we used a combination of bio-telemetry and underwater imagery to quantify the behaviour of Murray cod across their breeding cycle in a natural riverine environment. In most years, breeding behaviour including nest site selection was observed from early-August and spawning from late-August through to late-October, which is considerably earlier than previously reported. There was a positive correlation between the onset of breeding behaviour and week-of-year, and spawning was correlated with moon-phase. Whilst some nesting sites were amongst woody debris and in hollow logs, the majority were located in shallow water on hard substrate underneath undercuts along the riverbank edge. Nests were frequently established in isolated and disconnected pools with little or no measurable flow, suggesting that river hydraulics is not a key component driving spawning of Murray cod across at least some areas of its range. Larvae were observed actively swimming and controlling their position within and near nests and used a scatter tactic when dispersing. We also established that disturbing nesting Murray cod had a negative impact on egg and larval survival. We suggest a review of current regulations to safeguard the long-term conservation of the species across all sections of its range.
... Hydropeaking is classified as a hydrological impact. Anthropogenic changes in the natural flow dynamics (Poff et al., 1997) by the operation of storage HPPs can affect fish, macroinvertebrates and other aquatic or riparian organisms over comparatively long river stretches (Melcher at al.,2017;Bejarano et al., 2018;Schülting et al., 2019;Hayes et al., 2019). The main impact on aquatic organisms is considered to be involuntary drift and stranding due to the increase and decrease in flow, respectively (Greimel et al., 2018). ...
Due to the particularly favorable topographical and hydrological conditions in Austria, storage and pumped storage hydropower plants represent the backbone of Austria's electricity supply, boasting a share of 34% of the total installed power plant capacity. Austrian storage hydropower plants not only have a high system-relevant importance on the national level, but significantly support the efficient integration of fluctuating re-newable energies within the European electricity market too. The flexible operated storage hydropower plants, however, lead to artificial sub-daily flow fluctuations (hydropeaking). In Austria, overall, about 800 km of rivers are affected by hydropeaking. Anthropogenic changes in the natural flow dynamics by the operation of storage hydropower plants can affect fish, macroinvertebrates and other aquatic or riparian organisms over comparatively long river stretches. The main impact on aquatic organisms is considered to be involuntary drift and stranding due to the increase and decrease in flow, respectively. Moreover, habitat availability and quality are affected by these artificial unfavorable flow fluctuations. Combined with stranding and drift, these hydro-morphological changes can lead to a loss of early juvenile stages of fish and benthic invertebrates. The EU Water Framework Directive sets concrete environmental targets for water bodies that are affected by anthropogenic pressures such as hydropeaking. In Austria, most of the water bodies affected by hydropeaking are designated as "heavily modified water bodies" and must therefore achieve good ecological potential by implementing relevant measures. When determining the good ecological potential, on the one hand the ecolog-ical effectiveness of measures to reduce the impact of hydropeaking has to be considered. On the other hand, it has to be assessed the extent to which these measures are expected to restrict the hydroflexibility and use of the storage power plants or have adverse effect on the wider environment. This thesis focuses on the ecohydrological aspects of hydropeaking as an essential basis for a deeper under-standing of the ecological impacts as well as the development and standardized evaluation of measures to re-duce the impacts. The focus is exclusively on sub-daily flow fluctuations respectively on sub-daily flow re-gimes. The methods developed within the framework of this thesis provide the basis for a hydrological assess-ment and monitoring concept aimed at characterizing the sub-daily flow regime by describing its temporal variability, and comparing the conditions of several hydrographs or periods (publication 1). Moreover, they provide the hydrological basis for a standardized ecological and economical evaluation of hydropeaking miti-gation measures (publication 2). Both methodological approaches have been applied in practice and are intend-ed to make hydropower more sustainable. The algorithms developed within the scope of the dissertation (i.e., HydroPeak and PeakTrace) are available as open-source R packages. The newly developed methods are cur-rently being tested and validated by further applications and form the basis for a national hydropeaking man-agement guideline, in line with the ecosystem-based principles according to the EU Water Framework Di-rective. The methods presented in this thesis are also relevant for sustainable hydropeaking mitigation in line with the EU taxonomy for sustainable electricity from hydropower.
