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Articles
948 BioScience • December 2011 / Vol. 61 No. 12 www.biosciencemag.org
Articles
Large-scale Flow Experiments for
Managing River Systems
Christopher p. Konrad, Julian d. olden, david a. lytle, theodore s. Melis, John C. sChMidt,
erin n. Bray, Mary C. FreeMan, Keith B. Gido, nina p. heMphill, MarK J. Kennard, laura e.
M
c Mullen, Meryl C. MiMs, MarK pyron, Christopher t. roBinson, and John G. WilliaMs
Experimental manipulations of streamflow have been used globally in recent decades to mitigate the impacts of dam operations on river systems.
Rivers are challenging subjects for experimentation, because they are open systems that cannot be isolated from their social context. We identify
principles to address the challenges of conducting effective large-scale flow experiments. Flow experiments have both scientific and social value
when they help to resolve specific questions about the ecological action of flow with a clear nexus to water policies and decisions. Water managers
must integrate new information into operating policies for large-scale experiments to be effective. Modeling and monitoring can be integrated
with experiments to analyze long-term ecological responses. Experimental design should include spatially extensive observations and well-defined,
repeated treatments. Large-scale flow manipulations are only a part of dam operations that affect river systems. Scientists can ensure that experi-
mental manipulations continue to be a valuable approach for the scientifically based management of river systems.
Keywords: rivers, flow experiments, dams, ecosystem management
biological conditions in these systems may not be attrib-
uted solely to the level of streamflow during the experi-
ment. Unlike experiments on land, lakes, and small streams
in experimental watersheds, flow manipulations involving
whole rivers or estuaries can rarely, if ever, be isolated from
their social context. Stakeholders have diverse interests in
how water is used, and water managers operate facilities and
systems to achieve multiple objectives. The overarching issue
for scientists involved in large-scale flow experiments, then,
is to design scientifically credible and tractable investigations
that simultaneously inform water management about poli-
cies to achieve long-term objectives.
We review the global practice of flow manipulations in
rivers as large-scale experiments to guide future efforts in
this burgeoning area of interest using examples from over
40 systems (see the supplementary material, available online
at http://dx.doi.org/10.1525/bio.2011.61.12.5). We focus on
flow manipulations intended to achieve ecological objectives
because of their direct relevance for informing dam opera-
tions but recognize that investigations of natural flow events
and manipulations not intended for ecological outcomes
provide useful information for managing rivers and advanc-
ing river ecology. We identify how flow experiments have
elucidated and addressed facets of the complexity in river,
floodplain, and estuary ecosystems. These examples lead
us to a core set of challenges and principles for conducting
effective large-scale flow experiments that have both scien-
tific and social value.
M
anagement of water resources substantially alters
hydrologic regimes in freshwater and estuarine ecosys-
tems (Poff et al. 1997, Nilsson et al. 2005). Around the world,
changing societal values have compelled the modification of
dam operations and water diversions to mitigate physical
and biological impacts on aquatic systems (Williams RD
and Winget 1979, Travnichek et al. 1995, King JM et al. 1998,
Toth et al. 1998, Polet 2000, Rood et al. 2003, Hamerlynck
et al. 2005, Hall et al. 2011). Scientists have advocated for an
experimental framework to evaluate and develop operations
that provide ecological benefits, to create a more rational
basis for water-management decisions, and to advance
broader scientific knowledge (Walters et al. 1992, Poff
et al. 2003, Souchon et al. 2008). Indeed, flow experiments
(figure 1) have been used globally to evaluate the effects of
alternative dam operations on rivers, floodplains, and estu-
aries (Cambray et al. 1997, Bate and Adams 2000, Siebentritt
et al. 2004, Decker et al. 2008, Robinson and Uehlinger 2008,
Shafroth et al. 2010, King AJ et al. 2010, Schmidt and Grams
2011).
Rivers, floodplains, and estuaries are particularly chal-
lenging subjects for large-scale flow experiments, because
they are open systems with strong network connectivity,
spatial heterogeneity, and temporal variability arising in
large part from the action of streamflow. The influence of
streamflow on aquatic and riparian systems is not limited
to an experimental period but extends before and after
any experiment. Furthermore, the observed physical and
BioScience 61: 948–959. ISSN 0006-3568, electronic ISSN 1525-3244. © 2011 by American Institute of Biological Sciences. All rights reserved. Request
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reprintinfo.asp. doi:10.1525/bio.2011.61.12.5
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A taxonomy of flow experiments
A large-scale flow experiment can be defined broadly as field
observations and analysis used to test a hypothesis about
physical and biological responses to streamflow in a river,
floodplain, or estuary. Flow experiments are performed
over a defined period, with distinct streamflow character-
istics (the treatment) and observations or measurements of
physical or biological responses. Generally, the experimental
period encompasses a discrete event, such as a high-flow
pulse (Wilcock et al. 1996, Polet 2000, Henson et al. 2007),
reservoir drawdown (Moore et al. 2010), or other specified
flow (Bureau of Reclamation 2002), although experiments
can span longer-term step changes in dam operations that
increase minimum flow (Travnichek et al. 1995, Bednarek
and Hart 2005), reduce diurnal flow fluctuations (Connor
and Pflug 2004, Patterson and Smokorowski 2010), or restore
flow to bypassed reaches (Decker et al. 2008). Hypothesis
testing in large-scale experiments can be composed of either
a formal test of a predicted response based on observation or
the estimation of model parameters that relate responses to
treatments. Treatments in large-scale flow experiments are
not isolated to portions of a river, floodplain, or an estuary
and, in this way, differ from plot-scale field experiments or
mesocosms.
