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SPECIAL ISSUE PAPER
History and review of the habitat suitability criteria curve in
applied aquatic ecology
John M. Nestler
1
|Robert T. Milhous
2
|Thomas R. Payne
3
|David L. Smith
4
1
LimnoTech, LLC, assigned to
the Environmental Laboratory, U.S. Army
Engineer Research and Development Center,
CEERD‐EP‐W, Vicksburg, Mississippi
2
U.S. Geological Survey (retired), Colorado
3
Aquatic Habitat Analysts, Inc., California
4
Environmental Laboratory, U.S. Army
Engineer Research and Development Center,
CEERD‐EP‐W, Vicksburg, Mississippi
Correspondence
J. M. Nestler, LimnoTech, LLC, Environmental
Laboratory, U.S. Army Engineer Research and
Development Center, CEERD‐EP‐W, 3909
Halls Ferry Rd., Vicksburg, MS 39180‐6199.
Email: john.m.nestler@gmail.com
Funding information
U.S. Army Engineer Research and Develop-
ment Center
Abstract
Hydraulic microhabitat assessment is a category of environmental flow tools (e.g.,
Physical Habitat Simulation system and other methodologically similar software) that,
at its core, uses habitat suitability criteria (HSC) to link values of point hydraulic var-
iables (usually depth, velocity, and substrate/cover) to habitat values for target life
stages. Although this assessment tool has been used worldwide for decades, the his-
tory of the HSC curve is relatively unknown because the foundational information is
predominantly contained in obscure and often unpublished reports. We review the
history of the HSC concept in applied aquatic ecology to clarify its scientific pedigree,
ensure its proper use, and build a foundation for future research. We begin the
review with the formative decades of the 1950's through the 1970's, when
consumptive‐based western USA water law conflicted with conservation traditions
and natural resource management objectives, although water allocation issues date
back at least to the 19th century. By analysing the history of the HSC concept, we
aim to establish the biological, hydrologic, and geomorphological conditions that must
be met for the HSC concept to be successfully employed. In spite of its documented
assumptions and limitations, the HSC concept will likely continue to be a useful tool
to help address water resources allocation issues in defined hydrologic and geomor-
phic settings. We conclude that HSC‐based methodologies should be considered as
one of several environmental flow approaches involved in sustainable water
resources management.
KEYWORDS
aquatic habitat assessment, environmental flows, flow assessment, habitat concept, habitat
requirements, habitat suitability criteria, PHABSIM, river ecology
1|INTRODUCTION
Increased global needs for water have created a conundrum: how to
balance the competing water needs of human societies and natural sys-
tems (Arthington, Naiman, McClain, & Nilsson, 2010; Bunn &
Arthington, 2002; Davies et al., 2014; Richter, 2014). In response to
this conundrum, different types of aquatic assessment tools have pro-
liferated (Tharme, 2003) to address the environmental sustainability of
alternative water management plans. One category of tools, the
ecohydraulic assessment (also referred to as hydraulic microhabitat
assessment), attempts to describe river physical habitat at a spatial
scale assumed to be consistent with the behaviour of individual aquatic
organisms, usually fish. However, algae, aquatic insects, crustaceans,
mollusks, reptiles, amphibians, and birds have also been used as target
groups (Bovee, 1997). Ecohydraulic assessment tools include the
This article has been contributed to by US Government employees and their work is in the
public domain in the USA.
Received: 3 April 2019 Revised: 26 June 2019 Accepted: 27 June 2019
DOI: 10.1002/rra.3509
River Res Applic. 2019;1–26. © 2019 John Wiley & Sons, Ltd.wileyonlinelibrary.com/journal/rra 1
Physical Habitat Simulation (PHABSIM) system (Bovee, 1978a; Bovee,
1982; Milhous, Wegner, & Waddle, 1984), the most commonly used
aquatic habitat modelling software (Arthington et al., 2007; Tharme,
2003), as well as methodologically similar systems such as RHABSIM
(Payne, 1994), RHYHABSIM (Jowett, 2004), EVHA (Ginot, 1995), RSS
(Harby et al., 1999), and most recently SEFA (Payne & Jowett, 2012).
