BioScience Advance Access published May 2, 2016 

Overview Articles
 

Flow Management for Hydropower 

Extirpates Aquatic Insects, 

Undermining River Food Webs 

THEODORE A. KENNEDY, JEFFREY D. MUEHLBAUER, CHARLES B. YACKULIC, DAVID A. LYTLE, SCOTT W. MILLER, 

KIMBERLY L. DIBBLE, ERIC W. KORTENHOEVEN, ANYA N. METCALFE, COLDEN V. BAXTER 

Dams impound the majority of rivers and provide important societal benefits, especially daily water releases that enable on-peak hydroelectricity 

generation. Such “hydropeaking” is common worldwide, but its downstream impacts remain unclear. We evaluated the response of aquatic 

insects, a cornerstone of river food webs, to hydropeaking using a life history–hydrodynamic model. Our model predicts that aquatic-insect 

abundance will depend on a basic life-history trait—adult egg-laying behavior—such that open-water layers will be unaffected by hydropeaking, 

whereas ecologically important and widespread river-edge layers, such as mayflies, will be extirpated. These predictions are supported by a morethan-2500-sample, citizen-science data set of aquatic insects from the Colorado River in the Grand Canyon and by a survey of insect diversity 

and hydropeaking intensity across dammed rivers of the Western United States. Our study reveals a hydropeaking-related life history bottleneck 

that precludes viable populations of many aquatic insects from inhabiting regulated rivers. 

Keywords: biodiversity, dams, biological traits, river ecology, citizen science 

More than 800,000 dams exist worldwide  far exceeding the generation of all other renewable sources  (Richter and Thomas 2007), and the majority of combined (World Commission on Dams 2000, Zarfl et  al. 

Earth’s rivers are dammed (Nilsson et  al. 2005). Such flow 2015). Governments around the world are also planning 

management provides numerous societal benefits, including extensive new development of hydroelectricity potential 

water storage, flood control, and hydropower production in response to energy and climate crises (Kao et  al. 2014, 

(World Commission on Dams 2000). The ecological upshot Zarfl et  al. 2015), even though the damming of rivers has 

of all this damming is that rivers are some of the most exten- substantial environmental costs, including habitat loss, fishsively altered ecosystems on Earth (Carpenter et  al. 2011), ery collapse, and biological extinctions (Vörösmarty et  al. 

and flow regimes among large rivers are becoming homog- 2010, Liermann et  al. 2012). However, we contend that a 

enized as the timing and magnitude of impounded releases key dimension of these environmental costs—and a mechaare engineered to meet societal needs (Poff et  al. 2007). It nism underpinning many others—has been overlooked, 

is widely recognized that dams directly affect population contributing to an incomplete assessment of environmental 

viability for aquatic species by creating a barrier to move- trade-offs. As we demonstrate, this cost arises from a hithment and migration (Kareiva et al. 2000, Dugan et al. 2010). erto-unrecognized life-history bottleneck that flow manHowever, far less is known about the specific mechanisms agement for hydropower production imposes on aquatic 

underlying downstream ecological responses to hydrologic insects, ubiquitous organisms that play an essential role in 

flow modification (Bunn and Arthington 2002, Poff and food webs in and along the world’s rivers. 

Zimmerman 2010). Water releases from hydropower dams often vary at 

Hydroelectricity production is a primary purpose of hourly timescales in a practice known as load following or 

many dams, particularly the world’s largest and most iconic, hydropeaking (hereafter hydropeaking), whereby river flows 

including the Three Gorges Dam on the Yangtze River in are increased during the day when energy demand is high 

China, the Itaipu Dam on the Paraná River in Brazil and and decreased at night when demand is low (Førsund 2015). 

Paraguay, and the Hoover Dam on the Colorado River in Hourly changes in discharge associated with hydropeaking 

the United States (World Commission on Dams 2000). can be substantial, with discharge varying by a factor of 

Hydropower provides 19% of the world’s electricity supply, 10 or more within a day (Moog 1993, Topping et al. 2003). 

BioScience XX: 1–15. Published by Oxford University Press on behalf of American Institute of Biological Sciences 2016. This work is written by (a) US 

Government employee(s) and is in the public domain in the US. 

doi:10.1093/biosci/biw059 Advance Access publication XX XXXX, XXXX 

http://bioscience.oxfordjournals.org XXXX XXXX / Vol. XX No. X • BioScience 1 
Overview Articles
 

These large hourly changes in discharge produce kinematic 

waves that can propagate downstream for distances of more 

than 400 kilometers (km; Wiele and Smith 1996), creating 

an extensive intertidal zone along river shorelines that is 

absent from natural rivers and to which freshwater organisms are not adapted (Ward and Stanford 1979, Moog 1993). 

However, large dams impose multiple stressors on rivers, 

including altered flow, temperature, and sediment regimes 

(Poff et  al. 2007, Schmidt and Wilcock 2008, Olden and 

Naiman 2010), and prior studies have struggled to disentangle the effects of hydropeaking from these other stressors 

(Arthington et al. 2006, Poff and Zimmerman 2010). 

Hydropeaking and the artificial intertidal zone it creates 

have the potential to adversely affect numerous aquatic 

organisms, but strong deleterious impacts are particularly 

likely for organisms that rely on nearshore environments 

during one or more life stages. We selected aquatic insects 

for our investigation of hydropeaking impacts, because 

they are ubiquitous in freshwaters and represent the primary prey for myriad species of fish, birds, bats, and other 

wildlife living in and along rivers (Nakano and Murakami 

2001, Baxter et  al. 2005). These insect groups—especially mayflies, stoneflies, and caddisflies in the orders 

Ephemeroptera, Plecoptera, and Trichoptera (hereafter 

EPT)—are also widely used as bioindicators for rivers and 

are integral to freshwater regulatory policies in the United 

States, the European Union, and elsewhere (Fremling 1964, 

Lenat 1988, Carlisle et al. 2013). 

Most aquatic insects have complex life cycles, with a 

winged adult life stage that is terrestrial whereas the egg, 

larval, and pupal stages are aquatic. Nonetheless, the vast 

majority of investigations into aquatic insects have focused 

on the larval life stages in isolation, whereas other life stages 

are traditionally ignored. However, this approach is currently 

undergoing a transformation, because the conditions necessary to support aquatic insects throughout their life cycles 

(egg, larva, pupa, and adult) are increasingly recognized as 

crucial to species persistence (Encalada and Peckarsky 2012, 

Muehlbauer et al. 2014). 

The influence of recruitment (i.e., the number of eggs 

that develop into larvae) on population dynamics is poorly 

studied, particularly in the case of insects. Investigations 

into factors limiting the diversity or abundance of aquaticinsect assemblages have traditionally focused on studying 

larval life stages exclusively, because it was thought that the 

environmental conditions and biological interactions experienced by larvae (i.e., predation and competition) would 

invariably be the primary determinants of larval abundance. 