... Quantifying the effects of above mentioned hydraulic parameters in situ is challenging due to the variation in local characteristic and interacting external influences on biota (Hayes et al., 2019;Melcher et al., 2017;Pearce et al., 2019). Flume setups are common approaches to disentangle and quantify effects from multiple variables, since these allow precise control of the setting and a reduction of confounding factors (Bruno, Cashman, Maiolini, Biffi, & Zolezzi, 2016;Carolli, Bruno, Siviglia, & Maiolini, 2011;Imbert & Perry, 2000). ...
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The hydrological regime of many alpine rivers is heavily altered due hydroelectric power generation. Hydropeaking operation produces frequent and irregular discharge fluctuations. Depending on the operational changes of flow amplitude and/or up‐ramping rate as well as on river morphology, hydropeaking can lead to quick and strong variations in hydraulic stress affecting stream invertebrates and causing increased drift. In the present flume experimental study, we analyzed trait‐specific drift reactions to single and combined effects of increased flow amplitude and up‐ramping rate. We analyzed taxa according to their hydraulic habitat preference and flow exposure, as these traits seem to be indicative toward hydropeaking. The results show that the sudden increase in discharge and related flow velocity led to increased macroinvertebrate drift proportions in hydropeaking treatments, which differed significantly to parallel control runs (mean drift proportion in all hydropeaking setups: 13% compared to 5% in controls). Increasing flow amplitudes led to an increase in drift for most taxa and traits. This was particularly significant for taxa associated with lentic areas. The effect of the up‐ramping rate on macroinvertebrate drift was nonsignificant but showed strong interactive effects with the flow amplitude, especially for taxa dwelling on the substrate surface. Our results therefore indicate that discharge‐related parameters, such as flow velocity, primarily affect macroinvertebrate drift and the importance of the up‐ramping rate increases, if certain discharge‐related thresholds are exceeded. Vice versa, a reduction of the up‐ramping rate at hydropeaking events with high flow amplitudes may reduce the effect on macroinvertebrate drift. Flow‐exposed (surface) and flow‐sensitive (lentic) taxa showed distinct drift reactions following hydropeaking treatments, which were significantly higher compared to effects on taxa associated to lotic and interstital habitats. Therefore, we conclude that both traits (hydraulic and vertical habitat preference) have proven as promising for analyzing hydropeaking effects. The trait classifications should be extended to a higher number of taxa and to different life stages as these may show different drift patterns.
... Available evidence syntheses on the impacts of HPP on fish often focus on the effects of passage on behaviour, injury and/or mortality of fish due to HPP facilities [25][26][27], or on the alteration in abundance and diversity of fish populations resulting from specific types of hydropower operation (i.e., hydropeaking; Melcher et al. [28]) or design (i.e., impoundments; Turgeon et al. [29]). While reviews on ecological responses to altered flows have been done in the past [21,30,31] including a recent summary of the various aspects of ecohydrology river alterations on fish [32], there remains a need to update our understanding of specific fish-flow interactions using robust systematic review techniques. ...
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Altering the natural flow regime, an essential component of healthy fluvial systems, through hydropower operations has the potential to negatively impact freshwater fish populations. Establishing improved management of flow regimes requires better understanding of how fish respond to altered flow components, such as flow magnitude. Based on the results of a recent systematic map on the impacts of flow regime changes on direct outcomes of freshwater or estuarine fish productivity, evidence clusters on fish abundance and biomass responses were identified for full systematic review. The primary goal of this systematic review is to address one of those evidence clusters, with the following research question: how do changes in flow magnitude due to hydropower operations affect fish abundance and biomass?