Habitat suitability criteria (HSC; Bovee, 1986) are the core biolog-
ical component of an ecohydraulic flow assessment tool (Morhardt &
Hanson, 1988). The other components (e.g., linking subroutines that
couple independent programs into a system or algorithm that perform
tedious calculations) are either computational in nature and embody
no significant scientific issues or rely on well‐accepted methods of
hydraulic and hydrologic analysis. The definition of an HSC curve
can be derived from its primary assumption that “individuals of a spe-
cies will tend to select the most favorable conditions in a stream, but
will also use less favorable conditions within a defined range, with
the probability of use decreasing as conditions approach the end
points of the total range”(Bovee & Cochnauer, 1977, p. 2). Each point
on an HSC curve for a specific biological target translates a single
value of a selected hydraulic or substrate/cover variable on the
abscissa to a single suitability value (normalized to 1.0) on the ordinate
(Figure 1). The most important use of the HSC curve is as a compo-
nent of a broad habitat assessment that accumulates habitat values
of individual sample points into a single summary value for each dis-
charge of management interest. The resulting habitat versus discharge
relationship can be used as the foundational step of procedures to
determine the habitat value of different flows as part of an instream
flow negotiation or to relate the habitat value of different channel
configurations as part of a restoration project.
As the core concept of ecohydraulic assessment, HSC curves and
the manner in which they are generated determine the scientific
robustness of the ecohydraulic method in which they are used. Uncer-
tainties in HSC can heavily influence the results of habitat analyses
(Ayllon, Almodovar, Nicola, & Elvira, 2012). Ecohydraulic assessment
methods (primarily the PHABSIM system) have been reviewed
or critiqued and their predictive accuracy assessed by numerous
authors (described later in this manuscript). However, neither the ori-
gin nor the history of the HSC curve concept (for brevity referred to as
“the HSC curve”) has ever been described, so that any review of
ecohydraulic assessment methods must, by default, be incomplete.
As Stalnaker and Arnette (1976, p. IV) observe about the status of
methodologies to determine instream flows circa 1975, and as we
can confirm, the historical knowledge base documenting instream flow
methodologies is not organized into a consistently cited body
of literature and “valuable information exists in forgotten
files, unrelated data and techniques, or only in the minds of the
practitioners.”
As a consequence, users of ecohydraulic tools that depend on HSC
curves are unable to defend the scientific basis of their work and the
reliability of their findings, and decision‐makers will be unsure if their
decisions based on HSC meet scientific standards. Our goals are to
describe the history of the development of the HSC curve
and critically review its scientific underpinnings so that applied aquatic
ecologists can answer the following important questions about HSC:
1. What were the historical motivations behind the HSC curve, and
how did the motivations influence its form and development?
2. What were the origin and stages of development of HSC?
3. How should the output of an analysis using HSC curves be
interpreted? Can it really be used to predict standing crop of fish
or abundance of other aquatic taxa?
4. How scientifically robust is the HSC curve, and what are the new
research directions and applications?
5. Can lessons learned from the nearly 50‐year history of the devel-
opment of the HSC curve help guide the development of future
environmental flow methods?
2|HISTORICAL MOTIVATIONS FOR
DEVELOPMENT OF THE HSC CURVE
2.1 |Equitable distribution of water in the arid west
of the USA‐shaped instream flow technology
The early history of the HSC curve is tightly linked to institutional and
legal factors associated with the allocation of water at the state level
and with reservoir operation in the western states of the USA (Allred,
1976; Bradley, 1976; Caulfield, 1976). The genesis for development
of the HSC curve can be traced to two major shifts in water law in
the “arid west”during the westward expansion of the USA. The first
shift was a change from riparian to appropriated water laws. This sec-
ond shift cannot be fully understood without the context provided by
the first shift. Early policy makers understood that settling the western
USA required irrigation to reclaim arid land for agricultural production
FIGURE 1 An early probability of use curve (taken from Figure 2 of
Bovee & Cochnauer, 1977). Later appliers of the PHABSIM system
prefer the term “Velocity Suitability”over “Probability of Use.”
