# Safeguarding migratory fish via strategic planning of future small hydropower in Brazil

Thiago Couto, Mathis Loïc Messager, Julian Olden

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Thiago Couto, Mathis Loïc Messager, Julian Olden. Safeguarding migratory fish via strategic planning of future small hydropower in Brazil. Nature Sustainability, 2021, 4 (5), pp.409-416. 10.1038/s41893-020-00665-4 . hal-04750069

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# Safeguarding migratory fish via strategic planning of future small hydropower in Brazil

Thiago B. A. Couto<fnref n="1" />,<fnref n="2" />*, Mathis L. Messager<fnref n="1" />,<fnref n="3" /> & Julian D. Olden<fnref n="1" />

*Keywords:* Cumulative environmental impacts, migratory fish, river fragmentation, Small Hydropower Plants, energy policy.

<footnote n="1">¹ School of Aquatic and Fishery Sciences, University of Washington, Seattle, WA, USA</footnote>

<footnote n="2">² Current address: Institute of Environment, Florida International University, Miami, FL, USA</footnote>

<footnote n="3">³ Current address: Department of Geography, McGill University, Montreal, QC, Canada and National Research Institute for Agriculture, Food and Environment (INRAE), Villeurbanne, France</footnote>

<footnote>*Email: coutot@uw.edu</footnote>
# 1 Abstract

Small Hydropower Plants (SHPs) are proliferating globally, but their cumulative threat to blocking migratory fish and the fisheries they sustain has been underappreciated when compared to Large Hydropower Plants (LHPs). Here, we quantified the trade-offs between hydroelectric generation capacity and impacts on river connectivity for thousands of current and projected-future dams across Brazil. SHPs are the main source of river fragmentation, resulting in average connectivity losses fourfold greater than LHPs. Fragmentation by SHPs is projected to increase by 21% in the future, and two-thirds of the 191 migratory species assessed occupy basins that will experience greater connectivity losses by SHPs than LHPs. A Pareto frontier analysis identified future dam portfolios that halve the number of hydropower plants required to deliver the same generation capacity compared to least-favorable solutions, while simultaneously resulting in lower river fragmentation and protecting numerous undammed basins. Our results highlight the need for strategic planning that consider the unprecedented growth and cumulative effects of SHPs.

15

16

# 17 Introduction

Rapid expansion of hydropower dams threatens many remaining free-flowing rivers in biodiverse tropical regions of the world, interrupting the migrations of freshwater fishes on which millions of people rely directly for their livelihoods ¹⁻³. Consequently, there is a growing need to carefully balance trade-offs between hydropower production and ecosystem functioning to avoid biodiversity losses and ensure long-term food security and income ³,⁴. Considerable scientific activity and international scrutiny remain focused on Large Hydropower Plants (LHP), whereas the environmental repercussions of smaller projects have been largely overlooked in broad-scale energy policies and garner little public attention. Despite, and perhaps due to this oversight, a series of political and economic incentives to renewable energies have been implemented across the world to benefit projects labeled as Small Hydropower Plants (SHPs) – often defined as plants with generation capacity below 10 MW, but this threshold may vary among countries ⁵. These incentives assume that, in addition to the lower energy generation capacity of SHPs, “small” also equates to low environmental impacts. This has ultimately led to licensing exemptions, favored energy prices (e.g. feed-in tariffs), and subsidized loans that fuel increased construction ⁵⁻⁸. However, emerging scientific evidence suggests that the ecological impacts of SHPs can be disproportionally high when compared to their societal benefits ⁹,¹⁰.

The primary concern with rapid SHP development is that their number dwarfs that of LHPs. Today, more than 80,000 SHPs are operating in at least 150 countries (11 times the number of LHPs) and construction of tens of thousands of new dams is projected ⁵. Despite being widespread, the aggregate contributions of SHPs to electrical grids are often quite low. For instance, according to Brazilian regulations, SHPs are defined as those with a generation capacity below 30MW. Although they represent over 80% of the 1,341 hydropower plants operating in the country, they are responsible for only 5% of total generation capacity ¹¹. The
imbalance between the number of power plants and their generation capacity calls into question whether the marginal contributions by SHPs to national-level energy requirements are worth their potential environmental and societal costs. The cumulative impacts of SHPs on basin-wide hydrology and habitat connectivity may exceed that of LHPs when standardized by hydropower generation ⁹,¹⁰, thus challenging the majority of environmental policies that focus on the impacts of individual, and often large, dams ⁵,¹².