These scientific criteria for an experiment contrast with
both of the following common perceptions: that any
management action with an uncertain outcome is an
experiment and that an action
with predictable outcomes is
not an experiment. In our view,
the certainty of responses is
not central to whether a flow
manipulation is an experi-
ment, although it does bear on
whether an experiment is valu-
able and worth conducting.
Mensurative versus manipulative
experiments.
Following Hurl-
burt’s (1984) dichotomy of field
experiments, flow experiments
can be considered either mensu-
rative or manipulative, depen-
ding on how the treatment is
applied. In a mensurative flow
experiment, investigators do
not specify streamflow but, in-
stead, collect information about
ecosystem responses to stream-
flow observed over a defined
time period. In a manipulative
flow experiment, dam opera-
tion, diver sions, or ground-
water pumping are changed
during a defined time period in
order to modify streamflow in
a river, floodplain, or estuary while physical and biological
responses are observed.
Although Hairston (1989) argued against applying the
term experiment to observations of ecological responses
to natural events, mensurative investigations of flow can
be conducted in an experimental framework designed to
complement manipulative experiments (Kinsolving and
Bain 1993, Rood et al. 2003). Mensurative investigations
can incorporate events such as extended low flows or large
floods occurring outside the range of possible flow manipu-
lations and offer an opportunity for observing responses
to flow under different conditions (e.g., water tempera-
ture, turbidity) than would occur under an experimental
manipulation. Mensurative experiments may be most use-
ful in the initial stages of hypothesis testing (e.g., Do high
flows reduce macro phyte biomass?) and the development
of conceptual models, but they generally cannot resolve
questions with the level of precision and certainty needed
by water managers (e.g., Is there a threshold flow to scour
macrophytes? Will a longer duration flow result in lower
macrophyte cover?).
Manipulative flow experiments allow for more explicit
design than do mensurative investigations (Hairston 1989),
with stronger causal links between specific flow characteris-
tics and ecological responses. In manipulative flow experi-
ments, water can be released repeatedly with a specified rate,
duration, and timing (figure 2a, 2b), which thereby allows
Figure 1. (a) High-flow experiment at Glen Canyon Dam, Colorado River, in 2008.
(b) Flood pulse in the Bill Williams River, in Arizona, showing a major breach (more
than 3 meters wide) in a beaver dam after an experimental release from Alamo Dam
in 2007. The River Spöl below the dam at Punt dal Gall (Swiss National Park) (c) prior
to and (d) during an experimental flood release in 2000. Photographs: Thomas Ross
Reeve, Bureau of Reclamation (a); Patrick Shafroth (b); and Urs Uehlinger (c, d).
950 BioScience • December 2011 / Vol. 61 No. 12 www.biosciencemag.org
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the analysis of fixed treatments in a factorial design to dis-
entangle the effects of streamflow from other factors if those
factors can be controlled. Releases can be varied deliberately
and precisely in order to resolve the range and sensitivity of
responses to streamflow characteristics (e.g., Andersen and
Shafroth 2010, Schmidt and Grams 2011) and are repeated
in order to assess the influence of initial conditions (e.g.,
Robinson and Uehlinger 2008). The scheduling of manipu-
lations permits logistically complex data collection and the
coordination of interdisciplinary investigations (Wilcock
et al. 1996, Shafroth et al. 2010). The advantages of mani-
pulative experiments cannot be realized, however, without
recognizing and addressing common challenges.
Many large-scale experiments are hybrids. In practice, large-
scale flow experiments often have both mensurative and
manipulative elements. Water availability for experiments
in arid regions depends on recent precipitation where
manipulations may coincide with or supplement natural
events such as for the Kromme Estuary, South Africa (Stry-
dom and Whitfield 2000); Lake Ichkeul, Tunisia (Smart
2004); and the Murray-Darling, Australia (Siebentritt et al.
2004, King AJ et al. 2010). For longer-term experiments, pre-
scribed dam releases vary among wet, normal, and dry years,
such as in the Trinity River, in California (figure 2d), and
Klamath River, in Oregon, which leads to flow treatments
that are highly correlated with natural tributary inflows
(USFWS and Hoopa Valley Tribe 1999).
Most geophysical and phenological conditions are largely
uncontrolled even in manipulative experiments, including
the streamflow before and after the manipulations (fig-
ure 2b). The scheduling of experiments can be used to con-
trol some factors. For example, high-flow experiments on the
Colorado River in the Grand Canyon in 2004 and 2008 were
conducted after sediment inputs from the unregulated Paria
River (Schmidt and Grams 2011). Likewise, an unregulated
tributary downstream of a dam influences responses and
can effectively turn a manipulative experiment into one that
involves a hybrid of regulated and unregulated flows, as in
the Green River below its confluence with the Yampa River
in Utah (Vinson 2001). Removal of dikes from estuaries and
levee setbacks, such as along the Nueces Delta, Texas, rep-
resent another type of hybrid experiment, in which flow is
manipulated through structural modifications but in which
the subsequent characteristics of flow to the system are not
under experimental control (Montagna et al. 2002).