Figures 1–4 were digitally retraced to correct for poor resolution,
distortions, and discolorations from original hardcopy and archived
document scans available from the internet
2NESTLER ET AL.
(see histories in Grover, 1943; Frazier & Heckler, 1972; Brown, 2015).
Without irrigation, it was thought that much of the arid portion of the
continental USA could neither be self‐sustaining nor contribute signifi-
cantly to national economic development (Brown, 2015). Many of the
miners and farmers migrated to the west from regions of the USA
where a property owner had the right to water flowing through their
property and could exclude neighbours that did not have property next
to the stream from access to water (i.e., riparian water rights based on
common law; see Baxter, 1965). The riparian principle quickly proved
unsatisfactory in both mining camps and agriculture settlements during
water shortages. For example, in 1874, upstream users in the Cache la
Poudre River of Colorado diverted all the water they needed, depriving
older downstream diversions of water. Continued conflict eventually
led to the principle of prior appropriation (i.e., those who claim water
first have higher priority in times of water shortage; CFWE, 2004) being
enacted into law by the Colorado Legislature in 1879.
The 1879 water law in Colorado was possible because the Mining
Act of 1866 transferred administration of water to the states, and the
Desert Land Act of 1877 changed the riparian water rights principle to
allow landowners to obtain a property right to divert a portion of the
water in a stream that was not on or near their property (Baxter,
1965; Holbrook, 1922). The Mining Act confirmed water rights for min-
ing, agriculture, and other uses acquired by private parties on public
lands. Similarly, the Desert Land Act
“…provides for the sale of desert lands to persons who
agree to irrigate and cultivate such lands”and further
“… provides also that the right to the use of water on
such lands shall depend upon appropriation”. Of interest
to the development of the HSC curve, “all surplus water
over and above such actual appropriation and use,
together with the water of all lakes, rivers, and other
sources of water supply upon the public lands …shall
remain and be held free for the appropriation and use
of the public for irrigation, mining and manufacturing
purposes ….”(bold added for emphasis)
One of the foundations for managing and adjudicating water rights
was the principle of “beneficial use”as used in the Desert Land Act of
1877 and later refined by Bien (1905):
…the fundamental principles are few and well
established namely, that all the waters within the limits
of the State belong to the public and are subject to
appropriation for beneficial use, except from sources of
supply which are navigable; that the beneficial use of
water shall be the basis, the measure, and the limit of
the right ….”(bold added for emphasis)
Future court rulings would decree that “all source of supply
belongs to the public and that states have the responsibility of deter-
mining who uses the water and for what purpose (described in Oregon
State Water Resources Board, 1959). We present herein only the bar-
est outline of western USA water law sufficient to explain its influence
on the development of the HSC curve. The reader is encouraged to
consult the many authoritative sources on western water law for addi-
tional detail.
The first shift in water law created the need for new technologies
and new state government functions in the arid western USA. New
technologies were needed to inventory and map water available for
storage, diversion, and utilization in irrigation to help guide efficient
land reclamation (Grover, 1943). In 1888, the U.S. Congress
established the Irrigation Survey to “map drainage basins, measure
streamflow, and assess potential sites for irrigation canals and reser-
voirs.”Unfortunately, the technology to conduct flow measurements
in shallow western rivers was inadequate, and there was a shortage
of experienced hydrographers. To remedy these shortcomings, the
Irrigation Survey established a research, testing, and instruction camp
in late 1888 at Embudo, New Mexico (Brown, 2015). The work at
Embudo led to the stream‐gauging technologies necessary to rigor-
ously and systematically measure streamflow and inventory water
availability and served as the technology base upon which the HSC
curve was later developed. These technology developments enabled
each western state to establish a new agency to manage water rights
within its boundary. These agencies were typically staffed by engi-
neers, hydrologists, and economists, but not biologists.