Aquatic insects spend a disproportionate amount of their 

lives in the larval stage, and adult females can lay hundreds 

to thousands of eggs, so it seemed unlikely that larval abundance would ever be limited by egg abundance or the environmental conditions experienced by nonlarval stages per se. 

However, several recent studies have called this assumption 

into question and demonstrate that the abundance of larvae 

can be strongly influenced by the quality and availability of 

egg-laying substrates at the start of the cohort (Encalada and 

Peckarsky 2012, Lancaster and Downes 2014, Macqueen and 

Downes 2015). In one compelling study, investigators experimentally varied the number of Baetis mayfly eggs present in 

stream reaches by simply manipulating the number of emergent rocks that were present in different pristine streams 

(Encalada and Peckarsky 2012). Adult Baetis females land 

on these rocks in fast water and then crawl under the rock 

to cement their eggs to the underside. Nearly a year after the 

emergent rocks were manipulated, the abundance of latestage mayfly larvae had doubled in reaches where emergent 

rocks were added and declined by more than half in reaches 

where they had been eliminated. Therefore, recruitment 

dynamics—and the availability or quality of egg-laying 

substrates specifically—can be a primary determinant of 

aquatic-insect abundance in streams and rivers. 

When viewed through this lens of egg laying and rearing, it becomes apparent that aquatic-insect populations 

below hydropeaking dams could be especially vulnerable to 

recruitment limitation. Over the course of a single day, these 

rivers are subject to large changes in river depth, which may 

dramatically affect the availability and quality of egg-laying 

substrates used by aquatic insects. For taxa such as Baetis 

that cement eggs to substrates, for example, a partially submerged rock that is wetted during high water at the time of 

egg laying could subsequently be exposed to air a few hours 

later during low water, potentially killing the eggs. Scaling 

up, acute egg mortality caused by hydropeaking could be 

capable of regulating aquatic-insect abundance throughout 

river segments via recruitment limitation and reducing 

biodiversity by extirpating aquatic insects with river-edge 

egg-laying behaviors, with attendant consequences for populations of insectivorous wildlife (Nakano and Murakami 

2001, Baxter et al. 2005). 

Encapsulating all of these insights, assumptions, and 

inferences, we propose the following novel hypothesis concerning the intersection of hydropeaking and aquatic-insect 

life history: Hydropeaking limits the recruitment of aquatic 

insects via acute egg mortality, constraining the abundance 

and diversity of aquatic-insect populations downstream of 

hydropeaking dams. 

In the remainder of this article, we use multiple lines 

of evidence to test this novel hypothesis. We first formally 

test the two key assumptions implicit in this hypothesis: 

that most insects lay eggs in habitats vulnerable to drying 

by hydropeaking-induced water-level fluctuations and that 

drying reduces egg viability. Next, we develop a life history–hydrodynamic model that provides a predictive framework for exploring how different insect groups respond 

to hydropeaking, contingent on their specific egg-laying 

strategies. We then test predictions from this model using 

a large citizen-science data set of emergent insects collected 

throughout the 400-km-long Grand Canyon segment of 

the Colorado River. Finally, to investigate the consistency 

of hydropeaking effects on aquatic-insect assemblages, we 

compare insect diversity across 16 dammed rivers in the 

2 BioScience • XXXX XXXX / Vol. XX No. X http://bioscience.oxfordjournals.org 
Overview Articles
 

Portion of aquatic insects 

1.0 

.8 

.6 

.4 

.2 

0 

Cemented Broadcast Live birth Terrestrial Asexual 

Figure 1. Aquatic insects often cement eggs to substrates 

along river margins. Represented here are the egg-laying 

strategies for the most common aquatic invertebrate 

genera in North America and Europe, representing 96 and 

78 taxa, respectively. The error bars represent the standard 

error of the mean. Adapted with permission from Statzner 

and Beche 2010. 

Western United States that span a range of hydropeaking 

intensity. 

Where do aquatic insects lay their eggs? 

One major assumption that is implicit in our recruitmentlimitation hypothesis is that many aquatic insects lay eggs 

along river shorelines. We tested this assumption by drawing 

on a recent synthesis of biological trait databases (Statzner 

and Beche 2010). The use of biological traits as indicators 

has gained momentum as a powerful approach for disentangling the effects of multiple stressors in aquatic ecosystems 

(Poff et  al. 2006), and the development of comprehensive 

databases of biological traits for aquatic invertebrates is 

making this approach feasible on a continental scale (e.g., 

Vieira et al. 2006). For the purposes of evaluating egg-laying 

traits of aquatic insects, Statzner and Beche (2010) included 

descriptions for invertebrate genera spanning 96 North 

American taxa and 78 European taxa. 

These insect trait databases provide strong evidence that 

the majority of aquatic insects exhibit egg-laying behaviors that will be vulnerable to hydropeaking (figure 1). 

Specifically, more than three-quarters of aquatic insects 

exhibit the type of cementing behavior shown by females 

of Baetis that were studied in the Encalada and Peckarsky 

(2012) experiment described earlier. Significantly, this 

behavior generally requires the presence of substrates that 

are partially emerged out of the water surface on which 

females can alight before crawling under water or, alternatively, substrates that are in shallow water that females 

can swim to for the purposes of attaching their eggs. Both 

of these classes of substrates are vulnerable to hydropeaking-related flow variation, because partially submerged 

rocks and shallow habitats at high water can become fully 

exposed to air and dry out when the water is low. Although 

some taxa do exhibit egg-laying behaviors that will be 

unaffected by an artificial intertidal zone—such as species 

that give birth to live young (ovoviviparous) or broadcast 

eggs on the water surface—analysis of these traits databases indicates that such strategies are uncommon among 

aquatic insects (figure 1). Therefore, we conclude that this 

implicit assumption of our hypothesis is valid and well 

founded: The majority of insects lay eggs in habitats that 

are susceptible to drying via daily hydropeaking water-level 

fluctuations. 

Does desiccation reduce egg viability? 

In order for hydropeaking to limit aquatic-insect populations via acute egg mortality, desiccation on hydropeakingrelevant timescales must actually kill eggs. We directly tested 

this assumption by quantifying the viability of eggs that were 

exposed to brief periods of desiccation that are typical for 

hydropeaking rivers. Experiments were conducted using the 

eggs of a widespread genus of mayfly, Baetis (Baetidae), and 

two common, large river caddisfly genera, Brachycentrus 

(Brachycentridae) and Hydropsyche (Hydropsychidae). The 

experiments were conducted using insect eggs collected 

from the Green River downstream of the Flaming Gorge 

Dam, Utah, in June (caddisflies) and October (mayflies) 

of 2015. For caddisflies, egg masses attached to rocks were 

collected from nearshore habitats and assigned to one of 

four desiccation treatments: 0 (control), 4, 8, or 12 hours of 

streamside desiccation (n = 12–20, depending on the experiment and treatment level). Following the completion of each 

treatment, caddisfly egg masses were scraped from rocks, 

placed in individual vials, and stored in river water on ice. 