By virtue of their sheer number and geographic extent, the widespread proliferation of SHPs may be a significant, yet underappreciated, threat to the persistence of migratory fishes and the fisheries and diverse societal values they support ³,¹³. Dams constrain the movement of migratory fish along river networks and isolate critical habitats necessary for their life history (e.g., spawning and feeding grounds), resulting in local extinctions, population declines, and collapses of fishery stocks ¹⁴⁻¹⁶. Consequently, migratory fish species are among the most vulnerable organisms to hydropower development in the tropics ¹⁷, which is concerning considering their high ecological and socioeconomic importance. For instance, migratory species play key roles in food webs and ecosystem functioning ¹⁸,¹⁹, and comprise among the top-ranked inland fisheries in market values and cultural preferences ¹⁵,²⁰. Past and planned dam construction greatly exacerbates the loss and fragmentation of habitat in dendritic river networks through the cumulative effects of multiple barriers ²¹,²² and therefore threatens migratory fish persistence and the resilience of fisheries ²³.

Optimal siting of new dam construction is paramount when attempting to balance trade-offs between hydropower generation and river fragmentation that threatens migratory fish ²⁴, yet it is noticeably absent from many national energy plans. Managers and decision makers have a wide range of options to help guide where on the landscape prospective hydropower projects may be constructed ²⁵. Recent attention has focused on the utility of trade-off assessments of multiple objectives to identify win-win management opportunities and provide more prescriptive recommendations — both highly desirable in decision-making ²⁶. Trade-off analyses can evaluate the performance of prospective hydropower plant portfolios by comparing their cumulative ecological impacts and generation capacity gains to meet desired future energy demands ²⁷⁻²⁹. Therefore, they offer an alternative that looks beyond the “more is better” perspective ⁵,²⁵ to identify portfolios of SHPs and LHPs that maximize energy generation while minimizing the loss of river connectivity.

The duality of growing hydropower demand for future energy supply and the major socioecological value of migratory fish in developing countries necessitates a robust examination of the trade-offs between future SHP and LHP development and the resulting fragmentation of rivers. We explore this issue in Brazil, a global leader in hydropower development ⁵, where over 2,200 new hydropower projects are currently in different stages of the licensing and inventory processes. First, we compare the cumulative effects of current and projected-future SHPs and LHPs on the network connectivity of Brazilian rivers – cumulative effects defined here as the fragmentation by dams in aggregate. Second, we quantify the resulting range-wide loss of connectivity for 191 migratory fish species, including both those that are nationally red-listed and those highly valuable for commercial or recreational fisheries. Third, we explore trade-offs
between gains in energy generation capacity and loss in river connectivity to inform strategic planning for the development of new hydropower construction in the country.

In this study we adopted the Dendritic Connectivity Index (DCI) as our metric of connectivity²², an estimate of the probability of a fish being able to disperse between two random points in a given river network. Empirical research has shown that DCI can accurately describe the effects of river fragmentation and dispersal limitation on the spatial distribution of fish species³⁰,³¹, and it has been employed in different parts of the world to assess the impacts of fragmentation by dams ¹³,³²,³³. We computed DCI for all Brazilian basins according to the present-day (2018) and projected-future (circa 2050) distribution of dams, contrasting the respective contributions of SHPs and LHPs over time and space. Our results compose a national-level assessment of present and future fragmentation by hydropower that informs future energy and environmental policies through a trade-off analysis exploring a large number of possible future scenarios.

95

## 96 Results

### 97 Contributions of SHPs and LHPs to river fragmentation

The construction of hydropower dams has resulted in the widespread fragmentation of Brazilian river basins over the past century, with an average basin-level loss in river connectivity (DCI) from 100 to 83, representing a decrease of 17 units (DCI of 100 indicates a completely free-flowing basin with no artificial barriers) (Fig. 1). SHPs alone contributed to an average decrease of 14 ± 21 DCI units, four times more than that observed for the LHP-only scenario (4 ± 11 DCI units). The decrease in river connectivity associated with historical SHP construction mirrors the overall trends, indicating a disproportional contribution of SHPs to river fragmentation over the last century. Fragmentation rates remained relatively steady through time until a marked acceleration in recent decades (Fig. 1). Whereas fragmentation rates due to LHPs remained constant, the loss of connectivity caused by SHPs increased by seven-fold after the year 2000 – fueled by numerous governmental incentives⁵.

The 2,268 new hydropower projects currently in different stages of construction and potential approval that were evaluated in this study will compromise future river connectivity throughout Brazil, primarily driven by new SHP construction. Overall, future dam construction is expected to cause a 24% average loss in river connectivity (from a present-day average DCI of 83 to 62 projected by circa 2050), with decreases in individual basins ranging from 0 to 89% (Fig. 2a). This predicted loss is caused principally by anticipated SHP construction (average DCI decrease of 20%) compared to LHPs (4%). River fragmentation by hydropower has been historically more concentrated in basins located in the southeastern and southern portions of the country – in the Paraná, Uruguay and East Atlantic Basins (Fig. 3). Fragmentation by SHPs will continue to increase in these Basins, as well as expand in the Central-West and North regions over time.