2.2 |Expanding the definition of “beneficial use”to
include instream flows
The second shift in water law that affected the development of the
HSC curve was the expanded definition of “beneficial use”of water
from strictly economic uses to include noneconomic uses such as aes-
thetics, fishing and fish conservation, hunting, swimming, and scientific
and educational study (Estes & Orsborn, 1986; Lamb & Doerksen,
1987). Logically, the same agencies responsible for managing water dis-
tribution for economics‐based beneficial uses were authorized to
expand their responsibility to quantify the amount of water that should
be reserved for these newly defined instream uses. Unlike the national
program for development of gauging technologies and hydrologic
methods by the Irrigation Survey, there was no corresponding national
research effort to develop methods to quantify the beneficial use of
water for instream flow values. Water management responsibility had
already been delegated to the individual states, primarily by the Federal
Desert Land Act of 1894 (Holbrook, 1922), and in response, each state
had already created an agency to manage water distribution within its
boundaries. Without a clear mandate for federal intervention, such as
was used to create the Irrigation Survey, each state was left to develop
its own program to value instream uses of water (Lamb & Doerksen,
1987; Mckinney & Taylor, 1988; Stalnaker, 1982), a task for which they
were woefully ill‐prepared (Spence, 1975).
It is difficult now to fully appreciate the magnitude of impact of the
addition of instream values to the definition of beneficial use on those
responsible for the equitable distribution of water. For example, in
Oregon, one of the first states to establish instream flows as a benefi-
cial use in 1955 (Lamb & Doerksen, 1987; Thompson, 1972), the State
Water Resources Board was given the mandate to determine instream
NESTLER ET AL.3
flows to support aquatic life and minimize pollution (Oregon State
Water Resources Board, 1959). However, this board had neither biol-
ogists nor ecologists on staff to recommend values for instream flows
and so, logically, looked to the state fishery agencies for guidance with
the following result:
…the Board has repeatedly requested the fisheries
agencies to furnish criteria that could serve as
justification for the allocation of a substantial quantity
of the state's waters to a minimum flow program.
Criteria requested involves numbers of fish, species,
areas of spawning gravel, present fish population,
potential fish population that could be attained if
requested flows were established and maintained. This
information has not yet been submitted to the Board.
Present day biologists and ecologists recognize the extreme diffi-
culty of meeting the request of this board, particularly the tasks of
identifying “numbers of fish, species, present fish population, and
potential fish population that could be attained if requested flows
were established and maintained.”To the political appointees and
engineering staff of the board (and similar boards in other western
states), these tasks would seem reasonable because of the widespread
availability and acceptance of stream gauging methods, water supply
hydrology, and agricultural production economics. To them, it should
not be difficult to add one more production category of water use
(i.e., fish number or fish biomass produced per increment of water)
to what was already a relatively mature institutional process for equi-
tably distributing the benefits of water resources. This request to the
fisheries agencies was attended by a sense of urgency because many
western rivers were overappropriated (i.e., flows reserved to meet
the requirements of water right holders exceeded the amount of nat-
urally available water). For example, in Oregon,
…the known minimums in all cases analyzed are less
than the desirable base flows requested (by the fishery
agencies) without taking into consideration the effect of
additional depletion potential within the basin. (Oregon
Water Resources Board, 1959)
Similar stories unfolded in all of the western USA states (Lamb &
Doerksen, 1987).
2.2.1 |Federal power act reinforces beneficial uses
concept for instream flows below reservoirs
The Federal Power Act of 1920 (as amended by Chapter 687, August
26, 1935; 49 Stat. 803; Anonymous, 2017) required “adequate protec-
tion, mitigation and enhancement of fish and wildlife”(16 U.S.C.
803(a)) and required decision‐makers to “consider the recommenda-
tions from various sources, including fish and wildlife recommenda-
tions of affected Indian tribes.”State and federal agency attitudes
towards equitable water distribution for beneficial uses, including the
need for adequate instream flows, were beginning to be established
by the time the Federal Power Act (as amended in 1935) was enacted.