... Changes in hydrological and hydraulic conditions due to hydropeaking operations are well described (e.g., Harby & Noack, 2013;Saltveit, Halleraker, Arnekleiv, & Harby, 2001;Schneider et al., 2017), while associated rapid changes in water temperatures (thermopeaking, for example, Bakken, King, & Alfredsen, 2016;Bruno, Siviglia, Carolli, & Maiolini, 2013;Zolezzi, Siviglia, Toffolon, & Maiolini, 2011), gas saturation (saturopeaking, Pulg, Vollset, Velle, & Stranzl, 2016) and sound (soundpeaking, Lumsdon et al., 2018) are less investigated. Rapid and frequent changes in physical conditions lead to ecological impacts on aquatic plants (e.g., Bejarano, Jansson, & Nilsson, 2018), benthic invertebrates (e.g., Bruno, Maiolini, Carolli, & Silveri, 2010;Carolli, Bruno, Siviglia, & Maiolini, 2012;Gabbud, Bakker, Clémençon, & Lane, 2019), and fish species and communities (e.g., Auer, Zeiringer, Führer, Tonolla, & Schmutz, 2017;Melcher et al., 2017;Puffer, Berg, Einum, Saltveit, & Forseth, 2017;Schmutz et al., 2015), mainly due to flushing and stranding of organisms. Hydropeaking is assumed to represent an additional environmental pressure beyond traditional impacts from hydropower regulations (e.g., IPCC, 2011; World Commission on Dams, 2000), such as barriers and seasonal alterations of habitat conditions. ...
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This article proposes and demonstrates a new classification system of fish population level effects of hydropeaking operations in rivers. The classification of impacts is developed along two axes; first, the hydromorphological effect axis assesses the ecohydraulic alterations in rivers introduced by rapid and frequent variations in flow and water level, second the vulnerability axis assesses the site-specific vulnerability of the fish population. Finally, the population level impact is classified into four classes from small to very large by combining the two axes. The system was tested in four rivers in Norway exposed to hydropeaking, and they displayed a range of outcomes from small to very large impacts on the salmon populations. The river with a relatively high base flow and ramping restrictions scored better than rivers with the lower base flow or limited ramping restrictions, indicating that hydropeaking effects can be mitigated while maintaining high hydropower flexibility. Most effect factors could easily be calculated from timeseries of discharge and water level, whereas the use of hydraulic models to estimate potential stranding areas may require more work. The vulnerability factors are mainly qualitative and depend more heavily on expert judgments and are thus more uncertain. The system was deemed suitable for the purpose of supporting management decisions for rivers exposed to hydropeaking operations. It evaluates the severity of the additional pressures due to hydropeaking operations and proved useful to identify mitigating measures. While the system was developed for Atlantic salmon river systems, it could be adapted to other species or systems.
... These studies found that the magnitude, frequency, duration, timing and rate of change of hydropeaking significantly impact the age, growth, movement, migration, spawning and rearing of aquatic organisms (Resh et al. 1988, Harby et al. 2013, Anindito et al. 2019. For example, the relatively sudden flow decreases (rate of change-fall) can strand fish in isolated shallows and gravel-bar interstices as water level recedes , Melcher et al. 2017, Larrieu et al. 2020). Even though stranding may affect only a small portion of the fish population at a time, and may occur naturally, repeated flow fluctuations (frequency) can cause cumulative mortalities that can result in a significant fish loss (Young et al. 2011). ...