The treatments were initially conducted during both the day 

and the night, on the basis of the presumption that hotter air 

temperatures during the day would result in more rapid desiccation and egg mortality. However, both the day and night 

desiccation trials experienced extremely high mortality, and 

the ANOVA tests indicated that there was not a significant 

difference between day and night treatments (F-statistics 

= 0.001 and 0.042, and p-values = 0.978 and 0.837 for 

Brachycentrus and Hydropsyche, respectively). Therefore, the 

day and night treatments were combined for the remainder 

of the analysis, and the subsequent experiments on Baetis 

were conducted during the day only. The mayfly study also 

was conducted using eggs laid on slate tiles that had been 

placed throughout the stream rather than by scraping eggs 

off rocks, because low egg viability in the control groups 

for the caddisfly experiments indicated that scraping likely 

induced extraneous mortality. In addition, we added shorter 

duration treatments of one-half, 1, and 2 hours in the mayfly 

study to better identify critical desiccation exposure levels 

and conducted desiccation treatments in a laboratory setting instead of streamside. For both the caddisfly and mayfly 

experiments, the egg masses were reared at room temperature (23 degrees Celsius) on a 12-hour day–night light cycle 

for 5 weeks or until egg masses were no longer viable based 

on visual inspection. Egg viability was computed as the ratio 

of hatched eggs to the total number of eggs per egg mass. 

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Overview Articles
 

to hatch, they are in fact exposed to 

several cycles of inundation and drying 

prior to hatching. Therefore, our desiccation experiments should be viewed as 

conservative; field egg mortality after 

several such cycles is likely even higher 

than suggested by our laboratory rearing 

results. Conducting additional experiments on different aquatic-insect taxa 

(e.g., Plecoptera, stoneflies, or certain 

Diptera, true flies) would naturally be 

insightful and add nuance to the patterns 

we report here. However, our experiments included three globally common 

genera spanning two ecologically important aquatic-insect orders, and our results 

were unequivocal to the extent that they 

are likely to be broadly representative of 

other taxa. We therefore conclude that 

aquatic-insect eggs that are laid in the 

artificial intertidal zone created by daily 

hydropeaking will be subject to acute 

mortality later in the same day, when 

river stage is lower. 

A life-history bottleneck arising 

from hydropeaking 

Building on these egg-laying and desiccation results, we next developed a deterministic model to explore the interplay 

of hydropeaking waves with egg-laying Figure 2. Desiccation causes high mortality of aquatic-insect eggs. Mean egg strategies. Specifically, our model pro- viability (number of hatched eggs/total eggs) after desiccation for (a) Baetis, vides a predictive framework for under­ (b) Brachycentrus, and (c) Hydropsyche egg masses. The points represent the 

mean viability of egg masses across 12–20 replicate experiments, depending on standing how these variables interact to 

affect aquatic-insect populations along the treatment and taxon in question (see the main text for details). The error 

bars represent the standard error of the mean. 

These experiments provide convincing evidence that 

this assumption is also valid and well founded: Even brief 

desiccation markedly reduced egg viability (figure  2, 

ANOVA F-statistics = 21.500, 10.332, and 13.403 for Baetis, 

Brachycentrus, and Hydropsyche, respectively, and p-statistics < 0.001 for all). We did find that a half-hour of desiccation did not significantly reduce the viability of mayfly 

egg masses relative to undesiccated, control egg masses 

(t-statistic = 1.326, p-statistic = 0.099). However, after 1 hour 

or longer of desiccation, egg viability was significantly 

reduced for all three taxa compared with the control treatments, based on post hoc Tukey’s tests. In fact, egg viability 

in all of these desiccation treatments of 1 hour or longer did 

not differ significantly from 0 (i.e., total egg mortality), and 

no eggs hatched from 136 out of 164 (83%) of these treatments. It is worth noting that these treatments represent 

desiccation exposure for a single day along a hydropeaking 

river. Because many aquatic-insect eggs take days to weeks 

a hydropeaking river continuum. Lateral 

(shoreline) inundation due to hydropeaking can be approximated as a sine wave 

with magnitude h at distance x downstream from a dam as 

follows: 

(h max +h min � ) (h max -h min � ) h(x)= + ∗cos(f ∗ x) (1)

2 2 

where f determines the frequency of the wave, hmax is the 

maximum extent of lateral inundation, and hmin is the 

minimum extent of lateral inundation. As the sine wave 

propagates downstream, h(x) describes the degree of hydropeaking inundation that occurs on a given river segment 

at dusk, when most insect taxa are assumed to be laying 

their eggs (Pinder 1995, Merritt et al. 2008). Therefore, h(x) 

is an index of how severely hydropeaking affects recently 

deposited eggs at any location along the river. At locations 

with high values of h, eggs are laid when the river reach is 

at its daily high-flow peak, and deposited eggs will therefore 

be vulnerable to desiccation-induced mortality at low flow 

later that day and at the same time on each subsequent day. 

4 BioScience • XXXX XXXX / Vol. XX No. X http://bioscience.oxfordjournals.org 
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In contrast, when h(x) = 0, the river is at low flow during 

the time of egg laying, so newly deposited eggs will never be 

subjected to any flows lower than those present during egg 

laying, and these eggs will never be subject to desiccation. 

We assumed a trade-off between per-egg survival (s) and 

egg-laying location relative to the river edge (0  ≤  r ≤  1), 

such that eggs cemented to substrates along river edges 

experience the highest relative survival in the absence of 

hydropeaking: 

− ∗r c s r( )= e (2) 

where c controls the steepness of the relationship. Such a 

trade-off occurs if shallow edge habitats provide more favorable rearing conditions for eggs (e.g., slower water that will 

allow river-edge specialists to cement eggs directly on suitable substrates) relative to midchannel habitats (e.g., only 

open-water generalists that dip eggs directly into the water 

are able to use these habitats for egg laying, and many of 

these eggs may settle on unsuitable substrates). Note that 

r is measured relative to the river edge (i.e., the river edge 

occurs at r = 0 and river center at r = 1), and this will change 

in response to hydropeaking. Therefore, exclusive of any 

hydropeaking effects, maximum total egg survival, wmax, in 

a river reach is given by the area under equation 2 bounded 

by the portion of the river cross-section used for egg laying 

(r1 to r2), which is a species-specific quantity: 

r r c ∫ − ∗ w = 2 e dr (3) max r max 1 

Although any egg-laying strategy is possible, we consider 

the case where each insect species uses a characteristic portion of the river with respect to the river’s edge, and for 

simplicity, we treat the width of this proportion (r2 – r1) as 

constant. For example, river-edge specialists might lay eggs 

uniformly over the interval [0, 0.2] very near the shoreline, 

whereas open-water generalists might lay eggs uniformly in 

the thalweg [0.8, 1]. Under equation 3, the eggs of river-edge 

specialists will always have the highest potential survival 

(wmax) relative to other egg-laying strategies, but hydropeaking may strand some or all of the eggs deposited at a particular location such that the realized survival (w) is lower than 

wmax. The survival of eggs deposited within the zone affected 

by hydropeaking will be reduced by whydro, the portion of the 

egg-laying zone that is lost to desiccation: 

w w= max − whydro (4)

h k() −rc If r () (5) 1 < hk < r2 , whydro = ∫r e dr 1 

r −rc If h k() < r1, whydro = ∫

1 e dr = 0 (6) r1 

r −rc If () < r2 hk , whydro = ∫ e dr = wmax 

2

 (7) r1 

In equation 5, only part of the egg-laying zone is lost because 

of hydropeaking, with concomitant effects for population 

level egg survival. In contrast, equations 6 and 7 describe 

scenarios in which none or all of the egg-laying zone, respectively, is lost. 