The growing footprint of river fragmentation is predominantly the result of projected increases in new SHPs, more widespread over the country and with numbers that far exceed
those of LHPs (Fig. 3). Consequently, 424 basins that are currently free of hydropower will have new SHPs in the future, representing the primary driver for future nationwide river connectivity loss. A closer examination of basins that are currently free of hydropower reveals that river connectivity will decrease by over one third (35% ± 22) in response to future dam construction – an outcome again driven by SHPs (Fig. 2b). Decreased connectivity of basins already impacted by hydropower is also predicted (Fig. 2c).

## Implications of river connectivity loss for migratory fishes

Further river fragmentation caused by planned SHPs is also expected across the geographic range of migratory fishes, including species that are at risk of extinction and those that support highly valuable commercial or recreational fisheries. The respective contributions of future SHPs and LHPs to river fragmentation in the ranges of migratory species are relatively similar at present (Fig. 4a), but the relative fragmenting effects of SHPs are expected to rise. Close to two-thirds (62%) of all migratory species occupy basins that will experience greater fragmentation by SHP compared to LHP construction in the future (Fig. 4b).

The detrimental effects of fragmentation by planned SHPs are manifested in at least part of the geographic range of 14 red-listed species and 20 species of highest value for fisheries (out of 24 species for each category — see Methods), resulting in an expected DCI decrease of more than 10% in basins occupied by the majority of these species. For instance, a 28% decrease in river connectivity by SHPs is expected in the subset of the geographic range of *Brycon vermelha* where hydropower has been developed. This red-listed species is endemic to the upper portions of the rivers Mucuri and São Mateus, and SHPs were identified in conservation status assessments as one of the main threats to the persistence of the species³⁴. In addition, average decreases in river connectivity by SHPs over 10% are expected in basins occupied by 12 species of high value, including *Prochilodus lineatus*, *Megaleporinus obtusidens*, *Pseudoplatystoma corruscans*, and *Salminus brasiliensis* – top-ranked in market prices, recreational importance and yields in the Paraná, Paraguay and Uruguay Basins ¹⁵,³⁵. Results for all 191 species are found in the Supplementary Table 1. We also repeated the entire analysis with a modified measure of connectivity that accounts for situations where migratory fish populations are predominantly composed of external immigrants from downstream basins (see Supplementary Information). All results were similar to those presented above – aside from greater losses of river connectivity due to both planned LHP and SHP construction (Supplementary Figs. 1-6).

## Trade-offs between meeting energy demands and deteriorating river connectivity

Energy regulations stipulate that generation capacity is an indicator of the potential environmental impacts of a hydropower plant. However, there is little association between generation capacity of individual future hydropower projects and their predicted effect on river connectivity (Fig. 5). This is the case for all hydropower plants combined (Pearson R=0.23), only SHPs (R=0.18), and only LHPs (R=-0.01). Projected effect of each future hydropower project on river connectivity is provided in the Supplementary Table 2.

We simulated 4.3 million scenarios to evaluate portfolios of different numbers and identities of planned hydropower projects that optimize nationwide generation capacity gains while
balancing reductions in river connectivity. Although river connectivity and gains in generation capacity are inversely proportional, there is considerable variability in this relationship among dam portfolios (Fig. 6a). The Pareto frontier operator identified 207 favorable and 221 least-favorable dam portfolios (the remaining being considered intermediate solutions), of which 33 and 45, respectively, fall inside the range of projected hydropower demands for Brazil in the 2030-2040 period.

A closer examination of the projected generation capacity reveals that least-favorable portfolios generally involve the excess construction of hundreds of SHPs to achieve the same total energy generation capacity (Fig. 6b; Supplementary Table 3). Favorable portfolios have on average 430 SHPs (± 174) and 58 LHPs (± 21), whereas least-favorable portfolios have more than twice these numbers, averaging 1,138 SHPs (± 243) and 118 LHPs (± 30). In addition, the number of basins that remain free of hydropower in favorable portfolios is two-fold larger than in least-favorable ones (Fig. 6c); on average, 202 (± 67) and 394 (± 57) basins will no longer be free of hydropower in favorable and least-favorable portfolios, respectively. As an example, new input of 25 gigawatts can be achieved with 64% fewer SHPs (784), 57% fewer LHPs (77) and by damming 50% fewer free river basins (204), ultimately resulting in a two-fold lower reduction in river connectivity (DCI<sub>present-day</sub> = 83, DCI<sub>favorable</sub> = 77, DCI<sub>least-favorable</sub> = 68) when compared to a least-favorable portfolio. These results highlight significant scope for optimal siting of planned new dams in coming decades, thus supporting the importance of strategic planning and trade-off analysis to avoid excessive SHP construction that leads to needless loss in river connectivity.

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## Discussion

The nationwide proliferation of SHPs in Brazil has caused substantial disruption of river connectivity in recent decades, threatening the persistence of migratory fish species upon which ecosystems and humans depend. Losses in connectivity are predominantly driven by SHPs, exceeding the impacts of LHPs in most basins across Brazil. This finding supports the growing notion that SHPs in high densities can collectively outweigh the effects of LHPs on river connectivity despite the usual spatial centrality of larger dams in river networks<sup>5,9</sup>. As conflict over water resources increases under growing population and energy demands, ecologically sustainable hydropower development in developing countries will necessitate that rivers be managed for multiple co-benefits<sup>24</sup>.