Challenges in large-scale flow experiments
Classical experimentation requires the testing of alternative
hypotheses; documentation of initial conditions; sufficient
Figure 2. Hydrographs of (a) the daily mean streamflow for the Bill Williams River, in Arizona; (b) the daily mean
streamflow for the Savannah River, in Georgia and South Carolina; (c) the daily mean streamflow for the Susquehanna
River, in Maryland; and (d) the monthly (light line) and annual (dark line) mean streamflow for the Trinity River, in
California.
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observations to characterize responses; and the randomized
assignment of replicated treatments and untreated controls,
including the blocking of influential factors (Hairston 1989).
Flow manipulations face challenges in meeting these and
other experimental-design requirements common to large-
scale ecological experiments (Hurlbert 1984, Carpenter
1989). River systems have structure in space and time, which
compels extensive data collection at multiple time scales
to characterize the response. The structures are unique to
each system, which limits the value of using other systems
as replicates and controls. Flow can act along multiple path-
ways, influencing hydraulic conditions, sediment transport,
thermal regime, and other physical properties of water in
ecosystems that complicate the analysis of discrete, testable
hypotheses (e.g., Vinson 2001, Bednarek and Hart 2005,
Olden and Naiman 2010). Water-management issues pres-
ent another layer of challenges in terms of goals for large-
scale experiments and also introduce confounding factors
that must be addressed in analyses (Poff et al. 1997).
We identify five challenges in particular that must be
addressed for large-scale flow experiments to become an
effective part of river management (also see box 1). Aspects
of these challenges are common issues for river ecology in
general, so experiments that are successful in meeting these
challenges advance science, in addition to fulfilling manage-
ment needs.
Challenge 1. Large-scale flow manipulations are management
actions inseparable from their social context.
Manipulative
flow experiments are inseparable from their social context,
which includes competing uses for a public resources,
concerns about at-risk species, and impacts on human
activities in river ecosystems (Bureau of Reclamation 2002,
Hamerlynck and Duvail 2003, Jacobson and Galat 2008).
Scientists alone do not decide how much water to release
from a reservoir for an experiment (Hamerlynck et al. 2005,
Lind et al. 2007), even though they may influence those who
do make these decisions (Travnichek et al. 1995, Bate and
Adams 2000, Watts et al. 2010, Schmidt and Grams 2011).
In practice, the decisions of how much water to release and
when to release it reflect many constraints, including the
availability of water, the capacity of the hydraulic struc-
tures used to release flow, and risks to water users and
downstream residents. Stakeholder interests range from
maintaining the status quo to reshaping water resources
management, either of which may be supported or under-
mined by new information gained through experiments. As
a result of stakeholders’ interests and the resources required
for planning, implementing, and monitoring flow manipu-
lations, these types of experiments are perceived as high-
risk management actions.
Outcomes measured in terms of socially valued resources
are the principal motivation for water managers to manipu-
late flows and may be the only acceptable justification of the
costs and risks of such actions. Water managers may seek
to limit manipulations if they compromise other system
objectives (e.g., water delivery downstream for municipal
and agricultural use, management of tail-water fisheries,
providing recreational opportunities), even if the treat-
ment’s strength is reduced to a point that the results are
inconclusive. Failure to develop sufficient contrast between
the pretreatment period (or control system) and the treat-
ment period can lead to uninformative results, as well as to
the failure to achieve management objectives (box 2; Toth
et al. 1998, Strydom and Whitfield 2000, Webb et al. 2010).
Walters (1997) aptly noted the false economies for long-term
resource management of scaling back experiments to reduce
costs: A scaled-back or post hoc test may be inconclusive
Box 1. Challenges for large-scale flow experiments.
Flow manipulations are management actions intended to achieve outcomes rather than to provide learning opportunities. Because
large-scale flow experiments are not physically isolated from society, they cannot be designed with consideration only of scientific
issues. Flow manipulations are intended primarily to achieve ecological outcomes. Broad investigations driven by resource objectives
may not be conclusive or may not readily inform water-management decisions.
Experimental treatments and responses span multiple time scales. Treatments and other factors contributing to ecological responses
are difficult to control over long time periods and are difficult to repeat. Ecological responses exhibit varying time lags, including
legacy effects from past events, human impacts, and cumulative effects under serial treatments, which influence outcomes. Responses
integrate streamflow over time and may include the influence of nonexperimental operations.
Large-scale flow experiments are embedded in river networks. Treatments attenuate downstream and can also be modified through
interactions with in-channel responses. Tributaries contribute flow and sediment, change the physical properties of flow, and are a
source of colonists’ seeding for biological recovery.
Regulated systems are impacted by temperature and sediment regime shifts. The colonization, migration, and transport of
propagules or gametes may continue to be limited, despite flow manipulations. The influences of flow and other factors are difficult to
isolate with interacting and interdependent management interventions.