The Federal Power Act was a major component of the Federal Energy
Regulatory Commission licensing procedures during the peak in reser-
voir construction in the late 1940's and 1950's and during the
relicensing of hydropower dams after expiration of the initial license.
The Federal Power Act brought hydropower dam owners into the
same dialogue as irrigators on how instream flows should be consid-
ered to preserve natural values of rivers. The response of California‐
based Pacific Gas and Electric (PG&E) Company staff to the Federal
Power Act was particularly important to the development of HSC
(Kelley, Cordone, & Delisle, 1960; Waters, 1976).
2.2.2 |Effects of institutional and legal constraints
on development of instream flow methods
The acceptance of instream flows as a beneficial use was a watershed
event in water laws in the western USA. Unfortunately, there were no
methods available at the time to assign a value for beneficial use to a
specific instream flow, which prevented equitable water distribution
by the responsible state and Federal agencies. The challenge of con-
structing a defensible method to quantify the beneficial use of each
of a series of discharges spurred technological advancement, institu-
tional growth, and a profound change in regional social attitudes. From
this history, we infer five objectives that would have motivated the
earliest workers attempting to develop methods to determine environ-
mental flows for western USA rivers:
1. consistency with the methods used to quantify, analyse, and prior-
itize beneficial uses of water resources to support economic
development,
2. be incremental to support trade‐off analysis among competing
uses of water to ensure decisions were made in the public interest,
3. be sufficiently general for wide application by staff of state agen-
cies tasked with managing water resources,
4. near‐term availability because fisheries in overappropriated
streams were in crisis and equitable water resources decision‐
making (and therefore economic development and hydropower
licensing) would either have to cease or continue without consid-
ering instream flows, and
5. repeatability and defensibility to survive legal and scientific
challenge.
These objectives guided early workers towards methods that
linked the physical characteristics of channel form and streamflow pat-
tern to the requirements of a target aquatic taxon or waterborne
human activity, culminating in the development of the HSC curve.
3|CRITICAL HISTORY OF THE STAGES OF
HSC CURVE DEVELOPMENT
Any methodology based on HSC curves of multiple life stages over
many transects and flows is computationally intensive. The widespread
4NESTLER ET AL.
development and use of HSC curves did not occur until after the devel-
opment of the computer microprocessor in 1971 that allowed easy use
of hand calculators and interactive uses of mainframe computers (Atlan-
tic, 2007). The development of the HSC curve can generally be sepa-
rated into five periods, with the first two periods and the last two
periods are separated by the period when super computers became
readily available:
1. Incubation (1949 to 1955) when foundational concepts were cre-
ated that would become the basis for the form and use of the
modern HSC curve (e.g., Briggs, 1953).
2. Methodological Evolution (1956 to 1972) where the procedures
that included the HSC curve were developed within a broader
methodological framework (e.g., Kelley et al., 1960).
3. Development of Analytical Techniques (1973 to 1976) marked
the use of mainframe computers as a critical component of
computationally intensive instream flow analyses (e.g., Waters,
1976).
4. Golden Age (1977 to 1985) was enabled by new legal require-
ments, widespread availability of mainframe computers and appro-
priate software, methodological developments from the previous
decade, and an explosion of small‐hydroelectric project permit
applications (e.g., Bovee, 1982).
5. Period of Criticism and Reflection from 1986 to 2000 when the
HSC curve and the framework within which it was executed came
under increasing scientific scrutiny (e.g., Fausch, Hawkes, &
Parsons, 1988).
The following sections describe the contributions of each period,
followed by a description of its significant legacies and impacts on
subsequent periods.