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Hydropeaking, a hydroelectricity generation strategy involving rapid changes to flow releases from dams in response to fluctuations in hourly-adjusted electricity markets has been widely applied due to its economic efficiency. However, these operational practices produce sub-daily flow fluctuations that pose substantial hazards to riverine ecosystems and human activities. To ascertain the downstream impacts of hydropeaking, features of hydropeaking have been analyzed with respect to ecologically relevant hydrologic variables. However, since studies aiming to characterize hydropeaking regime often require manual feature extraction, they are commonly limited to small temporal and spatial scales. Additionally, riverine ecologists have commonly treated hydropeaking as a broadly similar flow-alteration pattern regardless of the complexities of the electricity market and differences in the natural settings where it is applied. Therefore, this study sought to determine whether significantly different hydropeaking patterns exist on a regional scale, as revealed by temporal variations in hydropeaking over a long temporal scale (> five years). To fulfill this goal, a new algorithm, the Hydropeaking Event Detection Algorithm (HEDA), was developed in R to automate the characterization of hydropeaking flow regimes. Clustering analyses were conducted to explore the similarities and differences of hydropeaking regimes among 33 sites in numerous hydrologic regions of California. Four distinct classes of hydropeaking flow regimes were identified and distinguished by the duration and frequency of hydropeaking. Meanwhile, rate of change, amplitude and timing of hdyropeaking played less important roles in the classification.
HIGHLIGHTS - Intensity of hydropeaking is still severe in many Norwegian rivers - An eco-peaking potential of over 19 GW can be utilised without river flow ramping - Ecological effects of hydropeaking are partly overlooked in management plans - A national hydropower mitigation strategy is needed to ensure sustainability - Flow ramping mitigation also relevant in small scale hydro, without large reservoirs
Accurate and informative methods for evidence synthesis that are also simple and inexpensive to implement would greatly increase our ability to use scientific research results to better manage natural environments. Across the world, pressure to use evidence-based environmental management is increasing, but little guidance exists as to what 'evidence-based' actually means. Methods for systematic review of literature evidence have been modified from those used in medical research, but the effort involved in using these methods means that they have not been widely adopted. We compared 3 independently developed approaches to 'rapid evidence synthesis' methods developed in an attempt to improve efficiency and responsiveness compared to standard methods for systematic review. Each method has features that should be included in an ideal rapid evidence-synthesis method and has potential for further development. Increasing standardization of methods for evidence extraction, quality assessment, and synthesis increases the transparency and repeatability of the results obtained. However, the most important consideration is that the methods are fit for purpose; i.e., that each method is good enough to do the job required. The methods presented in this BRIDGES cluster, and potentially a combined method derived from them, could reduce the effort and cost of evidence synthesis to the scales required for management decisions.
Water resources globally are affected by a complex mixture of stressors resulting from a range of drivers, including urban and agricultural land use, hydropower generation and climate change. Understanding how stressors interfere and impact upon ecological status and ecosystem services is essential for developing effective River Basin Management Plans and shaping future environmental policy. This paper details the nature of these problems for Europe's water resources and the need to find solutions at a range of spatial scales. In terms of the latter, we describe the aims and approaches of the EU-funded project MARS (Managing Aquatic ecosystems and water Resources under multiple Stress) and the conceptual and analytical framework that it is adopting to provide this knowledge, understanding and tools needed to address multiple stressors. MARS is operating at three scales: At the water body scale, the mechanistic understanding of stressor interactions and their impact upon water resources, ecological status and ecosystem services will be examined through multi-factorial experiments and the analysis of long time-series. At the river basin scale, modelling and empirical approaches will be adopted to characterise relationships between multiple stressors and ecological responses, functions, services and water resources. The effects of future land use and mitigation scenarios in 16 European river basins will be assessed. At the European scale, large-scale spatial analysis will be carried out to identify the relationships amongst stress intensity, ecological status and service provision, with a special focus on large transboundary rivers, lakes and fish. The project will support managers and policy makers in the practical implementation of the Water Framework Directive (WFD), of related legislation and of the Blueprint to Safeguard Europe's Water Resources by advising the 3rd River Basin Management Planning cycle, the revision of the WFD and by developing new tools for diagnosing and predicting multiple stressors.
Monitoring Ecological Impacts provides the tools needed by professional ecologists, scientists, engineers, planners and managers to design assessment programs that can reliably monitor, detect and allow management of human impacts on the natural environment. The procedures described are well grounded in inferential logic, and the statistical models needed to analyse complex data are given. Step-by-step guidelines and flow diagrams provide the reader with clear and useable protocols, which can be applied in any region of the world and to a wide range of human impacts. In addition, real examples are used to show how the theory can be put into practice. Although the context of this book is flowing water environments, especially rivers and streams, the advice for designing assessment programs can be applied to any ecosystem.