River fragmentation is on the rise in Brazil and accelerated in the early 2000s, when construction rates of SHPs increased 13-fold in comparison to the previous decade<sup>5</sup>. This boom in SHP construction followed a series of policy incentives that included new regulations for the energy market, a relaxation of size-classification criteria, simplified environmental licensing, and economic encouragements such as feed-in tariffs and subsidized loans by public banks<sup>5,36</sup>. For example, the first feed-in tariff policy in the country was implemented in 2004 and resulted in 59 new SHPs in just eight years<sup>37</sup>. Our results reveal that although basins located in the Paraná, Uruguay, and East Atlantic river systems have been more heavily fragmented by SHPs, river fragmentation in the northwestern Brazilian Highlands has increased dramatically in recent
decades. This includes upper mainstem portions and tributaries of important rivers such as the
Tocantins, Araguaia, Madeira, Tapajós, Xingu, São Francisco, Paraguay and Paraná.

Increased fragmentation of Brazilian rivers is imminent with thousands of SHPs currently
undergoing planning, licensing and construction. These dams, as compared to LHPs, are
expected to be the predominant source of future habitat fragmentation for two-thirds of the
migratory fish species in Brazil, including species of high conservation priority and
socioeconomic importance. In the Tapajós River Basin, among the most vulnerable rivers of the
Amazon Basin for hydropower development ³⁸⁻⁴⁰, growing conflicts and judicial battles have
arisen over the proliferation of SHPs in some of its tributaries ¹². One of the most controversial
cases is in the Juruena River in Mato Grosso, where the implementation of eight SHPs is
affecting the fisheries of the Enawene Nawe people – an essential component of their culture,
spiritual practices and food security ⁴¹. Here we report that 14 basins of the Tapajós Basin may
lose more than 50% of river connectivity due to planned SHP construction (eight of them part of
the Juruena Sub-basin), and the projected loss is as high as 85% for Cupari Leste and Buriti
basins.

Empirical evidence suggests that changes in the structural connectivity of rivers reduce
biodiversity and affect the composition of fish communities through dispersal limitation ³⁰,
highlighting the high vulnerability of migratory species to fragmentation ¹⁴. Our estimates
indicate that there will be a 40% decrease in river connectivity caused by future SHPs in basins
supporting the red-listed species: *Steindachneridion doceanum*, *Rhamdia jequitinhonha*, and *B. opalinus*. Moreover, the species *P. lineatus* – one of the most harvested species in the Paraná, Paraguay and Uruguay Basins where fisheries yields in some portions reach 30 thousand tons annually ¹⁵,³⁵,⁴² – occupy river basins expected to experience a 19% connectivity loss from SHPs alone. These examples call for heightened investigation of how rampant SHP development may compromise sustainable fisheries practices and threaten the persistence of migratory species in the future. Current understanding of fish migratory dynamics in Brazil is quite limited and did not allow us to refine our analysis to include the actual migratory routes of the species. We highlight this as a future research need to further inform the strategic planning of hydropower development.

Optimization approaches have provided renewed hope for reducing what have previously
been considered inevitable environmental and social conflicts involving hydropower
development ⁴,²⁶. Our study indicates that informed selection, or portfolios, of new dams can
both meet future energy demands of Brazil and concurrently minimize the resulting loss of river
connectivity. In fact, through a careful planning process, favorable dam portfolios more than
halve the number of both SHPs and LHPs compared to least-favorable dam portfolios, while
delivering the same total energy generation capacity. This amounts to hundreds fewer dams
being constructed, substantially lowering degradation of river connectivity, and effectively
protecting numerous basins that are currently free of hydropower.

Results from this study support previous investigations in other parts of the world that
demonstrate the strength of varied trade-off analyses in balancing hydropower production and
dam-induced effects on floodplain-fisheries, greenhouse gas emissions, and sediment trapping ²⁷⁻
242 243 244 245 246 247 248 249 250 251
²⁹. However, significant challenges persist. Our findings imply significant cost reductions as a result of substantially less SHP and LHP construction to achieve the same gains in energy capacity, yet a full accounting of the social, economic and geopolitical dimensions of different dam portfolios is a critical next step ⁴³,⁴⁴. In addition to assessing effects of dam construction on river connectivity, downstream hydrological and physical impacts vary among different dams, and should also be incorporated into any decision-making process. Finally, SHPs are still rarely considered in assessments of the cumulative ecological effects of future hydropower despite their global proliferation ⁵. For instance, 40% of the recent Cumulative Impact Assessments (CIAs) for hydropower in Brazilian basins completely ignore SHPs ¹²; this should be addressed in any future reforms of energy policy.