3.1 |Incubation: 1949 to 1955
The incubation period begins with the first state legal requirement to
protect the instream use of water for fishery protection per Section
FIGURE 2 A standard stream gauging method as narratively described in Corbett et al. (1943) and graphically described by Buchanan and
Somers (1969; Plot a) compared with standard method of describing the physical habitat of an aquatic organism (Bovee & Milhous, 1978; Plot b)
NESTLER ET AL.5
46 of the Washington State Fishery Code (Washington State
Legislature, 1949):
It is hereby declared to be the policy of this state that a
flow of water sufficient to support game fish and food
fish populations be maintained at all times in the
streams of this state.
This requirement and others like it in other western states were the
impetus for each state to begin the development of a method to esti-
mate instream flow requirements. Although the need to protect fresh-
water fisheries is clearly stated, the origin of the primary tool to
enable this protection is not clear. There is no single study or related
group of studies that unequivocally concludes that the use of depth
and velocity criteria in the form of a univariate HSC curve is the opti-
mum approach for relating fish habitat to streamflow. However, we
were able to find progenitor methods for each axis of the HSC concept.
3.1.1 |Relating the abscissa of the HSC curve to
stream gauging methods and technologies
The form of the abscissa of the HSC curve can be tied to the wide use
of stream gauging in western states. The basic concepts and technol-
ogies of water supply hydrology (e.g., stream gauging methods and
current meters suitable for deployment in western streams) and
methods to summarize water availability (e.g., the flow duration curve)
were first documented in 1904 (Murphy, Hoyt, & Hollister, 1904), for-
malized by the 1940s (Corbett, 1943), tested in the 1950s (Young,
1950), and well known by the 1960s (Buchanan & Somers, 1969).
Streamflow in a channel is measured by establishing a gauging tran-
sect perpendicular to water flow and then measuring water depth
and velocity at stations located at increments along the transect. The
flow within the stream channel is estimated by converting depth and
velocity station measurements into cell discharge estimates and sum-
ming the individual cell discharges to estimate total channel flow.
The convergence of stream gauging methods with the abscissa of an
HSC curve can be seen by comparing Figure 3 from Buchanan and
Somers (1969), which graphically describes a standard stream gauging
method to the nearly identical figure 5 from Bovee and Milhous
(1978), which describes hydraulic habitat measurement in support of
a PHABSIM study (Figure 2).
3.1.2 |Relating the ordinate of the HSC curve to
habitat use observations and measurements
The origin and meaning of the ordinate of an HSC curve are less clear
than those of the abscissa. Even the name of the ordinate has varied
over time, by user or by how it was derived. The generally accepted
term is “habitat suitability criteria curve”(Orth & Maughan, 1981;
Bovee, 1986), but it has been variously referred to as a habitat prefer-
ence (Briggs, 1953), physical criteria (Chambers, Allen, & Pressey,
1955), table of standards (Westgate, 1958), weighting factor (Waters,
1976), probability of use (Bovee & Cochnauer, 1977), preference
curve (Bovee, 1982), and habitat parameter suitability (Trihey, 1979).
HSC curves are unfortunately frequently still confused with the habi-
tat suitability index (HSI) of the habitat evaluations procedures (HEP)
even though HEP and PHABISM were clearly distinguished by
Armour, Fisher, and Terrell (1984). HEP are designed for quantifying
habitat values and for documenting impacts of physical or spatial hab-
itat changes on fish and wildlife resources in both aquatic and terres-
trial ecosystems. The HEP use HSI that can be composed of a wide
array of physical, chemical, and biotic variables to determine habitat
value for target biota. HSI can be formatted as regression equations
and simple equations or appear similar to HSC curves, but HEP are
not recommended for setting instream flows because HEP analyses
are static and do not include a hydraulic simulation module.
The HSC concept was first used to link river hydraulics to the
spawning requirements of salmon in streams along the Pacific Ocean
coast of North America. The first occurrence of a data display
resembling an HSC curve was associated with studies on salmon
reproduction. Salmon biologists of the time thought that populations
were limited by inefficient egg fertilization during spawning and
excessive egg and larval mortality (Briggs, 1953; Wales & Coots,
1955). This assumption was the underlying reason for the focus of
fishery managers, particularly in the USA, on early life stages and
the rationale for hatchery stocking practices as early as 1872 (Wales
& Coots, 1955).