Our life history–hydrodynamic model predicts that desiccation-induced egg mortality will extirpate river-edge egg 

layers from hydropeaking rivers (figure 3), particularly mayflies and other EPT taxa that cement eggs to substrates at the 

river margin or other zones subject to exposure under daily 

low hydropeaking flows (Statzner and Beche 2010, Encalada 

and Peckarsky 2012). In contrast, open-water egg layers, 

such as some blackfly species (e.g., Simulium arcticum; Adler 

et  al. 2004), will be minimally affected by hydropeaking 

(figure 3). These divergent egg-laying strategies represent 

an example of a classic life-history trade-off that may even 

provide a basis for why EPT taxa are useful as bioindicators 

in the first place: Eggs laid by such river-edge specialists are 

likely to experience high survival when conditions remain 

ideal but are susceptible to complete mortality from environmental perturbations that cause conditions to suddenly 

change (e.g., shoreline desiccation). On the other end of 

the spectrum, eggs broadcast on the water surface by openwater generalists are likely to always experience relatively 

high mortality because many eggs will settle on unsuitable 

substrates; nonetheless, some eggs will just as invariably 

settle on ideal substrates. Finally, some taxa, such as midges 

(Chironomidae), exhibit an intermediate strategy. Taken as a 

group, midges lay their eggs generically along shorelines in a 

fashion that is neither strictly dependent on emergent rocks 

and vegetation nor broadcasting over open water (Pinder 

1995). Specifically, most adult female midges tend to cement 

eggs to substrates, but some broadcast eggs freely (Williams 

1982, Pinder 1995). Furthermore, when eggs are cemented, 

midge behavior differs from that of EPT taxa such as Baetis 

in that midges land at or on the water surface to release their 

eggs from their abdomen and do not actually crawl under 

water (Branch 1928, Pinder 1995). During this time between 

egg laying and cementing, many eggs are lost to open water; 

indeed, egg drift due to this behavior is substantial and 

has been estimated at 4000 eggs per hour per square meter 

through a river cross-section (Williams 1982). For intermediate strategists such as these midges, the model predicts 

a spatial periodicity in abundance that correlates with the 

timing of daily hydropeaking waves. Specifically, population 

abundance should be greatest in river reaches where the lowest daily flow occurs coincident with the peak timing of egg 

laying, because eggs laid in these locations will remain wetted and will never be subject to desiccation, but these insects 

should also be found at low abundance throughout the river. 

Insect abundance and diversity along a hydropeaking 

river continuum 

We parameterized our life history–hydrodynamic model 

for the specific conditions present along the Colorado River 

downstream from the Glen Canyon Dam in the Grand 

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Oviposition strategy 

Distance from dam (km) 

River edge 

Open water 

0.0 

0.5 

1.0 0 

100 

200 

300 

400 

0.00 

0.05 

0.10 

Egg survival 

Figure 3. A life history–hydrodynamic model. Output from a life history–hydrodynamic model parameterized for the 

Colorado River in the Grand Canyon shows that as hydropeaking waves propagate downstream, the locations where 

daily flow minima occur at dusk are consistently located around river kilometer (km) 100 and 275. River-edge egg layers, 

such as mayflies (red line), are eliminated from hydropeaking rivers because of desiccation and the mortality of eggs. 

Open-water egg layers, such as Simulium arcticum (blackflies; green line) occur at all points along hydropeaking rivers. 

Intermediate strategists, such as midges (yellow line), exhibit spatial periodicity in abundance, with the highest abundance 

at locations where the timing of insect egg laying, generally at dusk, is in phase with daily flow minima. This figure 

illustrates equation 7, assuming r2 – r1 = 0.2, c = 2, and f = 0.033 (see the main text for details). 

Canyon, United States. Because of its canyon-bound setting 

and the absence of large tributaries, the daily hydropeaking 

wave released from the Glen Canyon Dam propagates far 

downstream, with hourly changes of 1 meter in river stage 

still present more than 400 km downstream, where the 

Colorado River enters its next reservoir, Lake Mead (Wiele 

and Smith 1996). This makes the Colorado River in the 

Grand Canyon an ideal case study because there is a wide 

range of variation in the degree of synchrony between the 

timing of egg laying (i.e., dusk) and the timing of minimum 

flows. Sampling along this remote segment of the Colorado 

River nonetheless presents logistical challenges, because 

most reaches are only accessible by two-week-long rafting trips. Therefore, to quantify aquatic-insect abundance 

throughout the Grand Canyon, we collaborated with river 

rafters to sample the adult life stage of aquatic insects, 

the abundance of which is directly related to larval abundance (Statzner and Resh 1993). Each evening in camp, 

these citizen scientists deployed fluorescent light traps for 

1 hour near the river’s edge, ultimately collecting more than 

2500 samples (see box 1, figure 4, and Acknowledgments). 

We recruited citizen scientists to the project using an outreach video and through advertisement in Grand Canyon 

rafting publications (see box 1). Citizen-science light trapping occurred from April to October in 2012, 2013, and 

2014. Each light trap consisted of a small plastic storage 

container and a fluorescent light that was placed along the 

short edge of the container (figure 4). Citizen scientists were 

provided with a table of sunset times to facilitate the consistent timing of trap deployment and were instructed to turn 

on light traps within 1 hour after nautical sunset. At the start 

of each deployment, citizen scientists poured the contents 

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Box 1. Little bugs, big data, and the Grand Canyon: The role of citizen science in Colorado River adaptive management. 

The closure of the Glen Canyon Dam in 1963 fundamentally altered the physical template of the 400-kilometer-long segment of the 

Colorado River in the Glen, Marble, and Grand Canyons (Schmidt et  al. 1998). For instance, water clarity is considerably higher 

because Lake Powell reservoir traps millions of tons of sediment and organic matter annually (Topping et al. 2000), hypolimnetic 

releases lead to relatively cold and constant water temperatures (Voichick and Wright 2007), and flow management dampens the 

annual low and high flow periods, whereas hydropeaking greatly increases hourly flow variation (Topping et al. 2003). These changes 

in the physical template, along with accidental and intentional introductions of organisms (Blinn and Cole 1991), have completely 

restructured aquatic food webs (Cross et al. 2013) and led to declines and local extirpations of native, endangered fish populations 

(Minckley 1991). Ongoing adaptive management of the Glen Canyon Dam releases seeks to rehabilitate the Grand Canyon ecosystem 

and benefit key resources, including native and desired nonnative fish populations (www.gcdamp.gov). 