252 253 254 255 256 257 258 259 260 261 262 263 264 265 266
Managing rivers for multiple, sustainable benefits requires integrating scientific, social, and policy perspectives into operational decision frameworks ²⁴,⁴⁵. This study challenges nationwide policies for SHP development in Brazil that consistently assume their low potential for causing environmental impacts and generally ignore their cumulative ecological effects. We found that the expansion of SHPs poses a persistent and emerging threat to the connectivity of Brazilian rivers, resulting in important negative consequences for migratory-fish conservation. In this regard, we demonstrate that strategic planning based on trade-off analysis can provide critical guidance for new construction to meet energy needs while ensuring the smallest loss of river connectivity. However, the actual implementation of comprehensive strategic planning is challenging due to the decentralized nature of SHP development and licensing, which are typically regulated at the state level. One possible solution is strengthening basin-level CIAs – a powerful policy required prior to licensing that is already mandatory, but poorly regulated for SHPs ¹²,⁴⁶,⁴⁷. Coordinated efforts between the SHP industry, local stakeholders, social organizations, and state/federal agencies can help inform CIAs by adjusting them to the social, economic, and environmental realities of the region.

267 268 269 270 271 272 273 274 275 276 277 278 279
Considering the spatial complexity of fish migrations and the socioeconomic importance of inland fisheries in Brazil, the current mechanism for hydropower siting is unsatisfactory from a socioecological perspective ⁴⁸,⁴⁹, needing scientifically sound and more transparent environmental criteria. For instance, although none of the SHPs of the Juruena River (Amazon Basin) are inside the boundaries of the Enawene Nawe lands, they have been reported to block fish migrations and cause declines in fish stocks that this indigenous group relies heavily upon ⁴¹. It is also important to highlight that national-level prescriptions of hydropower siting are unable to track all the site-level social, economic and environmental repercussions of dam construction, and local impact assessments and public consultations are indispensable even for SHPs. Improving strategic planning of hydropower development with environmentally-informed criteria, together with policy incentives to diversify renewable energy sources (e.g. solar and wind), are potential avenues to be explored ⁵⁰, and should minimize the adverse ecological effects of dam construction in tropical rivers.

280

281 Methods
282 Dam distributions and attributes

283 Our analysis combines hydrographic data with the spatial location of hydropower dams to
284 identify fragmented river networks and quantify river connectivity across Brazil. We used the
285 HydroSHEDS and HydroBASINS global hydrographic mapping products <sup>51,52</sup>, which contain 1.1
286 x 10<sup>6</sup> river reaches draining in and to Brazil, and polygons delimiting river basins at ten spatial
287 levels of organization, respectively. Next we retrieved a comprehensive national dataset of
288 hydropower plants from the repository of the Brazilian energy agency ANEEL <sup>53</sup>. This dataset
289 contains 3,795 plants with associated attributes that include size category (i.e. large, small, mini),
290 generation capacity, opening date, and status (i.e. operating, under construction,
291 decommissioned, inventory or licensing stage).

292 River connectivity in this study is an attribute of a river basin <sup>22</sup>. A “river reach” is
293 defined as a cartographic unit, represented by the line segment between two neighboring
294 confluences. A ‘river fragment’ is a subset of a basin’s network (i.e. a set of fully connected
295 reaches) that becomes disconnected from the rest of the network after the construction of a
296 barrier (dam). We conducted our analysis for level eight-basins (containing on average 731 km<sup>2</sup>,
297 and 314 km of rivers per basin), which corresponds to the scale of hydropower planning and
298 natural resource management that accounts for the scales of migration of most freshwater fish
299 species in Brazil <sup>35</sup>. River networks in coastal basins with two or more disconnected networks
300 flowing directly to the ocean were analyzed separately (as “sub-basins”) to ensure fully
301 connected networks.

302 Brazilian regulations classify hydropower dams as “small” when their generation
303 capacity is <30 MW and they impound reservoirs with surface areas <13 km², and as “mini”
304 when generation capacity is <1 MW; all other dams are labeled as “large” <sup>5</sup>. These classifications
305 are arbitrary choices not supported by scientific evidence – both small and mini dams receive
306 licensing exemptions. Therefore, we combined them in a single category of SHPs. We assumed
307 that all dams in operation in 2018 represent the current scenario, and all dams under construction
308 or in different stages of licensing or inventory (as of September 2019) represent the projected-
309 future scenario. The future scenario reflects a time period from present-day to mid-century (circa
310 2050) when all these dams were assumed to be built <sup>54</sup>. However, the timeline and the actual
311 number of dams that will get constructed (i.e. it can be greater or smaller) may change depending
312 on future policies, governance, and other local factors.

