Measures to safeguard and restore river connectivity

Abstract

Freshwater connectivity and the associated flow regime are critical components of the health of freshwater ecosystems. When freshwater ecosystems are fragmented, the movements and flows of species, nutrients, sediments, and water are altered, changing the natural dynamics of freshwater ecosystems. The consequences of these changes include declines and loss of freshwater species populations and freshwater ecosystems, and alterations in the delivery of certain ecosystem services, such as fisheries, buffering of flood events, healthy deltas, recreational and cultural values, and others. Measures exist that can maintain and restore connectivity or mitigate against its loss in the face of constructed barriers or other habitat alterations. These measures include system-scale planning for energy and water resources that includes options for limiting loss of freshwater connectivity; putting in place protections for keeping critically important freshwater habitats connected; mitigating impacts on freshwater ecosystems via barrier design, fish passage, or implementation of environmental flows; and restoring freshwaters via barrier removal and reconnection of rivers, wetlands, and floodplains and via active management of groundwater recharge. We present case studies of measures applied in Europe, Asia, Africa, and the Americas and reflect on the next generation of innovation needed to further enhance and advance the implementation of restoration and protection and the mitigation of freshwater connectivity impacts.

Full text

Measures to Safeguard and Restore River Connectivity
Each Author’s Name, Address and Email Address:
Thieme, Michele
World Wildlife Fund, 1250 24th Street, NW, Washington, DC, USA
Michele.thieme@wwfus.org
Birnie-Gauvin, Kim
Technical University of Denmark National Institute of Aquatic Resources, Freshwater Fisheries
and Ecology,
Silkeborg, DK
kbir@aqua.dtu.dk
Opperman, Jeffrey J.
World Wildlife Fund,
Washington, USA
Jeff.opperman@wwf.org
Franklin, Paul A.
National Institute of Water and Atmospheric Research Hamilton,
Hamilton, NZ
paul.franklin@niwa.co.nz
Richter, Holly
Resilient Rivers LLC.
Hereford, AZ, USA
hollyrichter@resilientrivers.com
Baumgartner, Lee
Charles Sturt University, Gubali Institute,
Albury, NSW, AUS
lbaumgartner@csu.edu.au
Ning, Nathan
Charles Sturt University, Gubali Institute,
Albury, NSW, AUS
nning@csu.edu.au
Vu, An Vi
Research Institute of Aquaculture no 2,
Ho Chi Minh City, VN
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Charles Sturt University, Gubali Institute,
Albury, NSW, AUS
vvu@csu.edu.au
Brink, Kerry
WWF-Netherlands,
Zeist, NL
kbrink@wwf.nl
Sakala, Michael
World Wide Fund for Nature Zambia,
Lusaka, ZM
msakala@wwfzam.org
O’Brien, Gordon C.
University of Mpumalanga,
Mbombela, Mpumalanga, ZA
Gordon.Obrien@ump.ac.za
Petersen, Robin
SANParks,
Pretoria, ZA
Robin.Petersen@sanparks.org
Tongchai, Pakkasem
IUCN Thai Country Program,
Bangkok, TH
pakasemtc@gmail.com
Cooke, Steven J
Carleton University Department of Biology, Biology,
Ottawa, ON, CAN
StevenCooke@cunet.carleton.ca
Corresponding Author’s Name: Steven Cooke, StevenCooke@cunet.carleton.ca
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Abstract
Freshwater connectivity and the associated flow regime are critical components of the health of
freshwater ecosystems. When freshwater ecosystems are fragmented, movements and flows of
species, nutrients, sediments, and water are altered, changing the natural dynamics of
freshwater ecosystems. The consequences of these changes include declines and loss of
freshwater species populations and freshwater ecosystems, and alterations in the delivery of
certain ecosystem services, such as fisheries, buffering of flood events, healthy deltas,
recreational and cultural values, and others. Measures exist that can maintain and restore
connectivity or mitigate against its loss in the face of constructed barriers or other habitat
alterations. These measures include system-scale planning for energy and water resources that
includes options for limiting loss of freshwater connectivity; putting in place protections for
keeping critically important freshwater habitats connected; mitigating impacts on freshwater
ecosystems via barrier design, fish passage or implementation of environmental flows; restoring
freshwaters via barrier removal and reconnection of rivers, wetlands and floodplains and via
active management of groundwater recharge. We present case studies of measures applied in
Europe, Asia, Africa and the Americas and reflect on the next generation of innovation needed
to further enhance and advance the implementation of restoration and protection and the
mitigation of freshwater connectivity impacts.
Introduction
Freshwater connectivity is fundamental for healthy land and riverscapes and to many of the
services that they provide to humanity. These services include fisheries production, water
regulation (i.e., groundwater recharge and buffering from flood events), nutrient and sediment
transport to downstream floodplains, fields, and deltas, and recreational and cultural values
(Durance et al. 2016). The ability of freshwater ecosystems to sustain biodiversity and deliver
many ecosystem services is governed by the degree to which their natural flow regime and
connectivity are maintained. River or fluvial connectivity extends in four dimensions:
longitudinally (up- and down-stream in the river channel including to estuarine and ocean
systems), laterally (between main channel, floodplain, and riparian areas), vertically (between
groundwater, river, and atmosphere) and temporally (natural flows that include seasonal
variations) (Ward 1989). Some hydrologic processes, such as the movement of groundwater
through an aquifer, are three-dimensional in nature. Alterations in any of the four dimensions
can affect fluvial processes and functions that span abiotic and biotic realms.
Fluvial connectivity has been significantly affected around the world with over two-thirds of long
(>1000 km) rivers no longer considered free-flowing. Similarly, more than 1.2 million barriers,
nearly 70% of which are less than two meters in height, are fragmenting Europe’s rivers (Belletti
et al. 2020) and hundreds of barriers fragment the Lower Mekong Basin (Baumgartner et al.
2021). Despite widespread recognition of the role that river systems play in providing ecological
connectivity and functionality across fluvial landscapes (Fausch et al. 2002), existing policy
mechanisms and measurements of the health of rivers and watersheds often fail to include
connectivity measures. Many governments focus their monitoring of freshwater ecosystems on
water quality measures - with measurements and metrics of the status of environmental flows
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and fluvial connectivity only having been introduced in recent decades (Harwood et al. 2018), if
at all. One recent example is the commitment by the European Union to restore 25,000 km of
river under its Biodiversity Strategy 2030 (European Commission 2020).
The Issue
A diverse range of species depend on freshwater connectivity within and between river reaches,
floodplain habitats, wetlands, lakes and estuaries, for foraging, reproduction or seeking refuge
(Lucas et al. 2001; McIntyre et al. 2016). A classic example is the outmigration of salmon smolts
from rivers to the ocean, and their subsequent return as adults to spawn, but the range of taxa
relying on freshwater ecosystems and their connectivity extends well beyond fishes to include
birds, mammals, herptiles, invertebrates and plants. For example, recent work on
hemimetabolous (i.e., relying on both freshwater and terrestrial habitats) damselfly showed that
habitat connectivity strongly influenced the proportion of colonized habitat patches (Streib et al.
2020).
In many ways, connectivity represents the template upon which freshwater species have
evolved (e.g., Lytle and Poff 2004). As such, any alterations to flows and connectivity will have
significant effects on freshwater species. In fact, the science on that matter is quite clear:
fragmented freshwater systems have negative consequences for freshwater biodiversity (Brauer
and Beheregaray 2020; Dias et al. 2017; Gido et al. 2015).
The impacts of fragmented longitudinal connectivity are well documented, particularly for fishes
and other strictly aquatic taxa. Barriers, both big and small, can affect the movement of aquatic
species as they migrate upstream and downstream (Winemiller et al. 2016), but they also alter
flows of water and organic matter that provide cues for seasonal movements, can flood
upstream and alter downstream habitats (Birnie-Gauvin et al. 2017b; Bunn and Arthington 2002;
Ligon et al. 1995), and prevent the exchange of individuals and genetic information between
populations (Raeymaekers et al. 2008; Wilkes et al. 2019). Even if fish or other animals
overcome a barrier, they likely 1) expend high levels of energy, 2) must swim further to find
suitable habitat, 3) spawn in unsuitable habitat (or not at all), likely resulting in low survival of
young, or 4) become injured or die. As such, fragmentation of longitudinal connectivity is about
much more than passage of aquatic species and has been directly linked to the extinctions, and
local extirpations, of species (e.g., see Table 1 in (Birnie‐Gauvin et al. 2019)). In contrast, dams
do not appear to impact the movement of freshwater birds as they can fly over them, nor do
they seem to impact their survival. In fact, reservoirs may provide habitats for birds that would
otherwise be seasonally dry. However, freshwater bird species are still declining, so barrier
impacts may still be negative in the long-term via impacts on water quality or prey items, for
example (McAllister et al. 2001). Dams and their reservoirs have been shown to fragment
populations of terrestrial and semi-aquatic vertebrates (e.g., platypus in Australia (Mijangos et
al. 2022) and terrestrial vertebrates in the Amazon (Benchimol and Peres 2015)).