Science plays an essential role in the adaptive management of the Glen Canyon Dam, but it has proven challenging for professional 

scientists to collect the type of high-resolution data needed to inform the management of this highly altered and remote ecosystem. 

Specifically, most reaches of the Colorado River are only accessible by two-week-long river trips that travel the entire 400-kilometerlong segment, which greatly limits the spatial and temporal extent of sampling regimens. However, starting in the early 2000s, physical 

scientists began using automated sensors to continuously monitor suspended sediment concentration and, for the first time, demonstrated a clear link between hydropeaking operations, sediment transport, and the erosion of beaches in the Grand Canyon (Topping 

et al. 2004, Wright et al. 2008). More recently, the continuous measurement of ecosystem metabolism using dissolved oxygen monitors 

revealed that hydropeaking operations have a significant, albeit minor, effect on algae production in the Grand Canyon (Hall et al. 

2015). Any similar solutions to the continuous monitoring of animal populations using sensors have proven elusive, however, so river 

trips staffed by professional scientists have remained the primary tool for investigations into animal population dynamics. However, 

testing predictions of our life history–hydrodynamic model using traditional benthic surveys of larval insects would have required 

dozens of such river trips through the Grand Canyon at a cost of millions of dollars. 

The involvement of citizen scientists, whereby members of the public conducted standardized light trapping of adult aquatic insects, 

played a vital role in overcoming these logistical challenges (figure 4). Unlike many citizen-science projects, we elected to pay the 

participants on a per-sample basis to ensure high-quality data and continued involvement by professional river guides. Even with the 

$15-per-sample cost, this approach has proven to be extremely cost effective, which is an important consideration in the development 

of long-term monitoring programs. Somewhat unexpectedly, the broader impacts of this project have also been especially significant. 

For instance, our citizen-science monitoring project has proven to be a powerful tool for outreach and overall engagement with the 

public (www.gcmrc.gov). In addition, collaboration with Grand Canyon Youth (www.gcyouth.org) and other education groups has provided a unique opportunity to train the next generation of scientists about the scientific process and the important role that science 

plays in adaptive ecosystem management. 

Figure 4. Citizen science in the Grand Canyon. Professional river guides, private boaters, educational groups, and 

other citizen scientists rafting the Colorado River in the Grand Canyon deployed light traps each night in camp 

between 2012 and 2014 (a); a Grand Canyon Youth river trip pictured (photo courtesy Freshwaters Illustrated/Dave 

Herasimtschuk), ultimately producing a data set of more than 2500 adult aquatic-insect samples (b). The spatial and 

temporal coverage of the citizen-science-derived data set (open black circles) far exceeds the scope of invertebrate 

sample data sets that can be collected using river trips staffed by professional scientists (e.g., blue filled circles, which 

represent the invertebrate sampling effort described in Cross et al. 2013). Abbreviation: km, kilometers. 

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Table 1. ΔBIC table comparing various univariate models for the three dominant insect taxa collected in the Grand 


Canyon light trap samples. 

Model Microcaddisflies Blackflies Midges 

Base model (temporal and spatial random effects) 52.9 11.9 27.4 

River slope 60.7 15.5 29.0 

Distance from dam 26.2 0.0 32.2 

Distance from tributaries 0.0 17.6 30.8 

Hydropeaking waves 60.5 16.2 0.0 

Note: The best-performing model for each taxa is underlined and bold. The multivariate models containing combinations of these variables are 

included in table S1; the univariate models listed here highlight the main driver of abundance for each taxon. 

of a 250-milliter bottle pre-filled with 95% ethanol into the 

plastic container and turned on the light. After one hour, 

lights were turned off, and the contents were poured back 

into the 250-milliliter bottle for storage. 

Three taxonomic groups were collected in sufficient numbers by citizen scientists to allow analysis: microcaddisflies in 

the family Hydroptilidae, blackflies (Simulium arcticum), and 

midges in the family Chironomidae. The abundance data for 

these taxa were fit to a series of mixed-effects models representing different hypothesized controls on spatial patterns. 

We had anticipated a priori that there would be a high degree 

of heterogeneity in these count data and that many samples 

would contain very few or no individuals; therefore, we used 

a negative binomial distribution for all models. Preliminary 

information content analysis indicated that a base model 

including random effects for both sample month of year (i.e., 

time) and sample location within the 12 distinct geomorphic reaches within the Grand Canyon (i.e., space; Schmidt 

and Graf 1990) was strongly preferred over other candidate 

base models containing only one of these random effects. 

Therefore, the base model with both random effects was 

used in all subsequent analysis. We then fit different models representing competing hypotheses for each taxonomic 

group, each consisting of the base model plus fixed effects for 

river surface slope, river kilometer, distance from a tributary, 

and hydropeaking wave phase, considered either alone or in 

combination. The river surface slope variable essentially differentiates rapids and other high-energy environments from 

low-energy environments such as pools, reflecting numerous 

empirical studies from the Grand Canyon that have found 

higher invertebrate abundance in higher-energy habitats 

(Cross et  al. 2013). Information on slope is derived from 

a comprehensive LIDAR data set representing over 2700 

individuals measurements throughout the Grand Canyon 

(Magirl et  al. 2005). Similarly, the river-kilometer variable 

functions as a proxy for environmental variables that vary as 

a function of distance from the dam, such as water temperature and turbidity. The model with distance from a tributary 

was considered on the basis of the possibility that taxa reared 

in tributaries may disperse as adults and appear in light traps 

set along the mainstem Colorado River, potentially inflating 

abundance estimates near these confluences. This model 

was based on the absolute distance between the trap and the 

nearest tributary with a baseflow of more than 1 cubic meter 

per second. Finally, the model with hydropeaking wave phase 

accounted for the difference in discharge at dusk (4 p.m.) 

relative to the daily minimum discharge for a given sample 

location; note that the low light conditions of dusk occur 

early in the Grand Canyon owing to shading by canyon walls 

(Yard et al. 2005). This variable reflected the likelihood that 

eggs laid along shorelines at dusk would be subsequently 

dewatered and subject to desiccation. Models were fit in 

R (version x64 3.1.2) using the glmmADMB package and 

compared using the Bayesian information criterion (BIC). 

Models with ΔBIC > 4 have very little statistical support with 

respect to the best model (Burnham and Anderson 2002). 