313 A series of spatial editing steps were performed on the hydropower plant dataset to
314 ensure that it was accurately linked to the HydroSHEDS river network. This included the
315 deletion of duplicate spatial records (for the purpose of this study, hydropower plants within 300
316 m of each other, keeping the plants with the largest generation capacity) and the automatic
317 snapping of plants within 200 m of the network to the nearest river reach. Plants beyond 200 m
318 of the river network were manually repositioned based on visual reference to high-resolution
319 imagery and to the Brazilian national cartographic dataset from IBGE 55. Plants were discarded
320 from the analysis in the rare occasion when they appeared to be disconnected from the main river
321 network. Lastly, we also ensured the positional accuracy of all LHPs and inspected plants whose
322 reported drainage area differed by more than 25% from the river reach in which they were
located or whose power generation capacity appeared high compared to the annual discharge of the reach. For the 3,193 (out of 3,514) current and future plants for which the geographic coordinates of the dam were available, the final position of the adjusted point was only 239 m (median =138 m) away, on average, from the reported location of the dam. Following these steps, our analysis included 3,063 SHPs (1,032 current and 2,031 future) and 451 LHPs (214 current and 237 future).

## 329 Migratory fish species

Although dams have been reported to modify habitats and limit dispersal and gene flow for fish species with a wide range of life histories³⁰,⁵⁶, we opted to focus on migratory species due to their higher vulnerability to river fragmentation and their socioeconomic importance ¹⁵,¹⁷,²⁰. We classified the 3,130 species of freshwater fish that are formally registered in Brazil as either migratory or non-migratory based on whether they demonstrate some level of longitudinal migrations in rivers to access feeding areas or to complete reproduction (i.e. upstream and downstream migrations along the main channel). Species whose migrations are restricted to lateral movements into floodplains were excluded³⁵. Unfortunately, basic knowledge on natural history and migratory behavior is lacking for most fish species⁵⁷. Therefore, we compiled a list of taxonomic groups (Family and Genus levels) reported as longitudinally migratory in South America³⁵, and extrapolated our definition of potentially migratory to individual species that belong to these groups, resulting in an initial list of 505 migratory species. This strategy is supported by strong associations between dispersal ability and morphological characteristics (e.g. body size and fecundity), and phylogenetic conservatism in parent-offspring dispersal distance reported in the literature⁵⁸,⁵⁹. This initial list was subsequently reviewed by two expert ichthyologists in Brazil (Jansen Zuanon and Roberto Reis, personal communication), resulting in a final list of 365 species that are highly likely to be migratory (Supplementary Table 1). Of these species, 24 are red-listed as Vulnerable, Threatened, Endangered or Critically Endangered according to national standards using IUCN criteria³⁴, and 24 are considered species of highest commercial and recreational value — species with annual yields of more than 10 tons and/or considered an important sport fisheries in Brazil ²⁰,⁶⁰,⁶¹. Point datasets of occurrence records for 335 species were acquired from the environmental agency ICMBio³⁴. Species occurrence data were then intersected with all river basins that have or will have at least one hydropower dam, resulting in a basin-level distribution range for 191 migratory fish species.

## 354 Quantifying river connectivity

The Dendritic Connectivity Index (DCI) is a metric of river connectivity that reflects the probability that a mobile organism (fish) can move between two randomly selected points from a network²². DCI ranges from 100 (i.e. completely free-flowing basin with no barriers) to zero and can be calculated for any size of stream network. The DCI equations can be adjusted according to the movement behavior of the fish species²². Here, we adopted the DCIₚ developed for potamodromous fish — the guild of migratory fishes that complete their life cycle, and thus migratory movement, exclusively in freshwater habitats⁶² — here assumed to be equally likely to move upstream as downstream. More specifically DCIₚ represents the stream length-weighted average connectivity among all fragment pairs within the network, where each barrier is assigned
a permeability value that represents the probability of a given fish passing through the barrier and connectivity is the joint probability that a fish can move between two fragments given the permeability of all barriers between the fragments. The DCIₚ is expressed as:

$$DCI_p = \sum_{i=1}^{n} \sum_{j=1}^{n} p^{m_{ij}} \frac{l_i l_j}{L L} \times 100$$

where n is the number of stream fragments in the network (i.e. equals to the number of barriers plus one), and the indexes i and j designate the fragment identities from 1 to n. The lengths of the fragments i and j are represented by $l_i$ and $l_j$, which are weighted by the total length of the network (L). $p^{m_{ij}}$ expresses the connectivity between the fragment pairs, with p being the two-way permeability of every barrier and $m_{ij}$ being the number of barriers between the fragment pairs i and j. We adopted 10% as the permeability for all dams (i.e. p = 0.1), which is a conservative estimate informed by research reporting very low efficiencies of fish passage structures. For instance, the efficiency of upstream passage was reported to range between 0.7 and 21% for non-salmonid fishes and for dams located in South American rivers⁶³,⁶⁴.