Lateral connectivity is important for flows to floodplain wetlands and the habitats that they
provide for freshwater and aquatic species. Floodplains are biologically diverse, with large
populations of waterbirds, invertebrates, and fish. However, altered flow regimes caused by
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dams or other barriers can disconnect floodplains and seriously decrease species biodiversity
and abundance (Boulton and Lloyd 1991; Gergel et al. 2002; Halse et al. 1998; Kingsford 2000;
Opperman et al. 2009). When wetlands receive water via lateral flow, they also receive energy,
matter and organisms, setting in motion a cycle that shapes the entire ecosystem (Jenkins and
Boulton 2003; Kingsford and Porter 1999), but dams alter this cycle. For example, reduced
flooding in the Chowilla floodplain (lower Murray River basin, Australia) has resulted in
decreased abundance of invertebrates, which will likely result in the decline of native fish and
waterbird species that rely on invertebrates to survive (Kingsford 2000). Dams and other forms
of river management in that system divert almost 10,000,000 ML every year, causing flows to
the Chowilla floodplain to be about half what they used to be (Maheshwari et al. 1995).
Vertical connectivity links the river channel to the hyporheic zone and plays a crucial function for
sustaining baseflows as long as the water table remains above the stream bed (Delleur 1999).
However, given the inherent complexity of groundwater aquifers and our inability to directly
observe them, the underlying importance of groundwater-surface water connectivity for
supporting freshwater biodiversity is commonly overlooked or misunderstood. The hyporheic
zone itself provides habitat for a range of microbes and invertebrates, sometimes several
kilometers away from the channel, which contribute to secondary production among other
functions (Boulton 2007; Marmonier et al. 1992; Stanford and Ward 1988). Regional
groundwater aquifers within a basin sustain the groundwater levels of near stream alluvial
aquifers that are essential for groundwater dependent ecosystems, including riparian and
wetland habitats. Humans have reduced both lateral and vertical connectivity by altering
patterns of water flow via water abstraction (particularly of groundwater) and dam building
(Hancock 2002), and through land use change (Gibert et al. 2009; Moldovan et al. 2012). This
includes changes in the hydrologic cycle such as those caused by increased levels of
impervious surfaces (Mojarrad et al. 2019) or reduced flows due to afforestation (Hughes et al.
2020), and sedimentation of waterways which causes infilling and blockage of the hyporheic
zone (Shrivastava et al. 2020). Although the effects of vertical fragmentation on higher trophic
species are not well documented, they do exist, as exemplified by the reduced survival of
salmonid eggs with reduced hyporheic flow (e.g., Bowerman et al. 2014).
The temporal dimension of connectivity is critical for the viability of many freshwater species.
Freshwater processes are often driven by seasonal changes in flow and the frequency of
flooding. Hermoso et al. (2012) highlighted the importance of considering temporal connectivity
for freshwater fish, waterbirds and turtles by demonstrating that integrating water residency time
(i.e., an estimate of the time during which connections between aquatic habitats were available
for them to access refugia) into prioritization processes (in addition to the usual spatial
connectivity) increased water residency time by 40% in priority areas. In essence, the
consideration of temporal connectivity helped to identify the periods with the longest spatial
connections, and thereby maximize the role of freshwater as refuge during dry periods.
Furthermore, freshwater biota often have seasonal reproductive cycles. Interrupting these
cycles has a dramatic impact on successful spawning and recruitment processes, which are
temporal events.
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Flows of sediment and other organic matter are also part of the natural flow regime and critical
in shaping the physical template for fluvial ecosystems and associated aquatic habitats
(Constantine et al. 2014; Harvey 1991). This affects, for example, the shape of riverbeds and
spawning habitat for fish and other species. Sediment capture by upstream dams and other
infrastructure can cause a cascade of impacts on fluvio-geomorphological dynamics and
processes far downstream, and reduce sediment delivery for floodplains and deltas alike,
ultimately impacting coastal morphology and ecosystems, leading to increased rates of delta
subsidence and coastal erosion (Petts and Gurnell 2005; Schmitt et al. 2017; Vörösmarty et al.
2003).
While many rivers and their floodplains around the world have been fragmented, there are still
many large and small rivers that maintain high levels of connectivity. The connectivity of these
systems is critical to remaining refuges for freshwater biodiversity (e.g., the Amazon and
tributaries, the Irrawaddy and Salween Rivers). Maintaining and restoring freshwater
connectivity is one of the six actions required to bend the curve for freshwater biodiversity
identified by Tickner et al. (2020). The main aim of the manuscript is to present a set of
measures to protect and restore connectivity (Figure 1) and to illustrate their potential for
addressing connectivity issues by providing case studies that highlight successful
implementation. A secondary aim is to provide a comprehensive overview of each of the
measures, including a review of the current state of understanding of effectiveness of the
measure and areas for further research. The measures were selected using collective
knowledge of the assembled authors and each expert conducted a review of scientific and grey
literature for their respective measure and area of expertise. The sequence of presentation of
the measures is in line with the mitigation hierarchy– i.e., first line actions should be focused on
avoiding loss of connectivity (i.e., planning siting of infrastructure in locations with no or minimal
impacts; putting in place protection mechanisms that safeguard connectivity of river corridors),
where that is not possible action can be taken to minimize or mitigate impacts (i.e., design of
dams and other water-related infrastructure; operating dams in line with environmental flows),
and finally where damage has already occurred actions can be taken to restore the system (i.e.,
dam or other water-related infrastructure removal) (Arlidge et al. 2018; Gann et al. 2019). We
also indicate both the step(s) of the mitigation hierarchy with which each measure is associated
and the dimension(s) of river connectivity that the measure has the potential to protect or
restore (Table 1).
Measures to Maintain and Restore River Connectivity
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Figure 1. Illustration of measures that can support maintaining and restoring river connectivity.
Strategic Planning for Energy, Water Resources & Biodiversity
Dams, and particularly dams with hydropower, have been a primary driver of the fragmentation
of large rivers worldwide, resulting in a loss of connectivity. The expansion of hydropower into
undammed river basins is a leading current cause for the loss of free-flowing rivers and threat of
future conversion (Thieme et al. 2021; Winemiller et al. 2016). Multi-purpose dams and irrigation
dams are also prevalent in basins around the world with anticipated continued additions as
climate change and aging infrastructure decreases available water storage (Baumgartner et al.
2021; McCartney et al. 2022). In their report for the World Bank, Ledec and Quintero (2003)
argue that, in terms of environmental and social impacts, project location is the single most
important decision about a proposed dam – and this is particularly true for connectivity impacts.
Regulatory and planning processes often require environmental assessments of dams that
entail quantifying potential negative impacts, evaluating tradeoffs, and informing decisions about
a proposed dam. However, in practice, environmental review tends to focus on single projects
and is often applied after major decisions, such as those about project size and location, have
been made. Because review often happens after major investments have already occurred, and
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political momentum has been generated, the process rarely results in rejection of a proposed
project (Sadler et al. 2000). Thus, as commonly applied around the world, environmental review
generally has little or no influence on project siting. Similarly, the current Hydropower
Sustainability Standard (Hydropower Sustainability Secretariat 2021), developed through a
process led by the International Hydropower Association, is generally focused, and applied, at
the level of single projects and often after decisions about location have already been made.
Due to the limitations of project-level assessment for influencing the location of proposed
projects, various organizations and researchers have recommended system-scale approaches
to dam planning, such as Strategic Environmental Assessment (Sadler et al. 2000), needs and
options assessments (World Commission on Dams 2000), and other basin-scale assessments
and planning processes that integrate both conservation planning and infrastructure planning
(Mekong River Commission 2016; Opperman et al. 2017a; Twardek et al. 2022). These
approaches are intended to assess multiple different options for siting dams, quantify their
performance across a range of social, economic, and environmental metrics, and, if possible,
identify options that perform well across multiple objectives.
The range of options can be greatly expanded—and thus the potential for identifying options
that perform well across multiple objectives can be increased—if the scale of planning extends
beyond siting to include other alternative options for meeting resource needs. In the case of
hydropower systems, this means expanding from hydropower planning to energy system
planning (Opperman et al. 2023). For example, energy master plans or Integrated Resource
Plans (IRPs; see case study below) provide a framework to compare pathways for meeting
projected energy demands, encompassing different generation technologies, storage options,
transmission as well as demand-side management, including energy efficiency and dynamic
demand management. Within these frameworks, hydropower technologies can be compared
against other technologies for meeting needs for generation and storage, allowing a broader
range of options to be compared in terms of environmental and social impacts as well as grid
performance and cost. Similarly, for the objective of water storage, achieving balanced
outcomes across objectives, including maintaining free-flowing rivers, will be more likely if
planning is expanded beyond that for storage dams to also include natural storage options, such
as managed aquifer recharge (Yu et al. 2021).