Flow affects different taxa through distinct mechanisms. The responses of some taxa may be mediated by competition and preda-
tion rather than directly through disturbance and habitat effects. Variable life histories (flow-dependent life stages, long-lived species,
reproduction, migration) result in taxa-specific responses. Invasive (nonnative, nuisance, upland) species may respond in a manner
different from that of native taxa or may modulate native taxa responses.
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(Bednarek and Hart 2005) or even misleading when it lacks
statistical power (Carpenter 1989).
Challenge 2. Experimental treatments and responses span mul-
tiple time scales.
Treatments in large-scale flow experiments
range from discrete events (figure 2a, 2b) that may last
for a few days or months (King JM et al. 1998, Bate and
Adam 2000, Smart 2004) to a series of events (Montagna
et al. 2002, Robinson and Uehlinger 2008) to revisions in
operating policies that modify releases for years (figure
2c, 2d; Weisberg and Burton 1993, Travnichek et al. 1995,
Connor and Pflug 2004, Decker et al. 2008). Discrete
manipulations generally fit within an experimental frame-
work comprising clearly defined treatments and responses
that can be attributed to the treatment with a high level of
certainty (Cambray et al. 1997, Siebentritt et al. 2004, Rolls
and Wilson 2010, Shafroth et al. 2010).
By contrast, changes in operating policies that affect res-
ervoir releases over years often do not present well-defined
treatments, because those policies may not be prescriptive,
which results in different release patterns depending, for
example, on whether the year is wet or dry (USFWS and
Hoopa Valley Tribe 1999, Konrad et al. 2011). Actual releases
under revised operating policies depend on natural inflows
and other uses of water. In cases in which flow is reintro-
duced to a dewatered reach or in which minimum flows are
increased substantially, changes in operating policies can
be investigated in an experimental framework but only in
highly regulated systems (Travnichek et al. 1995, Connor
and Pflug 2004, Hall et al. 2011).
Evaluating responses over long time scales is challeng-
ing for all types of flow experiments. Some responses to a
discrete manipulation extend over a long period and, as a
result, are influenced by subsequent operations or experi-
mental releases (figure 2b, 2d), which confound the attribu-
tion of observed responses to any specific flow characteristic
(Korman et al. 2011). For example, sandbars and backwater
habitats in the Grand Canyon that formed during high-flow
pulses on the Colorado River were eroded to a significant
extent in subsequent months under elevated discharges from
Glen Canyon Dam (Schmidt and Grams 2011). In other
cases, an experimental design may require years before any
effects can be demonstrated (Souchon et al. 2008).
Because organisms depend on various flows over their
life cycle and because the effects of flow are integrated with
other factors over time, longer-term responses in popula-
tions cannot commonly be attributed to a single isolated
manipulation of flow. Robinson and Uehlinger (2008)
demonstrated a shift in invertebrate community structure
in the River Spöl, in Switzerland, that only manifested after
years of repeated high-flow pulses. Rood and colleagues
(2003) described the sequence of flows over a period of
years needed to establish cottonwoods on the Truckee River,
in Nevada. Similarly, fish reproduction depends not only on
flows to induce spawning but also on flows during embryo
incubation and larval development (Strydom and Whitfield
2000, Connor and Pflug 2004).
Challenge 3. Large-scale flow experiments are embedded in river
networks exhibiting strong longitudinal and lateral connectivity.
Rivers are open systems in which sites are embedded in
networks, often with strong longitudinal and lateral con-
nectivity. Network structure and the lack of synchronous
responses in networks (figure 3) cannot be easily character-
ized by localized investigations or with randomized spatial
sampling. Sites along a river with similar local attributes
(e.g., gradient, valley confinement, bank vegetation) may
not be comparable as untreated sites for controls or may
display heterogeneous responses to flow manipulations
because of differences in their network positions. Therefore,
the responses to flow manipulations should be expected to
vary, depending on tributary inflow, valley characteristics
Box 2. Reconciling management objectives in large-scale flow experiments.
Ecological outcomes depend on strong manipulations and dam operations that continue after experiments. In Kromme Estuary, South
Africa, freshwater releases were not sufficient (low magnitude and duration) to increase larval fish abundance (Strydom and Whitfield
2000). In the Colorado River, in the United States, sandbars created by high flows were eroded by fluctuating flows for hydropower
peaking (Schmidt and Grams 2011). In the Mitta Mitta River, in Australia, under steady flows, nuisance algae recovered weeks after
scouring by high flows (Watts et al. 2010).
Repeated, frequent manipulations may be required to re-establish and maintain a community or system. In the Bill Williams River,
in Arizona, high flows flushed out beaver dams and restored lotic habitats, but beavers rebuild dams and re-establish lentic habitats
until the next high-flow event (Andersen and Shafroth 2010). In the River Spöl, in Switzerland, the abundances of blackflies, chiro-
nomids, stoneflies, and caddisflies increased in response to a series of high flows, which represents a shift in the invertebrate assemblage
and an increase in its resiliency to flood disturbance (Robinson and Uehlinger 2008).