We attributed the same permeability to SHPs and LHPs due to the lack of evidence relating the performance of fish passage structures and size-categories of hydropower⁶⁵. Although smaller dams may seem easier to pass over or bypass compared to LHPs, there are three main reasons why this is unlikely. First, size classifications based on hydropower generation capacity do not necessarily reflect ecologically-relevant attributes (e.g. dam height, reservoir area, and flow alteration) that constrain fish movement⁵,⁶⁶,⁶⁷. For instance, many SHPs operate as diversion schemes, dewatering long main channel sections and ultimately limiting fish movements in addition to the dams⁵,⁶⁸. Second, a high proportion of fish passages associated to SHPs have been reported to have critical structural and maintenance problems that completely eliminate their functionality⁶⁵. Third, SHPs are subjected to less rigorous requirements for environmental mitigation compared to LHPs, which means that the presence of fish passages and their monitoring and regular maintenance are also less likely⁵. In order to examine the effect of the permeability of SHPs and LHPs on our results, we ran a sensitivity analysis that revealed no major changes in the findings. Notably, SHPs still accounted for most projected-future DCI losses even when increasing the permeability of SHPs to 50% and decreasing the permeability of LHPs to 0% (Supplementary Fig. 7). Our analysis did not include potential natural barriers to fish migration like waterfalls due to the lack of a comprehensive national dataset and to the high diversity of migratory fish in the analysis – species respond differently to different sorts of natural barriers⁶⁹.

DCI analysis was conducted for scenarios containing SHPs alone, LHPs alone, and both dam types combined, in each of two time periods (current and future). Trends in DCI over the past century were also examined according to the opening dates of 1,211 (97%) dams from our dataset. We opted to separately explore scenarios of the existence of just SHPs or just LHPs because the contribution of each dam barrier to the basin-level DCI is non-additive. Therefore, we ran two separate DCI analysis, each of them containing a single dam type but ignoring the presence of the other (SHPs vs. LHPs). We only report DCI results (e.g. nationwide average) for
the 1,216 Brazilian basins that have or will have at least one hydropower dam in the future (Figs. 1-4). These basins represent 10% of all basins in Brazil (11,911 basins level 8 from HydroBASINS) and 15% of the total Brazilian river network by length. The remaining basins that are less attractive for hydropower (DCI = 100) were excluded from the analysis.

## 407 Selecting hydropower dam portfolios

We estimated the unique contribution of each planned dam on basin-level DCI using a permutation analysis that calculated the difference between DCIs when a given dam is present and absent in each basin, repeated for all possible combinations of future 2,031 SHPs and 237 LHPs across the country. The overall contribution of each planned dam was summarized as the mean and the range of marginal DCI loss when present in the basin, across all possible future dam development scenarios in that basin. In a second step, we examined 4.3 million future dam portfolios with respect to total nationwide generation capacity and average basin-level DCI. Each portfolio represents a random selection of a future number of dams that were sampled from the pool of 2,268 planned dams. Analyzing all possible portfolios (all permutations of dam sites in each basin) would necessitate a massive computational effort, as the number of all portfolios equals $5.4 \times 10^{682}$, calculated as $2^n$, where $n$ is the number of planned dams. We determined the set of favorable and least-favorable dam portfolios (i.e., the Pareto frontier) that best and worst traded-off gains in hydropower generation capacity with losses in nationwide river connectivity. We computed the Pareto frontier for future favorable and least-favorable scenarios using the function 'psel' of the R package 'rpref'. This function uses a top-k skyline operator to identify Pareto-optimal sets of data that meet the preference queries of an objective $^70$, which in our case are the dam portfolios that maximize capacity gains and minimize connectivity losses for the favorable portfolios and the opposite for the least-favorable. Future demands for hydropower generation capacity was defined as 120-141 GW from 2030 to 2040 according to an international report that estimates energy policy scenarios $^71$. Although not including all possible dam portfolios, this simulation exercise successfully captured the overall Pareto frontier expected from an optimization analysis of all possible solutions. This is supported by an overall stabilization of the Pareto front solutions beyond a threshold of a few million simulated portfolios (Supplementary Fig. 8). It is important to note that our analysis did not account for connection access to the national grid, generation efficiency, construction and maintenance costs, and many other social and economic factors that may play important roles on final decision-making. However, we assessed whether inefficiencies inherently associated with hydropower generation (e.g., due to local hydrology, turbine efficiency) could represent a source of bias to our results. We reproduced our analysis with estimates of capacity factors (i.e. proportion of the capacity that is actually translated into energy generation) for SHPs and LHPs. The results remained consistent with our analysis using generation capacity as the capacity factors of SHPs and LHPs do not markedly differ on average. (See explanation of methods in Supplementary Information and results in Supplementary Figs. 9-10).

Correspondence and requests for materials should be addressed to TBAC.
443

444 Acknowledgements

445 We thank J. Zuanon and R. Reis for validating our migratory species list and ICMBio for sharing
446 their dataset on fish species occurrence. We also thank David Li, Drielle Martins and Julio
447 Rocha for the assistance with the analysis. JDO was supported by H Mason Keeler Endowed
448 Professorship from the School of Aquatic and Fishery Sciences, University of Washington, also
449 supporting TBAC. TBAC received the CNPq/Science Without Borders Fellowship
450 (203991/2014-1), and research grants from Rufford Foundation and National Geographic
451 Society.