Research has demonstrated that energy system planning can identify grid expansion pathways
that are low carbon, cost-competitive and that minimize negative impacts to rivers. For example,
Shirley and Kammen (2015) found that decentralized generation technologies (along with more
realistic forecasts of future demand) could obviate the need for major new hydropower dams in
Sarawak that would have displaced indigenous people. Opperman et al. (2023) demonstrated
that Chile could meet energy demands without damming free-flowing rivers and that Uganda
could meet future demand without additional hydropower projects—in both cases for essentially
equivalent cost to options that involved greater expansion of hydropower. In essence, well-
designed energy plans at the system-scale can be an effective measure for planning needed
energy in ways that keep rivers free-flowing or well-connected.
Case study, IRP process in Zambia
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Zambia’s current electricity grid relies primarily on hydropower, which provides 80% of
generation from 2.4 GW of installed hydropower capacity (Energy Regulation Board 2021), with
much of that developed on the Zambezi River and its largest tributary, the Kafue River. Demand
for electricity is rising rapidly, between 150 and 200 MW per year, due to economic growth and
the need to dramatically expand electricity access from the current electrification rate of 25%.
Consequently, the government has proposed developing an additional 6,000 GW of hydropower
capacity. However, climate variability and drought have already begun to impact current
hydropower production, leading to energy shortages. To emphasize the criticality of the energy
security, on the 6th January, 2023, the Kariba Dam, designed to provide the bulk of electricity
consumed in both Zambia and Zimbabwe of 1080 MW and 1050 MW respectively, hit the record
low of 1.38 percent water level of usable water according to the Zambezi River Authority. The
Zambezi River Authority has been required to reduce generation activities (250 MW out of 1080
MW) until a further review of the substantive hydrological outlook at Kariba is undertaken. This
is the lowest record low water level experienced since the 1995/96 period. These shortages
have increased the urgency for developing new capacity as well as diversifying the energy mix.
As part of the government’s plans to increase hydropower capacity, the 240 MW Ndevu Gorge
Power Project was proposed for the Luangwa River, a tributary of the Zambezi and, at 1100 km,
one of the longest remaining free-flowing rivers in southern Africa. The Luangwa serves as a
key resource for 25 communities and flows through two iconic national parks that support
abundant wildlife. Because the proposed dam would have had major negative impacts on these
resources, the communities and conservation organizations opposed the dam and, in 2019, the
Zambian government canceled the pre-feasibility study and halted the project.
To guide expansion of Zambia’s power system, the Zambian Ministry of Energy initiated a
process to develop an Integrated Resource Plan (IRP) for the power sector for the next 30
years. The IRP process is focused on developing a long-term strategy to meet Zambia’s
projected energy demands in a sustainable, reliable and affordable pathway, including
diversification of the Zambian energy mix to include other generation technologies such as solar
and wind and storage, including Pumped Hydropower Storage. The inclusion of a broader range
of generation and storage options will make it more likely that Zambia can provide low cost and
low carbon power for its people and economy while minimizing additional damming of free-
flowing rivers. The IRP is also incorporating the climate change scenarios into its system-scale
planning approaches to support robust decision making about energy options.
River Protections that Safeguard Connectivity
A measure that has effectively safeguarded river connectivity in certain parts of the world is the
designation of rivers as protected or conserved. There are a range of designation types that
have protected rivers against fragmentation. In many countries IUCN category I or II protections,
like national parks, often prohibit building of infrastructure for commercial purposes and/or that
would significantly degrade natural ecosystems. In some countries, river-specific designations
have also been created that explicitly prohibit the building of dams and other infrastructure that
would degrade the free-flowing nature of the river. A newly emerging type of designation
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provides “rights for rivers”, legally granting rivers to be recognized as living entities with
inalienable rights.
National level programs that have been enacted into law to specifically keep rivers free-flowing
are relatively rare around the globe. The earliest example comes from the United States where
the US National Wild and Scenic Rivers Act of 1968 enabled the designation of free-flowing
rivers with outstanding natural resource values as Wild and Scenic Rivers (U.S. Congress
1968). As of 2019, over 21,565 km of 226 rivers in 41 states and the Commonwealth of Puerto
Rico were designated, representing less than one half of one percent of the nation's rivers
(NWSR 2019). Once designated, the free-flowing condition and essential characteristics of a
river that existed at the time of designation are to be preserved, and if possible enhanced.
Between the 1970s and 2010s several European nations have put in place legislation that
protects rivers from hydropower or other infrastructure development. These include the
protection of the remaining major free-flowing rivers in Sweden, designation of protected rivers
in Finland under the Rapids Protection Act, creation of a Natural River Reserves system in
Spain and a protection plan for watercourses across Norway (Schäfer 2019). Sub-national and
national laws aimed at designation of individual river protections have also occurred. For
example, a state law in Minas Gerais protects the Cipó, São Francisco, Pandeiros e Peruaçu,
Jequitinhonha, and Grande Rivers in Brazil, a 1976 national law protecting the Soča River and
tributaries in Slovenia (then part of the former Yugoslavia) has prevented hydropower
development, the Sarapiquí River in Costa Rica was protected from hydropower development
and further mining concessions for at least 25 years under a national law in 2022, and the
Bhagirathi River in India is protected by the 1986 Environmental Protection Act (Perry et al.
2021; RAFA 2022)
Rivers have been legally granted rights of personhood in several countries around the world.
Although the motivation for these rights is often due to water quality degradation, maintenance
of river connectivity may also be defensible under the designated rights. Examples of legal
designation of river rights come from New Zealand (Whanganui), Ecuador (Vilcabamba River),
India (Yamuna and Ganges), and Colombia (Atrato River) (Perry et al. 2021).
In addition to river-specific protections, allocations of environmental flows or water reserves, if
effectively implemented, can prevent the building of dams and other infrastructure that
fragments rivers. For example, the environmental water reserve for the San Pedro Mezquital
(decreed in 2014) has been part of efforts to prevent the Las Cruces hydropower dam from
being built. Part of the argument used to prevent the dam has included that the dam would
affect flows designated in the Environmental Water Reserve. In particular, the flows required to
reach the mangroves downstream in the Marismas Nacionales Ramsar site as well as those
required for social resources related to Indigenous People’s rights (Salinas-Rodríguez et al.
2021).
However, designation of a river as a protected or conserved area does not guarantee that the
river or river stretch will remain protected from development in perpetuity. An early example
comes from Yosemite National Park in the US where the Hetch Hetchy Dam was built within the
national park boundaries in 1913 to supply water to San Francisco. Evidence that this is not an
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isolated incident comes from Thieme et al. (2020), who documented a total of 342 dams that
had been built within protected areas around the world after establishment. Nevertheless, river
protection designations that explicitly safeguard river connectivity remain an important, and
increasingly implemented, measure to ensure the health of rivers and keep them connected and
free-flowing.
Case study: Vjosa National Park: Steps toward protecting a free-flowing stronghold in Europe
The Albanian government has recently taken several important steps towards ensuring the long
term protection of the free-flowing nature of the Vjosa River. The Vjosa, one of the longest
remaining free-flowing rivers in Europe, still flows 270 km nearly unimpeded from its source in
the Pindus mountains in Greece to the Adriatic Sea in Albania. It is considered an extremely
rare reference site for medium-sized rivers in Europe as its hydrological dynamics, including
sediment flows and floodplain characteristics, remain in a near-natural condition. The gravel-bed
river supports a uniquely intact river-dependent fauna typical of highly dynamic large rivers.
These types of rivers have lost a big proportion of their former distribution in Europe. Several
fish species native to the Vjosa depend on river connectivity for short and long distance
migrations to complete their life cycle, including the endangered European eel (Anguilla
anguilla). The river hosts 37 fish species, at least 267 distinct taxa of aquatic invertebrates and
hundreds of aquatic plants (Meulenbroek et al. 2021). The river has long been threatened by the
development of hydropower dams along its course but the Albanian government has recently
canceled the Kalivaç and Pocem dams that were to be built on the mainstem of the river after
intense pressure from civil society groups and local peoples. In June of 2022, the Ministry of
Tourism and Environment of Albania and the company, Patagonia, signed an agreement to
work together to upgrade the protection level of the Vjosa River Basin and its free-flowing
tributaries to an IUCN Category II Level National Park (Patagonia 2022). The intention is that
the river system itself is the subject of protection, along its entire course within Albania.
Barrier Design and Fish Passage
While the ‘effectiveness’ of individual fish passages have been highly variable (Bunt et al. 2012;
Hershey 2021; Noonan et al. 2012), biologically-informed design of instream structures to
account for the needs of migratory or otherwise mobile species has been shown to be able to
reduce their impact on this aspect of river connectivity (Larinier et al. 2002; Williams et al. 2012).