Achieving resource objectives depends on flow management over the life cycle of targeted taxa. In the Skagit River, in the state of
Washington, successful salmon reproduction depended on flows to cue spawning that were maintained throughout the incubation
period (Connor and Pflug 2006). In the Truckee River, in California and Nevada, cottonwood regeneration depended on high flows
for dispersal and germination followed by sufficient base flows to allow seedling establishment without high flows that would scour
young vegetation (Rood et al. 2003).
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Challenge 4: Serial discontinuity and multiple limiting factors
compromise large-scale flow experiments.
Flow manipulations
alone may not provide an adequate treatment when other as-
pects of serial discontinuity, including fragmentation of lotic
habitat, changes in thermal regime, reduction in sediment
supply and transport, and disruption in the migration and
transport of biota, are involved (box 4). As a result, aquatic
and riparian communities in regulated systems may not
respond to flow manipulation in ways analogous to responses
of unregulated systems to flow. Reduced hydrochory (the
water-borne supply of seeds, gametes, or propagules) is a
pervasive legacy of dams and diking that has limited the
efficacy of flow manipulations in restoring marsh vegeta-
tion and populations of fishes and mussels (Toth et al. 1998,
Siebentritt et al. 2004, Moles and Layzer 2008). Water
temperature affects mortality, growth, survival, migration
timing, and other behaviors in aquatic organisms and has
been a significant factor in large-scale flow experiments
around the world (Olden and Naiman 2010). Flow manipu-
lations in the Olifants River, in South Africa, and the Savan-
nah River, in Georgia, failed to promote fish migration and
spawning, because cold, hypolimentic water was released
(King JM et al. 1998, Konrad et al. 2011). Vinson (2001)
noted that despite modifications of Flaming Gorge Dam
to increase the temperature of releases to the Green River,
the water was not warm enough (i.e., the treatment was not
such as gradient and channel confinement, and in-channel
processes (box 3).
After flow is released from a dam as an experimental
manipulation, its influence will attenuate downstream
(figure 3; Travnichek et al. 1995, Connor and Pflug 2004).
En route, however, there may be a variety of changes that
occur. Releases from Camanche Dam on the Mokelumne
River, in California, in 2003 mobilized particulates, dissolved
nutrients, and bacteria, leading to variable water-quality
responses as the pulse translated downstream from the dam
(Henson et al. 2007).
Tributaries can have a strong influence on the results of
flow experiments, contributing biota, water with distinct
physical properties (e.g., temperature), and sediment. The
unregulated Geelhoutboom Tributary served as a source
of larval fish to the Kromme estuary during a high-flow
pulse (Strydom and Whitfield 2000). Invertebrate richness
in the Green River, in Utah, increased in a reach 18 kilo-
meters (km) downstream from Flaming Gorge Dam in
response to warmer-water releases (1978–1999) but not
in a reach immediately downstream of the dam (Vinson
2001). Vinson (2001) suggested that the dominance of
amphipods in the upstream benthic community and the
lack of colonists provided by downstream tributaries sup-
pressed the response of native invertebrates immediately
below the dam.
Box 3. Influences of connectivity on ecological responses to flow manipulations.
Flow treatments attenuate downstream of dams. In the Tallapoosa River, in Georgia, recovery of the warm-water fish assemblage was
limited 37 kilometers (km) downstream of the Thurlow Dam (Travnichek et al. 1995). In the Colorado River, during a high flow in
2008, sandbars were built about 160 km downstream of the Glen Canyon Dam but eroded further downstream (Schmidt and Grams
2011).
Responses to flow manipulations are influenced by the interaction of treatments with in-channel responses and by tributaries. In
the Mokelumne River, in California, the river channel was a source of particulate materials, dissolved nutrients, and bacteria that modi-
fied downstream water quality of a high-flow pulse (Henson et al. 2007). In the Green River, in Utah, Red Creek is a source of warmer
water, turbidity, and aquatic insects (Vinson 2001). In Kromme Estuary, South Africa, an unregulated tributary provided a refuge for
some estuary fish species (Strydom and Whitfield 2000).
Figure 3. River network with distinct system states in different locations (a) preexperiment, (b) during flow manipulation,
and (c) during a natural event.
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sufficiently strong) to elicit invertebrate responses until it
mixed with warmer inflows from downstream tributaries.
Resource managers may implement multiple interven-
tions (e.g., sequential flow manipulations, invasive species
removal, sediment augmentation, or water temperature
control) to increase the likelihood of achieving resource
goals under multiple stressors (Vinson 2001, Korman et al.
2011). Simultaneous or sequential treatments, however,
limit the potential to isolate effects and to attribute them
to specific flow characteristics. Bednarak and Hart (2005)
noted the difficulty in distinguishing the effects of flow
and dissolved-oxygen treatments for dams operated by the
Tennessee Valley Authority when those interventions were
applied as sequential step changes within a few years of each
other but without adequate time to document responses to
flow alone.
Challenge 5. Reconciling diverse taxonomic responses to large-
scale flow experiments.