Spatial variation in aquatic-insect abundance throughout 

the Grand Canyon closely matched predictions from our 

life history–hydrodynamic model, with variation among 

taxa being primarily related to egg-laying strategy. For 

instance, river-edge egg-laying specialists were exceedingly 

rare throughout the Grand Canyon, with only microcaddisflies—and no mayflies or stoneflies—present in sufficient 

abundance to analyze statistically (table 1, supplemental 

table S1, supplemental figure S1). Benthic larval surveys 

conducted over the past 30 years have also noted that EPT 

taxa are conspicuously absent from the Grand Canyon 

(Stevens et  al. 1997, Cross et  al. 2013), indicating that the 

scarcity of EPT taxa in light trap samples is not an artifact 

of our sampling methods. The abundance of microcaddisflies was generally low throughout the Grand Canyon and 

declined precipitously with distance from tributaries. As 

such, the distance from the tributary variable was by far the 

most important in improving the performance of microcaddisfly statistical models (table 1). Light traps that were 

proximate to tributaries (i.e., less than 1 km) sometimes 

captured thousands of microcaddisflies, whereas traps that 

were distant from tributaries (i.e., more than 20 km) never 

caught more than 300 microcaddisflies. This suggests that 

microcaddisflies are not well established in the mainstem 

Colorado River and that the majority of adult microcaddisflies captured in light traps actually dispersed from tributaries that do support diverse aquatic-insect populations 

(Oberlin et al. 1999). Therefore, citizen-science data indicate 

that taxa with river-edge egg-laying behaviors have been 

largely extirpated from the Grand Canyon and that extant 

8 BioScience • XXXX XXXX / Vol. XX No. X http://bioscience.oxfordjournals.org 
0 

2000 

4000 

6000 

8000 

Overview Articles 

Also as predicted, midges, as intermediate strategists, 

exhibited strong spatial periodicity in abundance that was 


related to the timing of hydropeaking waves; the best overall model was univariate and included only the fixed effect
 

related to the timing of hydropeaking waves (figure 5, table 1, 

table S1). In fact, locations where the daily flow minimum 

occurred at dusk when most midges lay their eggs (Pinder 

1995) had a threefold greater abundance relative to locations 

where the timing of midge egg laying was out of phase with 

daily tides (figure 5). Therefore, empirical citizen-science 

data from the 400-km-long Grand Canyon segment of the 

Colorado River are consistent with the hypothesis that 

High 

Low 

+0 

+50 

+100 

+150 

+200 

+250 

+300 

Relative egg survival

Discharge at dusk relative to daily

minimum (cubic meters per second) 

a

Average 

hydropeaking waves control the abundance and diversity of
 

aquatic insects via egg mortality, and these effects are con18000 b tingent on the egg-laying strategies of individual taxa. 

Average 

(2nd axis) 

0 100 200 300 400 

Midge abundance (number per hour) 16000 Hydropeaking as a lever on insect diversity 

800 throughout the American West
 

In addition, we tested predictions of our life history–hydrodynamic model by comparing insect diversity among large, 


dammed rivers in the Western United States that vary in 

the degree of hydropeaking. For this test, we quantified the 

degree of hydropeaking across rivers using a hydropeaking 

index: the daily coefficient of variation in discharge averaged over 5 or more years (Dibble et al. 2015). We obtained 


subdaily (15–, 30–, and 60-minute) discharge data and available raw benthic invertebrate data from government agency
 

Average midge abundance

(number per hour)

600 

400 

200 

0 

databases, hydroelectricity companies, private consulting
 

firms, universities, and individual scientists (see supplemental table S2). We used EPT percent abundance—computed 


as the mean of all available benthic samples across years—as 


our metric for assessing insect diversity. EPT percent abundance is widely used in river bioassessment investigations 


(Carlisle et al. 2013), and our use of it in this analysis is in 


line with earlier discussion about the propensity of these 


taxa to cement eggs on shallow substrates that are vulnerable 


to hydropeaking desiccation (Statzner and Beche 2010). To
 

ensure that the data set contained only high-quality data that
 

were comparable in terms of collection methods, sampling 


Distance from Dam (km) 

Figure 5. Spatial periodicity in insect abundance along a 


river continuum. Hydropeaking waves released from the 


Glen Canyon Dam propagate downstream through the 


Colorado River in the Grand Canyon, and the survival
 

of insect eggs will be highest at locations where daily 


flow minima occur at dusk, approximately 100 and 


275 kilometers (km) downstream from the Dam (a). 


Midge abundance exhibits spatial periodicity because of
 

variability in egg survival, with average abundance more
 

than three times greater at locations where the daily flow 


minima was in phase with peak egg-laying activity (b).
 effort, and spatial context, the suite of available raw data was 


populations of microcaddisflies are strongly tied to the presence of individuals originating from tributary streams. 

The open-water generalist blackfly Simulium arcticum was 

present throughout the Grand Canyon, and as predicted, 

there was not statistical support for a hydropeaking effect or 

any association with tributaries (table 1, table S1, figure S1). 

The best-supported model included a fixed effect for distance 

from the dam with a negative coefficient, indicating a general 

downstream decline in blackfly abundance. Prior investigations of invertebrate feeding habits in the Grand Canyon 

demonstrate that the quality of suspended organic matter 

consumed by filter feeding blackflies declines with distance 

downstream (Wellard Kelly et al. 2013), and it therefore seems 

likely that blackfly abundance is reflective of this gradient. 

subset to include only benthic data (i.e., no water column 

drift samples), only dams with at least three discrete samples, 

and only samples collected within 50 km downstream of the 

dam. This resulted in a final data set of 1267 samples across 

16 dams, 14 rivers, and 9 states throughout the Western 

United States (see table S2). 

We found that aquatic-insect diversity was strongly and 

negatively related to the degree of hydropeaking across the 

16 rivers for which adequate data were available (figure 6). 

Notably, this decline in diversity proceeded according to a 

logistic function (supplemental table S3), such that there 

was little association between insect diversity and hydropeaking index values less than 0.10. This indicates that 

diverse assemblages of aquatic insects can still persist in 

heavily altered, dammed rivers, so long as they lack large, 

artificial tides. At hydropeaking index values greater than 

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Figure 6. Hydropeaking is a lever on aquatic-insect diversity. We gathered available invertebrate data from dammed rivers 

throughout the Western United States (a). Insect diversity was strongly and negatively related to the degree of hydropeaking 

across the 16 rivers for which adequate data were available (b); the relative abundance of insect orders Ephemeroptera 

(mayflies), Plecoptera (stoneflies), and Trichoptera (caddisflies; EPT percent, y-axis) is a widely used aquatic-insect 

diversity metric. Inset (c) shows the discharge over a week in July 2013 for three well-studied river segments: the Green 

River below the Fontenelle Dam (green, low hydropeaking index value), the Green River below the Flaming Gorge Dam 

(yellow, moderate hydropeaking index value), and the Colorado River below the Glen Canyon Dam (red, high hydropeaking 

index value; note the log y-axis). Key to dams and rivers: A, the Hoover Dam, Colorado River; B, the Glen Canyon Dam, 

Colorado River; C, the Flaming Gorge Dam, Green River; D, the Deer Creek Dam, Lower Provo River; E, the Starvation 

Dam, Strawberry River; F, the Pine Flat Dam, Kings River; G, the Echo Dam, Weber River; H, the Wanship Dam, Weber 

River; I, the Causey Dam, S. Fork Ogden River; J, the Owyhee Dam, Owyhee River; K, the Libby Dam, Kootenai River; L, 

the Crystal Dam, Gunnison River; M, the Navajo Dam, San Juan River; N, the Fontenelle Dam, Green River; O, the Tongue 

River Dam, Tongue River; (p) the Holter Dam, Missouri River. See supplemental tables S2 and S3 for details. 