As far as practicable, all new structures should be designed in a way to eliminate or minimize
their impact on migratory species. Particularly in the case of small structures (e.g., culverts)
good design can effectively eliminate any barrier effect (Behlke et al. 1991). For larger
structures (e.g. dams), where elimination of impacts is rarely a realistic option, the impacts on
migratory species can be mitigated through, for example, integration of fishways (Bunt et al.
2012) or at facilities with turbines or other intakes, use of structures or technology that reduce
entrainment and related mortality (Algera et al. 2020). In the case of existing barriers, efforts can
be made to modify their design to reduce their impacts on freshwater biodiversity through
removal (see barrier removal section), replacement with fish ‘friendlier’ designs, or remediation.
There are also instances where barriers are intentionally created to fragment systems to impede
migration of invasive species (McLaughlin et al. 2013). So called “selective fragmentation”
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(Rahel and McLaughlin 2018) appears to be increasing in popularity and is a good reminder that
there may be specific instances where maintaining or restoring full ecological connectivity may
do more harm than good. Fortunately, there are a growing number of examples where facilities
have been designed that can pass desirable species while blocking undesirable species (Kerr et
al. 2021).
There is increasing evidence to show that small structures (e.g. culverts, fords and low-head
weirs) can have a disproportionate impact on river connectivity (Evans et al. 2015;
Januchowski-Hartley et al. 2013). However, in many cases careful design of these structures
can render them almost invisible to aquatic organisms and provide for continuity of geomorphic
processes. A good example is the ‘stream simulation’ approach to culvert design (Cenderelli et
al. 2011). Culverts have replaced bridges as the stream crossing of choice, particularly for
smaller waterways, largely due to their lower cost and ease of installation (Frankiewicz et al.
2021). However, traditional hydraulic design approaches often constrict the channel cross-
sectional area through the culvert, resulting in elevated water velocities fish are unable to swim
through, and cause erosion at the culvert outlet, creating drops that can be unsurpassable to
fishes. The stream simulation approach to culvert design rethinks crossing design to account for
both hydraulic conveyance requirements and the needs for aquatic organism passage and
maintenance of hydro-geomorphic processes.
For both small and large structures, constructed passage devices can be installed at the time of
construction, or retrospectively, to help mitigate or remediate their impacts and enable the safe
upstream and/or downstream movement of animals and sediments (just downstream).
Historically, this has focused on facilities designed to enable the upstream movement of fishes
past dams, and are most often referred to as “fishways” (Birnie‐Gauvin et al. 2019). However,
more recently attention has increased on providing solutions to improve upstream passage
efficiency at smaller structures, such as culverts and weirs (e.g., David et al. 2014; Goodrich et
al. 2018; Leng et al. 2019; Magaju et al. 2021), and structures designed to facilitate downstream
passage of sediments (and animals).
Fishways come in many forms (see Clay 1995) with highly engineered structures made of
reinforced concrete among the most common. Each type has various benefits and limitations.
For example, pool and weir style designs require jumping ability (e.g., salmonids; (Collins et al.
1962)) and, thus, are a poor choice for benthic species. Denil fishway and vertical slot fishway
designs have shown promise for passing more diverse fish communities including some smaller
bodied, weaker swimmers (e.g., Bunt et al. 2012). In some types of fishways, animals are
trapped and then hoisted or transported by land, boat or elevator past the barrier (e.g., Oldani
and Baigún 2002; Pompeu and Martinez 2007). Over the last few decades more nature-like
fishways have been designed with the intention of better emulating a stream or river. The
challenge with such designs is that a large footprint is needed to maintain low gradients and this
may not always be available, particularly at large dams or pre-existing dams. Small nature-like
passages for very small dams (e.g., <1 m head) can be constructed by volunteers using hand
tools and have proven effective for restoring connectivity for small-bodied fishes (Steffensen et
al. 2013). However, larger facilities are more common with one of the largest being at the Itaipu
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Dam in Brazil (Makrakis et al. 2011). In the latter example, some fish became resident in the
passage structure with only two species using it for complete passage, emphasizing the need to
understand the ecological requirements and migratory behavior of fishes when designing and
assessing facilities.
To be successful, passage facilities must first attract aquatic species to the device and then the
species must be able to fully pass through it (Bunt et al. 2012). Competing flows and complex
channels can confuse fish, which are often attracted to flows (rheotactic) and particularly so
during migration making it challenging to direct fish moving upstream towards the passage
entrance. Through adaptive management it has, however, been possible to optimize attraction
flows for a given species or assemblage (e.g., Bett et al. 2022). Assuming a fish finds the
entrance to the fishway, they then must be able to pass it. Fish body size, morphology,
condition, motivation and more influence whether a fish will be successful in passing a given
fishway (Bunt et al. 2012; Castro-Santos et al. 2009; Hershey 2021). So – even if a fish can
ascend a fishway but is unable to locate it – it will fail. And similarly, if a fish finds the fishway
entrance but is unable to ascend – it will fail.
A range of low-cost solutions for improving passage at small instream structures have also
emerged in recent years (Frankiewicz et al. 2021). A key focus has been baffle design (e.g.,
weir baffles, offset baffles) to facilitate the upstream movement of fishes through culverts. High
water velocities within the culvert barrel can exceed the swimming capabilities of fishes,
preventing them from passing upstream. Installation of baffles can reduce bulk velocities and
create low velocity boundary layers and resting zones that enable fish to successfully move
upstream. Efforts have also been made to design fish ramps to overcome drops at culvert
outlets (or low-head weirs). These include rock-ramp (e.g., Muraoka et al. 2017) and artificial
baffled ramp designs (e.g., Baker 2014) and have recently extended to exploring the relative
passage performance of native versus exotic species to design selective passages that
intentionally exclude exotic species (e.g., Franklin et al. 2021).
Enhancing the upstream movement of organisms has received most attention in fish passage
research and practice (Silva et al 2018) but developing solutions to restore downstream
passage are a priority and are receiving increasing attention (Lennox et al. 2019)).
When fish “go with the flow” downstream that often means entrainment in turbine or flood pump
intakes (Boys et al. 2021). Fish ‘friendlier’ turbine and pump designs can reduce fish mortality
and improve downstream connectivity (Buysse et al. 2014; Watson et al. 2022). Similarly,
operational changes can reduce entrainment and increase passage success (Baker et al. 2020).
Alternatively, bypass facilities have been built to collect and direct fish to safe paths and use of
various behavioral guidance methods (e.g., louvers, flashing lights) has proven somewhat
effective for guiding some fish toward safe areas. For example, Scruton et al. (2003) report on
the use of a louver system at a bypass to successfully guide most Atlantic salmon smolts to safe
passage. However, it is clear that behavioral guidance that exploits sensory physiology
mechanisms requires a nuanced understanding of a given species and contextual information
about a site, meaning their effectiveness can be highly variable (Elmer et al. 2021). Sediment
bypass tunnels have been constructed to enable the downstream movement of sediment (Boes
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et al. 2014 ; Kondolf et al. 2014) although they remain rare. Although conceptually it is possible
to have a single structure that passes both sediment and fish downstream (Foldvik et al. 2022),
to our knowledge no such facilities have been tested.
Most fish passage structures are built and then never formally studied to determine their
effectiveness (Katopodis and Williams 2012). Furthermore, where assessments are carried out
measures of ‘effectiveness’ are highly variable (Bunt et al. 2012; Hershey 2021; Noonan et al.
2012). Many of the first fishways and those that have been most studied were purpose-built for
salmonids, which are strong swimmers with good jumping abilities (Birnie-Gauvin et al. 2019).
Not surprisingly, passage devices designed for salmonids have not performed well for other
species (Mallen-Cooper and Brand 2007). As designs have diversified to meet the needs of
different types of species in the last decade, there is growing evidence that fish passage is
possible for non-salmonids (reviewed in (Bunt et al. 2012). There have also been several cases
where small modifications in design or operation of devices have greatly enhanced passage
(e.g.,Bunt 2001; Naughton et al. 2007). Engineers and biologists working collaboratively can
enhance the likelihood of developing effective fish passage solutions (Williams et al. 2012).
Fish passage can also have broader benefits to other taxa. For example, installation of a
fishway along an Australian River enabled recolonization by freshwater molluscs upstream
given the role of fish as glochidial hosts (Benson et al. 2018). There are relatively few examples
of purpose-built facilities for other taxa.
While fishways and other fish passage solutions have facilitated passage of aquatic species and
sediments in certain cases, their ability to fully mitigate the blockage of species and sediment
passage comprehensively is limited. Entrainment or impingement of aquatic species can also
result in direct losses including through mortality (e.g., turbine strikes; (Algera et al. 2020)).