The responses of different taxa to
streamflow reflect their habitat preferences, life histories,
competitive and trophic interactions with other taxa
(competition, predation), the degree to which species
are evolutionarily adapted to flow characteristics, and
the initial conditions of their populations prior to flow
manipulations (Toth et al. 1998, Propst and Gido 2004,
Shafroth et al. 2010). In the Tallapoosa River, in Alabama,
Travnichek and colleagues (1995) demonstrated increased
species richness and abundance of fluvial-specialist fishes
relative to habitat generalists in response to increased
minimum flows. A. J. King and colleagues (2010) observed
benefits to native fishes from managed high flows in the
Murray River, Australia. Different taxa responded through
distinct mechanisms: Some fishes increased spawning,
whereas others had high survivorship of larvae. Taxa-
specific responses may be mediated by habitat preferences
and availability (e.g., Propst and Gido 2004, Connor and
Pflug 2004, King AJ et al. 2010, Shafroth et al. 2010), or
they may be a result of trophic changes initiated by flow
manipulations (Weisberg and Burton 1993, Korman et al.
2011). Divergent taxonomic responses lead to community
shifts that, in turn, can affect community responses to sub-
sequent flow manipulations.
Life-history differences among taxa are important for
assessing responses to flow manipulations: Longer-lived spe-
cies are likely to be influenced strongly by survival or mortal-
ity of individuals extant before a flow manipulation, whereas
the response of shorter-lived species may be due primarily to
changes in growth and reproduction immediately after the
manipulation. From an evolutionary perspective, floods and
droughts that are predictable over time can exert primary
selective pressures that favor life histories synchronized to
avoid or exploit extreme flow events. Extreme flows that are
frequent and large in magnitude but unpredictable have low
selection strength for life-history timing, even though they
might inflict high mortality on populations (Lytle and Poff
2004). Robinson and Uehlinger (2008) observed distinct
response times of moss, periphyton, and invertebrates to
the high flows in the River Spöl, which led to differences in
cumulative effects from a series of 15 floods on these differ-
ent parts of the aquatic community. The invertebrate assem-
blage, for example, was increasingly dominated by species
adapted to flood disturbance, which enhanced the resiliency
of the assemblage to floods over time. Flow manipulations
may have little effect where a community has shifted to
another state because of the presence of nonnative species,
such as the shrub Ludwigia peruviana, which Toth (2010)
implicated in the failure of flow manipulations to restore
broadleaf marsh along the Kissimmee River, in Florida.
Principles for enhancing the scientific and social
value of large-scale flow experiments
Large-scale flow experiments can be effective tools for
advancing scientific knowledge and resource-management
goals when they address the challenges described above. We
present five principles that have been used to address these
challenges and to serve as guidance for scientists conducting
effective large-scale flow experiments in the future.
Principle 1. Experiments are for learning. Scientists, water man-
agers, and stakeholders should understand the motivations
Box 4. Serial discontinuity and multiple covariates.
Flow manipulations do not address all ecological impacts in regulated systems. In the Olifants River, South Africa, after high flows
cued spawning by Clanwilliam yellowfish, cold, hypolimentic water prevented the successful recruitment of embryos and larvae to
juvenile stages (King et al. 1998). In the Green River, in Utah, temperature control and flow manipulations were not sufficient for the
recovery of aquatic insects immediately downstream of Flaming Gorge Dam (Vinson 2001). In the Tennessee River basin, the relative
abundance and richness of sensitive aquatic insects increased more with a combination of flow manipulation and reaeration than with
flow manipulations alone (Bednarek and Hart 2005).
Hydrochory, reproduction, and recolonization depend on proximate sources of propagules, gamete, or colonists. In the Green
River, in Kentucky, mussel fertilization is limited immediately downstream of a dam in part because of the lack of an upstream supply
of gametes (Moles and Layzer 2008). In the Kissimmee River, in Florida, reestablishment of a seven-to-nine-month hydroperiod on
a floodplain allowed the recolonization of broadleaf marsh species because of a viable seed bank and remnant propagules (Toth et al.
1998). In the Murray River, in Australia, a flood did not result in the recruitment of new aquatic vegetation, possibly because of the
lack of a seed bank, although it did promote the growth of already established vegetation (Siebentritt et al. 2004).
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range of treatment strength, and can determine initial con-
ditions and covariates so that their influence can be assessed
(King JM et al. 1998, Rood et al. 2003, King AJ et al. 2010).
These alternatives should be considered in conjunction with
large-scale experimentation in developing approaches for
investigation.
Experiments by themselves are not sufficient to solve
resource issues without a framework for using new informa-
tion in dam operations. Managers and stakeholders must be
explicit about the level of evidence or certainty required to
make decisions so that scientists can design relevant experi-
ments that provide sufficiently strong evidence. Scientific
interpretations of experiments must extend to evaluating
the differences among management options, identifying
the conditions that determine when water managers should
conduct manipulations, and recommending how future
manipulations could be more effective (Bate and Adam
2000, King AJ et al. 2010, Watts et al. 2010, Konrad et al.
2011, Schmidt and Grams 2011). In regulated systems, con-
tinuing flow manipulations are needed in order to achieve
resource objectives (Bate and Adam 2000, Siebentritt et al.
2004, Watts et al. 2010), regardless of whether those manipu-
lations are treated as experiments.