A number of studies that influenced the development of the HSC
curve related hydraulic variables to spawning site selection by differ-
ent species of salmon. Early studies on salmon spawning during the
decade of the 50's adopted a planimetric mapping perspective (see
review of habitat mapping for background in Morrison, Marcot, &
Mannan, 2012) in which areas meeting depth, water velocity, and sub-
strate criteria were delineated. It is unclear how water velocities were
measured.
We describe a few of the more notable studies below and suggest
the reader consults the review by Fraser (1975) for additional informa-
tion. Briggs (1953), in studies begun in 1948, described the channel
location of redds and the associated depths and velocities (along with
average gravel size and egg depth). From these data, he prepared
tables describing the average depths and velocities where redds of sil-
ver (coho—Oncorhynchus kisutch), Chinook salmon (Oncorhynchus
tshawytscha), and steelhead (Oncorhynchus mykiss mykiss) were found.
Briggs referred to the depth–velocity hydraulic measurements at each
redd as the “preference”of spawning salmon. Chambers et al. (1955)
described the spawning sites of Chinook, sockeye (Oncorhynchus
nerka), and silver (coho) salmon for depth or velocity (and reported
temperature and dissolved oxygen) in several Washington Rivers in
studies conducted from 1953 to 1954. Importantly, the goal of studies
by Chambers et al. was not to develop instream flow recommendations
but to develop design criteria for artificial spawning channels to be
constructed as part of mitigation for loss of natural spawning sites
caused by dam construction. They summarized their findings by plot-
ting frequency distributions of redd locations for each site and species
and by depth or velocity (Figure 3) and referred to their findings as
“physical criteria.”Although the spawning criteria were not formatted
as HSC curves, they easily could have been by curve smoothing and
6NESTLER ET AL.
normalizing of the ordinate values. Later studies on spawning site
selection concluded that water velocity immediately above the redd
(usually at about the distance of the adult salmon lateral line above
the substrate, i.e., 0.3–0.4 ft. above the bottom) was more appropriate
than measuring surface or mean water velocity.
The assumption that spawning efficiency and egg and larval mortal-
ity limited salmonid populations was not shared by researchers in New
Zealand (Hobbs, 1940, 1948). Hobbs's work resulted in considerable
controversy in the late 1940's and early 1950's, culminating in a signif-
icant change in U.S. perspectives best voiced by Lagler (1949): “Stocking
is no longer regarded as the principal means for generally maintaining
and improving fishing; it has been shown at times to be unnecessary,
wasteful, and even harmful.”This change in perspective shifted the
focus of researchers in the mid and late 1950's to include life stages
other than spawning, egg incubation, and larval stages. Allen (1951,
1952) and his team conducted conceptually similar studies to those of
Briggs (1953) and Chambers et al. (1955) for a relatively small New
Zealand trout stream. Although limited in spatial extent, Allen's studies
were influential for their comprehensiveness and attention to detail. In
keeping with the perspective of New Zealand researchers, Allen was
interested in all aspects of trout ecology including all life stages, losses
to fishermen, and instream food production. He mapped hydrogeomor-
phic conditions in relatively uniform stream reaches into “pool, flat, run,
stickle (rapid), and cascade”and made multiple measurements of “width,
depth, current, and the nature of the bed”in each category to calculate
average reach habitat conditions. Conceptually, it is a small step to sep-
arate a river into weighted points along a transect and reduce the scale
of the habitat variables used by these early workers from a mesohabitat
scale to a microhabitat scale using a gauging perspective. Historical evi-
dence suggests that the integration of the ideas presented in Allen
(1952), Briggs (1953), Chambers et al. (1955), and similar studies with
widely available gauging and hydrologic summary methods was the
foundation that eventually led to the development and use of the mod-
ern HSC curve. We tried to trace back all of the earliest often‐cited
reports, but, unfortunately, a few with intriguing titles could not be
located and we fear they are lost to the scientific community.