Consequently, structure removal, or replacement with fish ‘friendlier’ designs, remains a priority
for restoring connectivity. Nonetheless, there continue to be efforts to improve the effectiveness
of mitigation for infrastructure that limits connectivity or contributes to mortality through facility
design. Increasingly solutions are emerging that function with a diverse fish community (or
broader assemblage of animals) in mind and more effort is going into evaluating the success of
remediation to support design and operational refinements (e.g., ensuring fishways are
maintained and do not become clogged with debris), such that fishways and other solutions can
play a role in mitigating some of the impacts of connectivity disruption.
Case study: Fishways in the Lower Mekong River in Laos
The Mekong River Basin is home to significant freshwater fisheries diversity (over 800 species)
which is currently under threat from river infrastructure and hydropower projects (Ferguson et al.
2011). These are blocking important migration pathways for migratory fish. There have been
significant efforts to improve riverine connectivity through the construction of fish passes (or
fishways) (Baumgartner et al. 2019). Fish passage technologies to assist connectivity were
initially piloted, and co-designed, with local communities (Baumgartner et al. 2012b). Work
focused, initially, on vertical slot fishways and expanded to submerged orifice and “cone”
designs. The experimental proof of concept was first undertaken to establish that fish would use
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the technology and a cone fishway was selected as the best candidate for a permanent
installation (Baumgartner et al. 2012a). The pilot stage was extremely successful. Over 100
species made use of the “cone” fishway and fishers began reporting catches of species which
had not been seen in over 20 years (Baumgartner et al. 2022).
Upon completion of the pilot stage, these structures are now being scaled into increasing
numbers of dam and irrigation projects (Campbell and Barlow 2020). Over thirty structures have
been put in place in the Lower Mekong Basin (Figure 2). The first-ever fishway in Vietnam has
just been completed. The case for implementing these measures more widely was based on
two major elements. First, there was a need to develop the financial case. A decision support
tool was developed to estimate, in economic terms, the return on investment and benefits to
local communities from increased protein provided by an improved fishery (Cooper et al. 2019).
Proponents could then quickly determine the relative viability of a project based on expected
returns. Second, there was a need to understand the motivations and abilities of actors in the
decision making ecosystem in order to make the business case in a way that resonated and
allowed for inclusion of this new approach into development programs (Conallin et al. 2022a;
Salter et al. 2020)
Sites where these interventions have been applied are experiencing significant increases in fish
biodiversity and contributing to community cohesion. Importantly, many fishers are also
reporting catching fish that had not been seen for many decades (Millar et al. 2019). Translating
these early successes, into region-scale donor investments, is what is needed to truly bend the
curve (Conallin et al. 2022b). Current initiatives are focused on building human and institutional
capacity to ensure decisions regarding the implementation of fish friendly solutions are
“automatically” considered in future infrastructure development projects (Baumgartner et al.
2017). Importantly, this program succeeded owing to the inclusion of local communities in
design, implementing a robust monitoring program, applying the principles of adaptive
management to future designs, and developing a comprehensive evidence base to support
enhanced decision making.
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Figure 2. Locations and types of fish passes planned and/or constructed in the Lower Mekong
Basin.
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Dam Operations for Environmental Flows
Environmental flows are defined as the hydrological regime required to sustain freshwater and
estuarine ecosystems and the human livelihoods and well-being that depend on them
(Acreman 2016). Environmental flows are often, although not exclusively, set within the context
of reoperation of existing infrastructure, such as dams and diversions. As such, they are an
important mitigation measure to reduce the impacts of infrastructure systems on the natural flow
regime and freshwater ecosystems. They can be a particularly useful mitigation measure for
restoring lateral connectivity. Societal choice plays a significant role in setting the objectives for
environmental flows. For example, changes in the flow regime can set to meet cultural (e.g.,
flow needs for certain spiritual activities), recreational (e.g., river rafting or kayaking), or
environmental (e.g., flood flows to cue spawning or to inundate floodplain nursery habitats)
objectives. We refer the reader to Arthington et al. (in this special issue) who review a set of
case studies where environmental flows have been successfully implemented and provide a
summary of key enabling factors for implementation.
Barrier Removal
Where barriers exist, their removal is the only solution that fully restores all aspects of
connectivity (Bednarek 2001; Birnie‐Gauvin et al. 2019). Removing barriers, specifically those
that are unsafe, obsolete, financially unviable, or even those whose ecological impacts are too
great to ignore, provides an unprecedented opportunity to restore connectivity across its four
dimensions. Removing barriers restores the natural flow regime (Bednarek 2001; Poff et al.
1997), and the habitat that a multitude of organisms depend on to feed, spawn and seek refuge
(Birnie-Gauvin et al. 2017a; Bubb et al. 2021). It restores substrate, macrophyte growth (Hill et
al. 1993), the flow of sediments (Poff et al. 1997) and temperature regimes (Bednarek 2001;
Yeager 1994). Importantly, barrier removal increases the abundance and (often) the diversity of
fish and invertebrate species (e.g., Birnie‐Gauvin et al. 2018; Bubb et al. 2021; Ding et al. 2019;
Hill et al. 1993) as well as riparian vegetation (Brown et al. 2022). Moreover, it reconnects
aquatic habitats and the passage of organisms within and between these habitats
(Birnie‐Gauvin et al. 2018; Dynesius and Nilsson 1994; Kukuła and Bylak 2022; Weigel et al.
2013). In essence, barrier removal means restoring a river to its natural state, so it is perhaps
not surprising that the dam removal movement is growing (see for example Dam Removal
Europe, www.damremoval.eu).
Of course, restoring connectivity through removal can also permit the movement of invasive
species into a system, or farther upstream within an already invaded system, creating
challenges for managing fragmentation that maintains hydrological connectivity while blocking
invasion from non-native species (Rahel 2013). Although barriers designed to deter aquatic
invasive species have shown moderate success, most studies have been too short to detect
adequate ecological impacts, highlighting the need for refining design and operation of such
barriers, particularly if they are to enable the passage of native fauna (Jones et al. 2021). For
example, the eradication of non-native rainbow trout above a barrier has enabled native
mountain galaxias to recolonize the area above the barrier, but not below, where rainbow trout
were still present (Lintermans 2000). So, although barriers may prevent the movements of
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invasives, they may also affect those of native species, so careful consideration is required in
these instances.
Barrier removal is a restoration measure that can have direct ecological, social and economic
benefits (American Rivers 2019; Bednarek 2001; Bellmore et al. 2017; O'Connor et al. 2015;
Schiermeier 2018 ). Additional to the ecological ramifications of damming, safety hazards of
aging barriers and economic considerations are among the top reasons for removing barriers –
that is, removal is typically less costly in the long-term than the costs of maintaining a barrier
(Doyle et al. 2003; Pejchar and Warner 2001; Silva et al. 2018). A return-on-investment analysis
of barrier removal projects in the North American Great Lakes found that removing both dams
and road culverts had the greatest potential to benefit fishes, and demonstrates the importance
of both small and large removal projects (Fitzpatrick and Neeson 2018). Moreover, there now
exists several systematic methods for prioritization of barrier removal including cost-benefit
analyses, spatial graphs for habitat suitability modeling and several other publicly available tools
(Branco et al. 2014; Garcia de Leaniz and O'Hanley 2022; Hermoso et al. 2021; Kemp and
O'Hanley 2010; Whitelaw and Macmullan 2002).
Removal of barriers as a policy priority has been limited. The EU policy (the EU Biodiversity
Strategy for 2030 has a target that at least 25 000 km of rivers will be restored into free-flowing
rivers by 2030) and the recently passed US infrastructure bill included US$2.3 billion for
increasing hydropower capacity without adding new dams (through retrofits and powering non-
powered dams) and for removal of aging dams to restore rivers and improve public safety.. In
Southeast Asia and Oceania, there has also been a rise in barrier removal cases over the past
two decades (Ding et al. 2019). In Australia, New South Wales implemented the ‘fish
superhighways’ project which is the largest fish passage remediation program in Australia.
Despite this movement, a recent review indicates that within the U.S., less than 10% of the 1200
removed dams have been scientifically evaluated. Thus, the need for long-term monitoring and
robust study designs must be addressed to predict the impacts of removal and inform decision-
making (Bellmore et al. 2017). Stakeholder involvement is crucial to successful removal projects
and can even lead to restorative environmental justice in some cases. The Ottaway, Penobscot
and Elwha rivers are examples where Native American Tribes have played a key role in
removals by bringing cultural, economic and legal resources into the process.
Barrier removal Case Studies: Dam removal in Denmark and South Africa
Denmark is considered a leader in river restoration with no new barriers having been built since
1973 and hundreds removed. Given its relatively flat landscape, most barriers in Denmark are
small, and yet have important repercussions on habitat availability (Birnie-Gauvin et al. 2017a)
and fish populations (Birnie-Gauvin et al. 2017a). Removal of these barriers – primarily small
hydropower dams and weirs used in the fish farming industry – has led to rapid and impressive
benefits at both local and catchment scales, as well as across the lifecycle of salmonids (brown
trout Salmo trutta and Atlantic salmon Salmo salar; (Birnie-Gauvin et al. 2017a; Birnie-Gauvin et
al. 2017b; Bubb et al. 2021). River Villestrup, where 6 weirs were removed, is a striking example
where the trout smolt run went from 1600 to just under 20000, and the spawning population
from 350 to 3600 (Birnie‐Gauvin et al. 2018). The Trend, and Idom Rivers are just two of the
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many rivers where young-of-the-year trout were essentially absent, but where their density
exploded in the 1-2 years following removal (Figure 3).