Principle 2. Modeling and monitoring can be integrated
with experiments to evaluate long-term outcomes of flow mani-
pulations.
Experiments become more difficult to control,
interpret, and repeat as they span longer time periods.
Experiments that examine flow manipulations that last
weeks or longer must deal with potentially numerous,
interacting effects of streamflow during that period (figure 2c,
2d). The causal link between any specific characteristic of
that time series (e.g., peak flow rate) and the ecological
outcomes is tenuous, because the outcomes integrate the
effects of the entire sequence of flows with other ecological
processes. In a few cases, experiments have extended over
multiple years with documentation of baseline conditions
and sufficient contrast in dam operations between the base-
line and the experimental periods to overcome climatic dif-
ferences among those periods (e.g., Robinson and Uehlinger
2008, Hall et al. 2011). Effective long-term experiments are
only possible in highly regulated systems in which the influ-
ences from confounding factors (e.g., weather patterns) can
be controlled and in which trials can be repeated.
Despite the advantages of short-term, discrete experiments
in which the mechanistic actions of flow are examined, short
trials may be inadequate to produce the predicted responses
(Strydom and Whitfield 2000). Moreover, the relevance
of such experiments to long-term resource-management
objectives, such as the recovery of viable fish populations,
increased native biodiversity, or the reestablishment of the
structure of communities with long-lived species, may not
be evident.
A challenge for scientists, then, is to design experiments
that can be scaled in order to understand the broader or
longer-term impacts. In the Columbia River, in the state of
for conducting large-scale experiments and should dis-
tinguish between what is needed for learning and what is
needed to achieve management objectives. Even though
manipulative experiments are management actions, they are
not surrogates for the operational changes needed to achieve
long-term resource-management objectives. Flow manipu-
lations that are intended principally to achieve management
objectives should not be considered experiments unless there
is an explicit design that permits learning (e.g., estimation
of model parameters, refutation of hypotheses). Even flow
manipulations that failed to achieve management objectives
have been effective experiments when they informed future
water-management decisions (King JM et al. 1998, Strydom
and Whitfield 2000, Rolls and Wilson 2010).
Scientists and water managers bear the responsibility
for justifying an experimental approach in terms of its
practical ability to help resolve uncertainty (e.g., What
management question will be informed by an experi-
ment?) and the likelihood of outcomes that will achieve
management goals weighed against the costs and risks of
proposed manipulations. Flow experiments can be effective
for answering specific questions to inform decisionmaking
in water management, including the following: Did the
expected direct response occur as a result of a flow
manipulation? Are there threshold effects that depend on
precise manipulations? Were there unintended, negative
outcomes from the manipulations? Resource objectives
simply rephrased as hypotheses, however, require years to
evaluate in many cases, and it may not even be possible to
test them experimentally.
Linkages between flow manipulations and management
objectives are numerous, indirect, and uncertain. Scientists
need to better articulate their understanding of ecological
functions involving flow, the time scales of those func-
tions, and the likely outcomes of flow manipulations. The
potential risks of the experiment to valued resources (e.g.,
endangered species, bank at risk of erosion, water quality,
property) should be acknowledged (Kondolf and Wilcock
1996, Bureau of Reclamation 2002), but they should be com-
pared with actual risks under the status quo.
Scientists have alternatives to large-scale experiments,
including mesocosm experiments, simulation models, or
mensurative investigations, that may be more efficient for
learning. Conceptual, statistical, or simulation models of
ecosystems can be used to identify the most uncertain link-
ages between management actions and resource goals that
may need to be resolved experimentally. Scientists can use
models to determine the treatment strength needed to pro-
duce measurable responses and to predict the differences
among the outcomes of possible treatments (Alexander
et al. 2006, Jacobson and Galat 2008, Schuwirth et al. 2008,
Webb et al. 2010). Mensurative investigations (Rood et al.
2003, Smart 2004) can serve as benchmarks for the potential
benefits from future flow manipulations. Combining men-
surative and manipulative approaches (figure 3b, 3c) can
increase the number of events examined, can expand the
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Replication and the randomized assignment of treatments
and controls is not a feasible approach for flow experiments
involving whole systems (figure 3) and cannot be achieved
by monitoring at multiple sites within one system or by the
repeated application of treatments to one system (Hurlbert
1984). Regression analysis (e.g., Propst and Gido 2004) and
parameter estimation (e.g., Webb et al. 2010) offer alterna-
tives to fixed-effect hypothesis testing (e.g., Lind et al. 2007)
for analyzing repeated trials (manipulations or natural
events). Maximum likelihood and Bayesian methods, in
particular, are suited for comparing the performance of
competing models or estimating model parameters given
weak contrasts and a lack of replication (Reckhow 1990).
Paired-system studies comprising a treated system and an
untreated control system, which Carpenter (1989) suggested
as a feasible alternative to large-scale ecological experiments
with replication, have not been used widely in large-scale
flow experiments (Connor and Pflug 2004, Patterson and
Smokorowski 2010), so it is difficult to assess their value rel-
ative to the simple and widely used before-and-after design.