0.10, however, insect diversity declined rapidly and eventually approached zero. In fact, downstream of both the 

Hoover and Glen Canyon Dams, which had the two largest 

hydropeaking values in our data set, few to no mayflies, 

stoneflies, or caddisflies were present. We also compared 

candidate models containing other variables that might 

affect insect diversity downstream of dams for which data 

could be compiled: dam height (a proxy for downstream 

water quality including temperature) and channel width (a 

proxy for ecosystem size). The univariate model containing 

only the hydropeaking index performed better than univariate or multivariate models containing these other variables. 

Therefore, strong agreement between predictions of our life 

history–hydrodynamic model and two large validation data 

sets implicate hydropeaking as a primary factor limiting 

aquatic-insect diversity and abundance in dammed rivers. 

Hydropeaking and the specter of multiple stressors 

It is widely recognized that flow, temperature, and sediment alterations associated with river regulation can lead 

to invertebrate assemblages that are ecologically unhealthy 

(Arthington et  al. 2006). Specifically, insect diversity 

and richness are often low downstream of dams, and 

invertebrate assemblages are often dominated by noninsects 

such as worms, snails, and amphipod crustaceans (Vinson 

2001, Cross et  al. 2011, Robinson 2012). However, the 

virtual absence of mayflies, stoneflies, and caddisflies in 

the Colorado River downstream of the Glen Canyon and 

Hoover Dams is unusual even relative to other dammed rivers in the region (figure 7). One possible explanation for the 

low diversity of aquatic insects in the mainstem Colorado 

River may be that invertebrate assemblages have never been 

diverse, even before the construction of these dams. The 

physical template of the unregulated Colorado River did 

have a dramatic disturbance regime with many distinctive 

characteristics that may have made life difficult for aquatic 

insects (Resh et al. 1988), including large snowmelt floods, 

extremely high suspended sediment concentrations, and 

low water clarity (Schmidt and Grams 2011). However, 

the Green and Colorado Rivers located near or within the 

Cataract Canyon, approximately 200 km upriver of the Glen 

Canyon Dam, maintain similar characteristics to the unimpounded Colorado River, and the invertebrate assemblage of 

these reaches is dominated by EPT taxa, including 16 genera 

of mayflies, 7 genera of stoneflies, and 7 genera of caddisflies (Haden et al. 2003). Tributaries in the Grand Canyon 

10 BioScience • XXXX XXXX / Vol. XX No. X http://bioscience.oxfordjournals.org 
Overview Articles
 

Figure 7. Noninsects dominate below hydropeaking dams. A photo collage showing genus-level invertebrate richness for 

three well-studied Western US rivers. The Green River below the Fontenelle Dam (left) has a low hydropeaking index 

value, and invertebrate assemblages comprise 54 unique genera, including many insects. The Green River below the 

Flaming Gorge Dam (middle) has a moderate hydropeaking index value and contains 47 unique invertebrate genera. The 

Colorado River below the Glen Canyon Dam (right) has a high hydropeaking index value and supports only 12 unique 

invertebrate genera, most of which are noninsects. See supplemental table S4 for a list of genera. 

segment of the Colorado River between the Glen Canyon these regulated segments of the mainstem Colorado River 

and Hoover Dams also support diverse invertebrate assem- and that some stressors exist that prevent these insect groups 

blages that include many species of mayflies, stoneflies, from recolonizing the river. 

and caddisflies (Oberlin et al. 1999, Whiting et al. 2014). It Decreases in river temperature due to cold, stenothertherefore seems probable that EPT taxa were extirpated from mic releases from large dams are often considered to be 

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Overview Articles
 

the dominant driver of invertebrate community assembly 

in tailwaters (Olden and Naiman 2010), including for the 

Colorado River ecosystem downstream of the Glen Canyon 

Dam (Stevens et al. 1997). However, this thermal-alteration 

hypothesis alone cannot adequately explain why taxa from 

cool-water tributaries do not colonize these hydropeaking 

river segments. In the Colorado River in the Grand Canyon, 

for example, the Tapeats Creek inflow closely approximates 

temperatures in the mainstem (see supplemental figure S2). 

In spite of this thermal similarity, the 11 EPT taxa present in 

the Tapeats Creek (Oberlin et al. 1999) have not colonized 

the mainstem in the decades since completion of the Glen 

Canyon Dam. However, the Tapeats Creek also happens to 

be located at river kilometer 240, situating it at a segment of 

the mainstem river where daily flows are elevated at dusk 

(figure 5), thereby making it an unlikely source of colonists 

from the perspective of hydropeaking wave-induced egg 

mortality. 

Using multiple lines of evidence, we have demonstrated 

that hydropeaking represents a significant stressor that is 

capable of constraining both the abundance and diversity 

of aquatic insects via egg mortality. Although interactions 

among stressors have undoubtedly played a role in extirpating aquatic insects from these segments (e.g., flow and 

temperature; Olden and Naiman 2010, Statzner and Beche 

2010), managers have limited ability to mitigate many stressors. For example, the high cost (more than $200 million) and 

risks (i.e., invasion by warm-water nonnatives fishes) associated with the installation of a temperature-control device on 

the Glen Canyon Dam render the restoration of Colorado 

River temperature regimes improbable. Each of the stressors 

that large dams impose on rivers likely represents an environmental filter that limits invertebrate populations to some 

extent (Poff 1997, Statzner and Beche 2010), and cumulatively, all of the stressors that the Glen Canyon and Hoover 

Dams impose on the Colorado River are responsible for the 

extremely low insect diversity and productivity conditions 

we observed. Nonetheless, identifying the specific lifehistory bottleneck that hydropeaking imposes on aquatic 

insects represents an important step toward understanding how to potentially improve the health of invertebrate 

assemblages downstream of hydropeaking dams along the 

Colorado River and throughout the world. 

Ecosystem consequences of biodiversity loss 

and a path forward 

Although some may argue about the extent to which aquatic 

insects have value or importance in their own right, in practical terms, unhealthy aquatic invertebrate assemblages do 

have tangible negative impacts on wildlife populations that 

are universally valued by society (figure 8). For instance, 

recent food-web and bioenergetics studies demonstrate that 

in the popular Lees Ferry sport fishery downstream of the 

Glen Canyon Dam, the maximum size and growth of rainbow trout are limited by the abundance and overall small 

size of their invertebrate prey (Cross et al. 2011, Dodrill et al. 

2016). In addition, the simple food-web architecture in Lees 

Ferry results in rainbow trout populations that are inherently unstable and not resistant to controlled flood experiments that are released from the Dam (Cross et  al. 2013). 

Farther downstream in the Grand Canyon, food-web studies 

indicate that the abundance and distribution of native fishes, 

including endangered humpback chub (Gila cypha), are also 

limited by the availability of insect prey; native fish populations consume virtually all of the available annual midge and 

blackfly production (Cross et al. 2013). 