Figure 3. Density (no. of fish per m2) of young-of-the-year (green) and older (blue) brown trout
(Salmo trutta) in river Trend (left), and Atlantic salmon (Salmo salar) in river Idom both before
and after removal.
In some regions, particularly in developing countries of Africa and South America, dam removal
has gained little momentum, if any. In Africa, the only known barrier removals are in the Kruger
National Park (KNP), a biodiversity hotspot in South Africa, where conservation managers have
removed 26 obsolete dams. These removals form part of the water management plan that
allows for removing redundant barriers with the aim of improving natural fish migration patterns
and restoring natural aquatic ecosystem processes. The KNP is given the mandate to
independently manage the region which is a biodiversity hotspot in Southern Africa and an
important tourist attraction for international travelers.
The Shingwedzi River in the KNP is a biodiversity hotspot with 27 fish species. However, in
1977 following the construction of the Kanniedood dam the natural fish migration patterns were
restricted. A simple weir-pool fish pass was built on the dam but was largely (90%) inoperable
(Heath et al., 2005; Olivier, 2003). In 2012 part of the dam was damaged by a flood and in 2017
with assistance from the South African National Defense Force it was demolished. Following the
removal of the dam a significantly greater diversity of cyprinids and siluriform fishes has been
observed increasing the diversity of fish upstream of the barrier from <10 species between 1978
and 2018 to >25 species within three seasons after the removal, and this diversity has been
maintained into 2019 and is expected to increase. The important increase in diversity of fish in
the Shingwedzi has made a considerable contribution to the conservation efforts of South
African National Parks and demonstrated the value of connectivity management in the region.
Floodplain Protection and Reconnection
Floodplains are among the most productive and diverse ecosystems on Earth, with productivity
and diversity both strongly influenced by hydrological connectivity between floodplains and river
channels (Opperman et al. 2017b). Floodplains, defined as landscape features that are
periodically inundated by water from adjacent rivers (Opperman et al. 2010), also have a variety
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of characteristics that have long drawn people to live and work on them, including flat
topography, fertile soils and proximity to rivers. Due to intensive settlement and development of
floodplains, river floods have become one of the costliest forms of natural disaster in recent
years, with $82 billion in damages in 2019 (Aon 2019). A common response to managing flood
risk has been the construction of infrastructure intended to avoid inundation by hydrologically
disconnecting floodplains from rivers through physical barriers (levees or tide gates) or by
lowering discharge to reduce flood peaks (dams with reservoirs managed to regulate floods).
Due to widespread construction of dams and levees, floodplains have become disconnected
from their rivers across much of the world, particularly in temperate regions. Due to this
widespread hydrological disconnection—and subsequent conversion to agricultural or urban
land uses—Tockner and Stanford (2002) have described floodplains as among the most
converted and threatened ecosystem types on the planet.
Although dams and levees are crucial infrastructure for public safety in many places, flood
managers are increasingly acknowledging that narrow reliance on grey infrastructure can create
problems. For example, infrastructure can give people the impression that flood risk has been
eliminated, not just reduced, leading to dramatic increases in population within areas affected by
levees, and therefore greatly increasing the people and property at risk should a levee fail or be
overtopped (the “levee effect”; Tobin 1995). Further, in many countries, including the U.S.,
budgets have failed to keep pace with the maintenance needs of aging infrastructure and, as a
result, there is a considerable backlog of maintenance needed to ensure that dams and levees
are safe and effective. For example, the American Society of Civil Engineers gives both dams
and levees in the U.S. a letter grade of “D,” with dams requiring approximately 100 billion USD
in rehabilitation costs and federally managed levees needing 21 billion USD in rehabilitation
costs (the cost for non-federal levees is unknown but likely far more) (American Society of Civil
Engineers 2021).
Due to the limitations of strict reliance on grey infrastructure, flood managers now promote a
“diverse portfolio” of flood-management approaches, including Nature-based Solutions (NbS)
that involve strategic reconnection of floodplains—or maintaining existing connectivity. By
strategically allowing floodplains to flood they can provide conveyance or storage of floodwater
to reduce flood risk for other locations, such as cities or valuable farmland. Maintenance of
existing connected floodplains can be achieved through zoning or acquisition, including
easements, of floodplain areas to keep them in land uses compatible with flooding. For
example, protection of the 10,000-acre Otter Creek Swamp Complex along Otter Creek
(Vermont) has proved to be successful at reducing flood risk for downstream communities.
During Hurricane Irene, which produced record-breaking flood levels and damage across
Vermont in 2011, the flood peak in the town of Middlebury was cut in half, and delayed by a
week, because floodwaters spread out across the Otter Creek Swamp Complex. Researchers
estimated this saved millions of dollars in damages (Watson et al. 2016).
Reconnection of floodplains can occur through setting levees back from the river, increasing the
area of floodplain available for conveyance, or through features such as flood bypasses or
floodways. Levee setbacks have proven to reduce flood risks through various applications in the
United States and Europe and can have added benefits for biodiversity. Serra-Llobet et al.
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(2022) describe case studies from Germany and California. Flood bypasses are crucial
components of flood-management systems for the lede and Sacramento rivers (Opperman et al.
2017b; Sommer et al. 2001).
NbS are intended to provide multiple values beyond their primary objective. For example, in
addition to reducing flood risk, NbS projects involving floodplain reconnection or protection can
provide diverse benefits, including groundwater recharge, water quality improvements, and
provision of a range of resources for people (fish, wildlife, materials), as illustrated in the case
study below.
Case Study: Protecting Floodplains along the Ing River in Thailand for Community Benefits
In 2015, the federal government of Thailand proposed developing a Special Economic Zone
(SEZ) that would have resulted in the conversion of 500 hectares of floodplain forest, and filling
of wetlands, along the lower Ing River, a tributary of the Mekong River. In response, the Boon
Rueang Wetland Forest Conservation Group (BRWFCG) pushed for protection of the
seasonally flooded forest. The villagers and BRWFCG demonstrated that this forest is essential
for their livelihoods, contributed to the local economy, and provided diverse social, cultural and
ecosystem services, including flood-risk reduction. The Regional Community Forestry Training
Center for Asia and the Pacific (RECOFTC), an international organization focused on
community management of forests (United Nations Development Programme (UNDP) 2021),
calculated the ecosystem services of the floodplain forest would cost $4 million annually to
replace, considering both substitute services and the loss of livelihood and revenue. The
floodplain provides habitat for native wildlife and fish (Living River Association 2015, 2017) and
sequesters carbon (Living River Association 2021). Further, the floodplain stores and conveys
floodwaters and was credited with sparing the village from inundation during a major flood in
2010. By documenting the multiple benefits provided by a floodplain, including flood-risk
reduction, the village and BRWFCG were successful in convincing the government to withdraw
the SEZ proposal in 2018. In 2020, BRWFCG received the United Nation’s Equator Prize for
“outstanding community efforts to reduce poverty through the conservation and sustainable use
of biodiversity.” The villagers and BRWFCG are currently working to protect other floodplain
forests along the Ing River into a riverine wetland recognized under the Ramsar Convention.
Groundwater Management/Recharge
Hyporheic zones include the streambeds, banks, and floodplains where the mixing of water that
is episodically (naturally) recharged from flood flows, mixes with the more constant inflows of
groundwater from surrounding aquifers. Hyporheic exchange flows can be hotspots for
biogeochemical processing (Boano et al. 2014), sustaining unique fauna (Boulton 2007),
providing thermal refugia (Casas-Mulet et al. 2020), and contributing to the success of specific
life cycle stages such as fish spawning (Malcolm et al. 2008). Streamflow depletion through
direct exploitation of groundwaters reduces inflows to surface waters and, in more extreme
cases, drives surface flow intermittency (de Graaf et al. 2019). Channel modification (e.g.
physical straightening or bed armoring) lessens vertical connectivity by shrinking and reducing
the porosity of the hyporheic zone (Kondolf et al. 2006). Efforts to sustain and restore vertical
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and horizontal connectivity to support ecosystem health must, therefore, address both
groundwater use and river geomorphology (Boulton 2007).
Changes in connectivity, can be controlled by conjunctive management of groundwater and
surface waters within sustainable limits (Gleeson and Richter 2018; Zipper et al. 2022).