452

453 Author contributions

454 TBAC and JDO designed the study, and all authors led the writing. TBAC and MLM worked on
455 the data acquisition, processing, and analysis in R, and MLM coded the spatial analysis in
456 Python.

457

458 Data availability

459 All the analyses were based on governmental (ANEEL, IBGE, ICMBio) or open source datasets
460 like HydroSHEDS and HydroBASINS. All references are included in the text. A repository with
461 a research compendium including non-reproduceable data sources, intermediate products, scripts,
462 and guidance to reproduce the results is available at
463 https://figshare.com/s/5ba67b7f58ccc812ae70. Supplementary tables include the output data
464 generated by our analysis.

465

466 Code availability

467 The code used to analyze the data and generate figures are available at:
468 https://github.com/messamat/BrazilDCI_Python and https://github.com/messamat/BrazilDCI_R.

469

470 References

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628
<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Temporal trends in river connectivity in Brazil according to changes in DCI over the past century and future projections due to ongoing and planned dam construction. Colored lines represent yearly estimates of DCI for 1,216 individual basins, with lines indicating the unique contribution of SHPs (red) and LHPs (blue). Black lines represent average values for SHPs (dotted), LHPs (dashed), and all types of hydropower combined (solid). “Future” reflects ongoing and planned construction that is projected to occur in the mid-century (circa 2050); the exact year is subject to change depending on local-level factors, future policies, and governance.</figcaption></figure>

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<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Predicted future change (%) in river connectivity (DCI) from present-day (2018) to mid-century (circa 2050) for the Brazilian basins according to the separate contributions of SHPs (red), LHPs (blue), or all types of hydropower combined (purple). Black bars indicate average values and black lines indicate 95% confidence intervals. Results are shown for (A) all 1,216 basins, (B) 573 basins that are currently free of hydropower, and (C) 643 basins currently regulated by hydropower.</figcaption></figure>

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<figure><img src="image_4.png" /><figcaption>**Fig. 3.** (A) Present (2018), (B) future (circa 2050) and (C) projected percentage change in river connectivity (DCI) for the Brazilian basins according to the contributions of SHPs (red shades) and LHPs (blue shades). Color pallets represent (A-B) values of absolute DCI loss from free-flowing state (i.e. 100 - DCI) or (C) percentage loss between periods (negative change in all instances). The black lines delimit the boundaries of the major Brazilian hydrographic units.</figcaption></figure>

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<figure><img src="image_5.png" /><figcaption>**Fig. 4.** Effects of SHPs and LHPs on river connectivity (DCI) in basins occupied by 191 migratory fish species according to (A) present-day and (B) projected future (mid-century) dam distributions. Average river connectivity across each species' geographic range (i.e. occupied basins out of the 1,216 basins) are represented by colored circles (95% confidence intervals displayed as light gray lines). Colors differentiate the species that are red-listed (24, orange) or highly valuable for fisheries (24, yellow) from the remaining species (gray). Black triangles and black lines indicate the overall averages with associated 95% confidence intervals. The main taxonomic groups classified as migratory are illustrated in (A). From the top down: Bryconidae, Prochilodontidae, Anostomidae, Curimatidae, Hemiodontidae, Triportheidae, Serrasalmidae, *Raphiodon* (Cynodontidae), *Pterodoras* (Doradidae), *Rhamdia* (Heptapteridae), and Pimelodidae (full species list available at Supplementary Table 1).</figcaption></figure>

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<figure><img src="image_6.png" /><figcaption>**Fig. 5.** Relationship between the generation capacity of each future hydropower project and its effect on river connectivity (DCI) at the basin level. Estimates of average DCI loss (the average difference across all basin-level future scenarios with and without each dam) for planned SHPs (red) and LHPs (blue) are represented by circles, and the gray lines indicate the range. The 24 planned LHPs with future capacity below 30 MW were classified as such by the energy agency ANEEL, following other criteria such as reservoir area (i.e., > 13 km²).</figcaption></figure>

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<figure><img src="image_7.png" /><figcaption>**Fig. 6.** (A) Future projections of nationwide river connectivity (average DCI across all 1,216 basins) and total additional generation capacity (based on a 2018 baseline) for 4.3 million bootstrap samples (circles) from the 2,268 planned dams in Brazil. Vertical dashed lines delineate the projected additional power need for Brazil by the 2030-2040 decade. The colors of the circles indicate the favorable (black), least-favorable (white), and remaining (purple) dam portfolios computed using a Pareto frontier operator. The inset boxplots contrast (B) the number of new SHP and LHP projects to be built and (C) the number of basins that will no longer be free of hydropower in the favorable and least-favorable dam portfolios located within the 2030-2040 projected power demand.</figcaption></figure>