The lack of replication is not a design flaw: Large-scale
flow manipulations are motivated primarily by site-specific
objectives rather than by the goal of increasing general
knowledge about river ecosystems. In this context, repeated
manipulations over time in a single system are useful for
understanding the influence of initial conditions and factors
other than flow (Lind et al. 2007, Robinson and Uehlinger
2008, King AJ et al. 2010, Schmidt and Grams 2011). A series
of variable manipulations can be used to assess responses as
a function of treatment strength, as in the case of the flood
characteristics needed to breach beaver dams in the Bill
Williams River, in Arizona (Andersen and Shafroth 2010).
Principle 5. Effective flow manipulations depend on other manage-
ment actions.
Long-lived aquatic and riparian organisms
integrate the legacy effects of past water management and
historical land use, multiple flow treatments, and other
(nonexperimental) managed or natural flows. Even though
experiments may be most informative when they can be iso-
lated from the confounding effects of dam operations before
and after the experiment, this type of experiment would
depict only a part of the broader dam operations affecting
river systems (Schmidt and Grams 2011). Flow manipula-
tions that achieve short-term ecological objectives (e.g.,
seedling germination, fish spawning, sediment deposition)
must be followed by subsequent flows that support the next
stage in the life history of biological targets (box 2; Rood
et al. 2003, Connor and Pflug 2004).
Resource managers can manipulate flow to target the dif-
ferential responses of taxa on the basis of their life histories
in order to suppress invasive species, although these efforts
have had mixed results. High-flow pulses promoted cot-
tonwood and willow germination and recruitment and sup-
pressed tamarisk along the Bill Williams River, in Arizona
(Shafroth et al. 2010). In the San Juan River, in Colorado and
Utah, native fish densities increased in response to elevated
Washington, tests that route water over dam spillways rather
than through turbines have produced statistically significant
decreases in the in-reservoir travel time for juvenile salmon
but only represent hours over a 3-kilometer reach of a total
journey that may take weeks and cover hundreds of kilo-
meters. The systemwide passage of fish through spillways
and spillway chutes, however, has decreased travel time by as
much as four days in recent years. In this case, experimental
evidence of the direct, short-term responses to flow manipu-
lations had to be integrated with broader monitoring and
modeling to demonstrate the significance of flow at the level
of populations (Williams JG et al. 2005).
Principle 3. Spatially explicit observations are needed to
define the spatial extent and gradients in treatments and
responses in large-scale flow experiments.
At the most
basic level, data collection in large-scale flow experiments
should define the extent of treatmaents and responses in
rivers and estuaries. Gradient analyses may be needed in
order to address the longitudinal (downstream or seaward)
variation of treatments and responses in which, for exam-
ple, mixed-effect regression models were used, rather than
analyses of variance, to integrate the results from different
sampling locations (Kinsolving and Bain 1993, Webb et al.
2010).
Spatially extensive monitoring is needed in order to
account for the influence of connectivity on experimental
results, especially with regard to tributaries (box 3). If it
is possible to do so, tributaries can be incorporated in the
experimental design in order to determine whether the
introduction of biota and sediment or the modification of
physical characteristics of water, such as temperature, influ-
ences responses to flow manipulations (Strydom and Whit-
field 2000, Vinson 2001, Schmidt and Grams 2011).
Principle 4. Experiments with well-defined treatments, repeated
over time, can isolate the ecological influences of flow.
Repeated, discrete flow manipulations will generally be
more informative for the adaptive management of dams
than will investigations limited to periodic monitoring.
Infrequent, nonsystematic, or variable manipulations with
interacting responses cannot isolate the effects of different
factors or attribute responses to specific flow characteris-
tics. Flow manipulations should be applied individually
through separate trials to rule out alternative hypotheses
without other simultaneous management interventions
(Korman et al. 2011).
Although concurrent management actions, such as
variable-elevation intakes for temperature control or sedi-
ment augmentation, are difficult to analyze, they may be the
best possible approach for improving downstream resources
(King JM et al. 1998, Bednarek and Hart 2005, Olden and
Naiman 2010). If resource managers have committed to the
implementation of multiple approaches, there may be no
need to assess the effects of flow alone, but such situations
do not constitute effective experiments.
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for the development of ecologically sustainable water-
management schemes that address shifting water supply
and demand caused by climate change and an increasing
human population. Although rigorous large-scale experi-
ments require substantial commitment by scientists, man-
agers, and stakeholders, they may offer the only practical
approach to inform water policies and decisions with the
level of certainty and precision needed for the management
of one of our most vital and increasingly scarce resources.
By recognizing the challenges and adopting the principles
demonstrated in this article for large-scale flow experiments,
scientists will ensure that such experiments continue to be a
valuable approach for the scientifically based management
of river systems.
Acknowledgments
This contribution is based on the international workshop
Evaluating Responses of Freshwater Ecosystems to Experi-
mental Water Management, funded by the National Cen-
ter for Ecological Analysis and Synthesis (NCEAS Project
12374). The manuscript was improved by comments from
Jonathan Higgins, Anne Brasher, and three anonymous
reviewers. JDO was funded by the US Environmental Pro-
tection Agency Science to Achieve Results program (Grant
No. 833834).
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