Approaches to manage or protect groundwater sources, include the acquisition of land, water
rights, or conservation easements, or water transactions- including fallowing agreements,
leases, or incentive payments used to restrict the magnitude, timing or location of pumping. On-
farm irrigation efficiency measures, crop switching, and municipal water conservation policies
and programs may also be used to reduce groundwater withdrawals.
In addition, managed aquifer recharge projects can also be designed specifically to sustain
freshwater systems using predictive hydrologic models to meet the specific water needs of
aquatic, riparian or wetland freshwater species or communities (Lacher et al. 2014; Leake et al.
2008; Saito et al. 2021). Possible sources of water for aquifer recharge include high quality
treated effluent and the capture of stormwater runoff from urbanized areas or watersheds in
poor condition. This can improve water quality, reduce erosion and sedimentation, and enhance
groundwater storage for natural systems in downstream locations.
Successful implementation of these strategies relies on effective groundwater governance (FAO
et al. 2016). Key characteristics of successful governance include effective institutions that
integrate co-management with stakeholders; policies and resourcing that support local, regional,
and global management goals; legal systems with the capacity to create and, importantly,
implement and enforce laws effectively; and local knowledge and scientific understanding of
groundwater systems (Closas and Villholth 2020; FAO et al. 2016; Gleeson et al. 2020; Molle
and Closas 2020a, 2020b). Mechanisms include comprehensive plans, municipal zoning, the
transfer of development rights, government requirements for regional groundwater
sustainability, and reserved water rights that pertain to groundwater.
Alongside managing the effects of depletion, restoring geomorphological controls on vertical
connectivity have received increasing attention. Flushing of fine sediments from interstitial
spaces to mitigate colmation and restore natural vertical connectivity can be achieved using
flushing flows (e.g., Mathers et al. 2021) or by direct removal (e.g., Ward et al. 2018).
Increasingly, efforts are being made to create ‘engineered hyporheic zones’ (sensu Tewari et al.
2021) by modifying stream channels to induce hyporheic flows. This includes creation of
bedform structures like riffles and gravel bars, and instream geomorphic structures such as
meanders, log jams and cross vanes (Tewari et al. 2021). Addition of large wood generates
vertical mixing between the stream channel and hyporheic zone (Krause et al. 2014) and has
been shown to alter hyporheic meiofauna communities (Magliozzi et al. 2019a) and stream
temperatures (Klaar et al. 2020). Similarly, installation of cross-vanes (Daniluk et al. 2013) and
beaver dam analogues (Wade et al. 2020) have been shown to increase vertical water fluxes
and improve groundwater-surface water interactions, while hyporheic exchange flows were
generated by constructed riffles in a lowland stream (Kasahara and Hill 2006). The
effectiveness of these interventions can likely be improved through strategic spatial planning of
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restoration efforts by aligning interventions with natural areas of hyporheic exchange (Magliozzi
et al. 2019b).
Case study: San Pedro Riparian National Conservation Area, Arizona, USA
A collaborative group of 21 local, state and federal entities (uppersanpedropartnership.org)
developed a regional hydrologic monitoring program (Gungle et al. 2016) and predictive models
(Leake et al. 2008; Pool and Dickinson 2007) to evaluate a wide range of groundwater
management alternatives to protect and enhance baseflows and shallow groundwater required
for the San Pedro Riparian National Conservation Area (SPRNCA) in Arizona. Hydrologic
monitoring data were used to calibrate modeling scenarios (Lacher et al. 2014) that estimated
the impact and benefits to riparian ecosystem health from proposed water policies, land and
water protection, and infrastructure projects.
Over 20 years, permanent protection of more than 6,000 acres of hydrologically sensitive areas
was put in place along 25 miles of the SPRNCA, in addition to regional land use policies and
ordinances, and federal, municipal and agricultural water efficiency measures that reduced the
annual groundwater deficit in the region from 10,700 acre feet to 3,600 acre feet per year, even
with a more than 9.4% population growth rate during that same period (USGS 2022). As of
2022, three managed aquifer recharge projects were in operation, and two more were under
design to help sustain the ecological values of the SPRNCA using high-quality treated effluent
and stormwater runoff. Between 2015-2021, 31,000 acre-feet of groundwater was recharged or
retired from pumping from strategic locations (https://ccrnsanpedro.org/about/cite). Lastly, an
overarching adaptive management framework for regional groundwater management, which
addresses future monitoring, modeling, and needs for additional projects or policies, was put in
place by a Memorandum of Understanding between federal and local governments, and is
available at https://uppersanpedropartnership.org/.
Overcoming Implementation Challenges
Actions to maintain and improve river connectivity should aim to avoid and minimize the creation
of new barriers that fragment rivers (aside from specific instances when essential for selective
fragmentation for invasive species control), while also addressing the significant legacy of
existing structures that restrict the movements of aquatic organisms and disrupt hydro-
geomorphic processes. Challenges to achieving these goals include a lack of understanding of
the problem, weak policy directives, trade-offs with apparently conflicting objectives (e.g.,
increasing renewable energy associated with achieving net zero targets), status quo bias in
structure siting, design and operation, and inadequate resourcing.
As we look toward the future, we know that there will be increasing demand for new
infrastructure, which will fragment freshwater environments if a business-as-usual approach
continues. For example, projections show an estimated $90 trillion is expected to be invested in
new infrastructure globally by 2030 (Bhattacharya et al. 2015). This includes networks of new
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roads and associated culverts, levees and dams for energy production and water storage
(Alamgir et al. 2017; GWP and IWMI 2021; IEA 2021). Ensuring efficient use of resources –
both energy and water – should be a front line response of governments and utilities to, in the
first instance, limit the rate of increase in demand and, thus, the number of new structures
needed for energy production and water storage (IEA 2021). Integrated water storage, which
takes advantage of both built and natural storage will similarly be important for ensuring that
natural functions and services are maintained in ways that serve societal and nature’s needs
(Dillon et al. 2022). Siting, structure design and operation, where new infrastructure (culverts,
levees, dams and others) is deemed necessary, will be critically important for minimizing
impacts.
Reconnection of freshwater systems that have already been fragmented can result in dramatic
and quick improvements to the health and productivity of aquatic ecosystems (Tonra et al.
2015). Decision makers, including public agencies and elected officials, are much more willing
to solve the problems that they believe are actually “solvable” (as opposed to intractable), if only
incrementally at first. However, the public, policy makers and managers are often unaware that
fragmentation is even a problem. Part of the reason for this is that there are no routinely used
measures of connectivity in the state of environment monitoring and reporting in many parts of
the world. There are also often limited publicly accessible barrier databases and standardized
methodologies to assess impacts. Putting these in place and sharing them with the public helps
raise the profile of the issue of dis-connectivity and to understand the scale of the problem.
Efforts to build better barrier inventories (e.g., Belletti et al. 2020; Franklin et al. 2022; USACE
2022) help, especially when they're accessible. Interactive tools like the AMBER barrier tracker
app and the NZ Fish Passage Assessment Tool app are making it easy for people to assess
and report structures as well. However, there has also been a history of mitigation (e.g.,
construction of fish passage), yet no monitoring to determine if such devices actually are
effective (Cooke and Hinch 2013). This has led to concerns about further investment in
expensive fish passage facilities that may not work. There is need to further build the evidence-
based to ensure that mitigation efforts achieve desired conservation gains (Silva et al. 2018).
The social context behind restoration and conservation should not be ignored. Communities and
individuals can be catalysts for ensuring implementation, though this requires a change in the
perception of what rivers can actually offer; rivers are not merely resources fulfilling human
needs for drinking water, but are home to a huge diversity of organisms (Birnie-Gauvin et al.
2023). Changing this perception is key to successfully engage with society’s diverse groups.
Empowering communities with knowledge and understanding of the situation is fundamental for
enabling them to act. For example, the Ndevu Gorge Dam was proposed on the Luangwa River
in Zambia, which would have major impacts on the South Luangwa National Park, communities,
and the tourism sector in the region. Local communities and chiefdoms were engaged in an
educational campaign about the proposed dam; a model of the potential reservoir was created
to demonstrate the extent of flooding; and leaders and citizens were mobilized to speak out
about the dam. After months of engagement and outreach, the government ultimately canceled
the dam and has continued its efforts to identify more sustainable options (WWF 2019).
Monitoring programs and other community-based partnership efforts that directly engage
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community scientists, community leaders, and others can also build such a common
understanding over time, especially when diverse perspectives and knowledge systems are
engaged. Evidence suggests that increased intensity of local participation tends to generate
policies with increased quality, and even when the outcomes are not ‘better’, stakeholders learn,
and feel empowered (Kochskämper et al. 2016).
In the absence of awareness and understanding of the issue, as well as good data to
characterize the size of the problem, there is often little incentive for strong policy. Strong
policies backed by implementation make a difference. In some parts of the world freshwater
connectivity is beginning to be incorporated into policy. For example, New Zealand has
established national environmental standards for the design of new structures (New Zealand
Government 2020) and compulsory policy objectives to maintain or improve fish passage and
develop fish passage action plans setting out actions to remediate existing structures (Ministry
for the Environment 2020).