Hydropower plants are an important source of renewable energy. In the near future, high-head storage hydropower plants will gain further importance as a key element of large-scale electricity production systems. However, these power plants can cause hydropeaking which is characterized by intense unnatural discharge fluctuations in downstream river reaches. Consequences on environmental conditions in these sections are diverse and include changes to the hydrology, hydraulics and sediment regime on very short time scales. These altered conditions affect river ecosystems and biota, for instance due to drift and stranding of fishes and invertebrates. Several structural and operational measures exist to mitigate hydropeaking and the adverse effects on ecosystems, but estimating and predicting their ecological benefit remains challenging. We developed a conceptual framework to support the ecological evaluation of hydropeaking mitigation measures based on current mitigation projects in Switzerland and the scientific literature. We refined this framework with an international panel of hydropeaking experts. The framework is based on a set of indicators, which covers all hydrological phases of hydropeaking and the most important affected abiotic and biotic processes. Effects of mitigation measures on these indicators can be predicted quantitatively using prediction tools such as discharge scenarios and numerical habitat models. Our framework allows a comparison of hydropeaking effects among alternative mitigation measures, to the pre-mitigation situation, and to reference river sections. We further identified key issues that should be addressed to increase the efficiency of current and future projects. They include the spatial and temporal context of mitigation projects, the interactions of river morphology with hydropeaking effects, and the role of appropriate monitoring to evaluate the success of mitigation projects.
Dams impound the majority of rivers and provide important societal benefits, especially daily water releases that enable on-peak hydroelectricity generation. Such “hydropeaking” is common worldwide, but its downstream impacts remain unclear. We evaluated the response of aquatic insects, a cornerstone of river food webs, to hydropeaking using a life history–hydrodynamic model. Our model predicts that aquatic-insect abundance will depend on a basic life-history trait—adult egg-laying behavior—such that open-water layers will be unaffected by hydropeaking, whereas ecologically important and widespread river-edge layers, such as mayflies, will be extirpated. These predictions are supported by a more-than-2500-sample, citizen-science data set of aquatic insects from the Colorado River in the Grand Canyon and by a survey of insect diversity and hydropeaking intensity across dammed rivers of the Western United States. Our study reveals a hydropeaking-related life history bottleneck that precludes viable populations of many aquatic insects from inhabiting regulated rivers.
Hydro peaking causes an important environmental impact on running water ecosystems. Many affected rivers have a poor ecological status. In rivers affected by hydro peaking, the flow conditions are highly complex and difficult to grasp. To develop a general framework for detecting and characterizing sub-daily flow fluctuations, we analysed more than 500 Austrian hydrographs, covering the whole range from unimpacted to heavily impacted rivers. Different fluctuation types could be identified according to the potential source: e.g. sub-daily flow fluctuations caused by hydro peaking, rainfall or snow and glacier melt. Additionally, the term 'hydro fibrillation' was established, to indicate frequently occurring artificial fluctuations with comparably low intensities. An automatic procedure was used to detect frequency and intensity of each flow fluctuation. Using variables based on duration curves of flow fluctuation rates (ramping rates), amplitudes, flow ratios, durations and daily numbers of fluctuations, a predictive model (linear discriminant analysis) was fitted to classify hydrographs into predominant fluctuation regimes. This is the basis for a detailed investigation of present sub-daily flow regimes and to analyse the differences between the regimes. Based on the results, we finally propose a general framework that enables a standardized assessment of flow fluctuations regarding event intensities and/or event timing. The proposed framework offers a standardized selection of particular flow fluctuations referring to increase and decrease events separately. The selection of specific flow fluctuations can be defined with respect to several research questions (e.g. ecologically relevant fluctuations), which offers a wide range of applications.