River managers are increasingly conducting environmental-flow experiments that mimic key aspects of natural flow 

regimes to rehabilitate river ecosystems or achieve specific 

ecological outcomes (Acreman et al. 2014, Olden et al. 2014). 

Incorporating knowledge of life stage–specific requirements 

for fish is an axiomatic component of such environmental 

flows. For instance, controlled floods are released from many 

dams to clean gravels of fine sediment and improve spawning habitat for salmonids (Olden et al. 2014). Similarly, river 

managers are designing environmental flows that enhance 

the flow conditions experienced by insect larvae. However, 

because the importance of recruitment dynamics to insect 

populations has only recently been demonstrated (Encalada 

and Peckarsky 2012, Lancaster and Downes 2014, Macqueen 

and Downes 2015), the flow conditions necessary to sustain 

other life stages have not yet been considered. Furthermore, 

implementing environmental flows is challenging in highly 

managed rivers because of the juxtaposition of uncertain 

environmental benefits against the readily quantified economic losses associated with reduced hydroelectricity generation (Jacobson and Galat 2008). 

Our study informs the design of cost-effective environmental flows that should enhance biodiversity and 

ecosystem function downstream of hydropower dams on 

the Colorado River and elsewhere. Specifically, our study 

suggests that reducing hourly discharge variation during 

periods of peak aquatic-insect egg laying should alleviate 

the life-history bottleneck that arises from hydropeaking 

operations. Aquatic insects can be extremely fecund, with a 

single female laying hundreds to thousands of eggs (Merritt 

et al. 2008, Statzner and Beche 2010). Therefore, alleviating 

the life-history bottleneck that hydropeaking imposes on 

aquatic insects may not require long blocks of stable flow 

releases that completely preclude hydropeaking. For example, hydropeaking is less lucrative on weekends when electricity demands are low (Førsund 2015), so a cost-effective 

environmental flow could involve releasing stable and low 

flows every weekend during periods of peak aquatic-insect 

egg laying. Releasing low flows every weekend would ensure 

that eggs laid on weekends remained wetted and were never 

subject to desiccation prior to hatching, which typically 

occurs after days to weeks of incubation (Merritt et al. 2008, 

Statzner and Beche 2010). This type of environmental flow 

would minimally affect hydroelectricity revenues (USBR 

2016) while substantially improving the quality of substrates available for aquatic-insect egg laying and rearing. 

12 BioScience • XXXX XXXX / Vol. XX No. X http://bioscience.oxfordjournals.org 
Overview Articles
 

Figure 8. Aquatic insects play an essential role in river and riparian food webs. Aquatic insects are ubiquitous in 

freshwaters and are the primary prey for myriad species of wildlife living in and along rivers. These insects have complex 

life cycles that include a terrestrial winged adult life stage, whereas egg, larval, and pupal stages are aquatic. Ecologically 

important insect groups such as mayflies, stoneflies, and caddisflies cement their eggs along river-edge habitats, making 

them especially sensitive to dam water management practices such as hydropeaking that affect these edge habitats. 

Environmental flows such as this may allow highly managed 

river ecosystems to continue meeting societal needs for 

renewable hydroelectricity while ensuring the sustainable 

provisioning of critical ecosystem services that are provided 

by the world’s rivers. 

Acknowledgments 

This article is dedicated to the professional river guides, 

private boaters, and organizations that participated in 

or facilitated the citizen-science light trapping project: 

Arizona Raft Adventures, Arizona River Runners, Arizona 

Game and Fish Department, Eric Baade, Matt Bryan, 

Riley Burch, Kirk Burnett, Canyon Expeditions, Canyon 

REO, Canyoneers, David Cassidy, Ceiba Adventures, 

Laura Chamberlain, Colorado River and Trail Expeditions, 

Jerry Cox, Bob Dye, Grand Canyon Expeditions, Grand 

Canyon National Park, Grand Canyon River Guides, Grand 

Canyon Youth, Amy Harmon, Hatch River Expeditions, Scott 

Jernigan, Ethan Johnson, Walker Mackay, Kelly McGrath, 

Moenkopi Riverworks, Ariel Neill, Outdoors Unlimited, 

Marc Perkins, Wade Permar, Katie Proctor, Professional 

River Outfitters, Matt Robinson, Orea Roussis, Gibney 

Siemion, Connie Tibbitts, John Toner, Jamie Townsend, 

Tour West, Kelly Wagner, Wilderness River Adventures, 

Kelly Williams, Kelsey Wogan, Kiki Wykstra, and Glade 

Zarn. We thank Lynn Hamilton of Grand Canyon River 

Guides and Emma Wharton of Grand Canyon Youth for 

their ongoing support and engagement. We thank Daren 

Carlisle (US Geological Survey), Mark Nelson (Bureau of 

Reclamation), Raymond Perkins (Oregon Department of 

Fish and Wildlife), Andrew Welch (PPL Montana), and 

Marc Wethington (New Mexico Department of Game and 

Fish) for providing invertebrate data used in the regional 

analysis. Jeremy Monroe of Freshwaters Illustrated developed 

the conceptual model (figure 8). We thank Matt Schroer and 

Jesse Fleri for conducting the egg desiccation experiments. 

http://bioscience.oxfordjournals.org XXXX XXXX / Vol. XX No. X • BioScience 13 
 

Overview Articles
 

TAK thanks Jack Schmidt for the opportunity to participate 

in the 2011 Lodore Canyon river trip that inspired this 

project. Funding for this study was provided by the Bureau 

of Reclamation’s Glen Canyon Dam Adaptive Management 

Program, the US Geological Survey’s Southwest Biological 

Science Center, and the Department of Energy’s Western 

Area Power Administration. Finally, we are grateful for 

comments from Shahid Naeem, Sarah Hobbie, Jacques 

Finlay, Jane Lubchenco, Daren Carlisle, Brendan McKie, 

and Jack Schmidt on earlier drafts that greatly improved this 

manuscript. Any use of trade, product, or firm names is for 

descriptive purposes only and does not imply endorsement 

by the US government. 

Supplemental material 

The supplemental material is available online at http:// 

bioscience.oxfordjournals.org/lookup/suppl/doi:10.1093/biosci/ 

biw059/-/DC1 . Data associated with this article are available 

at http://dx.doi.org/10.5066/F7WM1BH4. 

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Theodore A. Kennedy (tkennedy@usgs.gov), Jeffrey D. Muehlbauer, 

Charles B. Yackulic, Kimberly L. Dibble, Eric W. Kortenhoeven, and 

Anya N. Metcalfe are affiliated with the US Geological Survey, Southwest 

Biological Science Center, Grand Canyon Monitoring and Research Center, 

in Flagstaff, Arizona. David A. Lytle is with the Department of Integrative 

Biology at Oregon State University, in Corvallis. Scott W. Miller is affiliated 

with the Department of Watershed Sciences at Utah State University, in 

Logan. Colden V. Baxter is with the Department of Biology at Idaho State 

University, in Pocatello. 

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