Even where strong policies exist, there will always be tradeoffs against other policy imperatives.
In most cases, infrastructure projects are assessed and planned on a project by project basis
without considering the context of the basin and the needs for movements of water, species,
and organic matter across the landscape. Decisions that consider portfolios of projects and
scenarios of impacts on the supply of ecosystem services, biodiversity, energy, water and other
variables, will be most likely to be beneficial to society and nature over the long term. Examples
of such planning processes are few and far between but should be held up as examples of what
is possible. For example, a hydropower relicensing process in the Penobscot Basin in Maine,
USA expanded beyond single dams to include the hydropower system in the lower basin. By
seeking solutions at that expanded scale, the process resulted in the removal of two aging
dams, fish passage improvement on two other dams, and equipment and operational changes
at several other dams. These changes resulted in total electricity generation from the Penobscot
Basin remaining equal (or slightly increasing) to the level prior to dam removal, but with
dramatic increases in habitat available to migratory fish; numbers of river herring increased from
20,000 before dam removal to nearly 2 million in 2016 (Opperman et al. 2011). Science
(including structured decision making approaches; Dolson et al. 2021) can and should be used
to directly support these processes. The use of predictive models and other related tools should
not be restricted to only defining future problems, but also for exploring and identifying solution-
based alternatives that benefit biodiversity/connectivity, while also being socially and
economically acceptable to local communities.
In certain instances, mindsets and thinking are resistant to change, creating a barrier for
implementation (Jørgensen and Renöfält 2012). For example, dam removal on the Selune River
in France was protested due to fear of what would happen after the removal (Birnie-Gauvin,
pers comm). Human nature often tends to bias keeping of the status quo and reverting to known
approaches or designs. Visual rendering of what the site will look like can help alleviate some of
the concerns from the public. Sharing stories of the “early adopters” who have successfully
implemented required changes and their lessons learned, can also go a long way toward
building the confidence of other communities and decision makers to try innovative approaches.
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To create a large-scale shift in perspectives among technical experts educational programs and
training for engineers and planners regarding planning, siting, and design of both grey and
natural infrastructure is needed. Several examples of these types of programs already exist,
such as the US Army Corps of Engineering’s Engineering with Nature program, Northwestern
University’s Center for Engineering Sustainability and Resilience and University of Georgia’s
Institute for Resilient Infrastructure Systems. In New Zealand, establishment of a multi
stakeholder, cross-sectoral fish passage advisory group that undertook codesign of guidance
and tools on how to mitigate impacts through structure design and operation has proven
successful in allowing greater uptake of new approaches. Wider dissemination and uptake of
new approaches to planning, siting, design and operation across a broader section of experts,
educational stages, geographies and governmental and private entities will be necessary for
wholescale adoption.
Where policies exist and decision makers have been convinced either to maintain or reconnect
freshwater systems, a lack of resources is inevitably a barrier. Hence, there is a need for
prioritizing either the barriers that will be removed or the freshwater systems that should remain
connected. Numerous tools and approaches have been developed that can support decision
makers in prioritizing efforts (e.g., Garcia de Leaniz and O'Hanley 2022; Kemp and O'Hanley
2010; King et al. 2017; O'Hanley and Tomberlin 2005). For example, analyses can be
undertaken that prioritize certain rivers or river stretches to keep connected for a number of the
services that they provide (e.g., swimways for migratory species, flows of sediments to
downstream floodplains and deltas, areas of high freshwater biodiversity) (e.g., Caldas et al.
2023; Hermoso et al. 2009; Winemiller et al. 2016; Worthington et al. 2022). Tools and methods
also exist for optimizing freshwater connectivity within a basin via removal of specific barriers to
achieve certain social or environmental objectives (Garcia de Leaniz and O'Hanley 2022;
Hermoso et al. 2021; Null and Lund 2012; Null et al. 2014). These approaches have advanced
significantly in recent years and provide decision makers with the ability to make more informed
decisions about which parts of the system are most strategic to restore or maintain connectivity.
Conclusion
We have presented here an array of measures that are currently possible for maintaining and
restoring connectivity in the face of constructed barriers or other habitat alterations that impact
freshwater connectivity. These measures span across the mitigation hierarchy from 1)
avoidance, i.e., via avoiding barriers in the most harmful locations or via alternative options
and/or via protections for critically important freshwater habitats from the impacts of built
infrastructure to 2) mitigation, i.e. mitigating impacts via barrier design or dam reoperation for
environmental flows to 3) restoration, i.e., via barrier removal. Although these measures provide
a certain level of effectiveness, most have limitations in terms of fully reducing impacts on
aquatic species, and thus, bending the curve for freshwater biodiversity. Moreover, there may
be trade-offs involved, such as where removal of a barrier makes habitat accessible to invasive
species (Rahel and McLaughlin 2018). As such, efforts to improve connectivity cannot be done
in a vacuum without considering the ways in which interactions may occur with other threats and
conservation measures.
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It is apparent from the literature as well as conservation practice that we need better post-
implementation monitoring to understand which measures work best under different locations,
species assemblages and other circumstances. For example, only ten percent of over 37,000
river restoration projects in the US had implemented monitoring programs (Bernhardt et al.
2005). We do have decent evidence for some measures’ effectiveness (e.g., removal of barriers
shows clear improvements for certain species) and the limitations of others (e.g., ill-planned
fishways designed only to move big salmon upstream). Knowing these limitations requires
taking an evidence-based approach and avoiding transfer of technology or solutions without
local testing. To the extent possible, maintaining important connectivity corridors through good
planning decisions is the best approach. The emerging efforts to identify “swimways” for
migratory freshwater species will, for example, support a better understanding of where those
corridors are critical for the viability of migratory species populations (Worthington et al. 2022).
Whereas longitudinal connectivity is the most often recognized dimension of freshwater
connectivity, the other three dimensions (lateral, vertical, and temporal/seasonal) are equally
important. Measures that impact one dimension often will also benefit another and, in some
places, there will be synergies among actions taken (e.g., protection of headwaters may support
downstream water quality and temporality of flows). In some locations, taking action to maintain
or restore connectivity may be needed alongside other ones highlighted in this special issue.
Finally, we see the need for greater levels of innovation and expertise to support development
of new designs and a next generation of barriers that have minimal or no impacts on the
connectivity of freshwater systems. Doing so will require interdisciplinary collaborations (e.g.,
among hydraulic engineers, limnologists, fluvial geomorphologists, and biologists) and an
adaptive management framework with explicit monitoring components to inform future
refinements. Dams that generate sufficient electricity or store water but that are off channel, or
at an existing, non-passable waterfall, or only partially block a river channel, or are permeable to
flows of aquatic biota and organic matter and levees that can be shifted in space over time–
these constitute the needed design challenge for development of that next generation of
technologies that can support bending the curve for freshwater biodiversity.
The ‘Field of Dreams Hypothesis’ suggests that if we restore habitats, species will recolonize
them (Palmer et al. 1997). As such, habitat restoration or protection can be viewed as a
fundamental aspect of bending the curve for freshwater biodiversity. However, the ability to
recolonize restored habitats depends on more than the appropriate conditions being present: it
also depends on the ability of organisms to get there. In this way connectivity of freshwater
systems is fundamental and a co-equal with the other actions needed and presented in this
special issue and introduced in Tickner et al. (2020) to support the recovery of freshwater
species.
Competing Interests Statement: The authors have no competing interests.
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Data Availability Statement: Data generated or analyzed during this study are available from the
corresponding author upon reasonable request.
Acknowledgements: The authors thank World Wildlife Fund and the Natural Sciences and
Engineering Research Council of Canada for their support of this project. They also thank Olsi
Nika from EcoAlbania and Cornelius Wieser from River Watch EU for their review of the Vjosa
case study and Katarina Jin from World Wildlife Fund for creation of Figure 1.
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Measure
Mitigation Hierarchy Step
Dimension(s) of River Connectivity
Potentially Affected
Strategic Planning for Energy,
Water Resources & Biodiversity
Avoid
Longitudinal, Lateral, Temporal,
Vertical
River Protections that Safeguard
Connectivity
Avoid
Longitudinal, Lateral, Temporal,
Vertical
Barrier Design and Fish Passage
Minimize
Longitudinal
Dam Operations for
Environmental Flows
Minimize
Longitudinal, Lateral, Temporal,
Vertical
Barrier Removal
Restore
Longitudinal, Lateral, Temporal,
Vertical
Floodplain Protection and
Reconnection
Restore, Avoid
Lateral, Vertical
Groundwater
Management/Recharge
Restore, Avoid
Vertical
Table 1. River Connectivity Measures and their associated step(s) in the mitigation hierarchy
and the dimension(s) of river connectivity that is potentially affected by the measure.
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