3.1.3 |Legacies of the incubation period: 1949–1955
The 1950s established important concepts in ecohydraulic assessment
that helped fuel future methodological development. However, this
decade was also responsible for technology legacies that later became
institutionalized without critical evaluation of their origin or scientific
robustness.
Focus on regional hydrology—the minimum low flow
The hydrology of western USA salmon streams may not be represen-
tative of streams in other parts of the world (Bain & Boltz, 1989; Nes-
tler, 1993; Poff & Ward, 1989). Many western streams have an
extended base flow period because of groundwater inputs or snow-
melt that coincides with the peak crop growing season. Predictable
flows from groundwater and spring‐run‐off are a boon to irrigators
because they can be used to reliably forecast water, thereby reducing
agriculture risk. These streams are also biologically important because
they do not exhibit the low‐flow population bottleneck characteristic
of streams with more variable flow patterns (Giger, 1973). The con-
cept of the minimum low flow is easy to understand and implement
but of uncertain value when exported to streams not exhibiting
extended periods of predictable flow.
Assumption that increased habitat equates to increased fish
production
State fishery agencies during this time conducted habitat studies in
trout and salmon streams to identify and remove population bottle-
necks and thereby improve fish harvest. In a similar vein, water
resource managers often perceived habitat as a commodity that could
be equivalenced to fish harvest, manufacturing output, or agricultural
production. These perspectives led to the expectation that
FIGURE 3 Early step in the development of
an HSC curve: example frequency distribution
of redd locations by depth (in feet) for
individual redds (upper graph) or mass
spawners (lower graph). Similarly formatted
graphs depict frequency distribution for
velocity (Figure 4 from Chambers et al., 1955)
NESTLER ET AL.7
implementation of findings from relatively simple habitat assessments
designed to quantify beneficial use of instream flows should have a
simple relation between aquatic habitat and fish harvest. As stream
ecologists understand now, the factors that determine fish abundance
can be complex, interactive, and their relative importance change from
year to year (Schlosser, 1991; Lancaster & Belyea, 2006).
Focus on streams with low species abundance
An HSC curve approach may be adequate to conserve salmon species
or other keystone species whose abundance can be tied to flow in
streams with depauperate fish faunas. However, it clearly represents
a reductionist approach for assessing the effects of flow alterations
on streams with diverse fish faunas. The early emphasis on develop-
ment and use of the HSC curve may have interfered with the develop-
ment of more holistic approaches that would be better suited to
streams supporting diverse fish faunas.
Limited channel complexity of western streams
Early hydraulic habitat studies focused on the main channel because
many western mountain streams do not have extensive floodplains,
although they may have extensive wetlands and riparian areas. How-
ever, spawning and rearing of trout and salmon occur in the main
channels, and, consequently, habitats outside of the main channel
were ignored in aquatic habitat assessments. This early focus on
main channel habitat was particularly detrimental for management
of warm water fish in floodplain rivers in other parts of the USA
(Stalnaker, 1990) and the world (Bunn & Arthington, 2002; Junk &
Wantzen, 2004; Nestler et al., 2012; Welcomme & Halls, 2004)
when overly simple habitat models were used to recommend envi-
ronmental flows.
Lack of studies describing methods to determine juvenile and
resident adult habitat
Unlike the firm scientific foundation that related spawning and egg
incubation to physical parameters (e.g., Burner, 1951; Reiser &
Wesche, 1977), we could find no foundational scientific studies on
how to best describe habitat for juveniles and nonanadromous adults,
as also noted by Tennant (1972), Thompson (1972), and Hooper
(1973). The methods used for describing habitat requirements for
early salmon life stages appeared to be effective and widely accepted
so they were simply extended to describe juvenile and resident adult
habitat with very little supporting research. The majority of early
instream flow studies dealt with either spawning criteria or flows
needed to sustain sport fisheries for trout and other species in tailwa-
ters immediately below dams (Giger, 1973; Leathe & Nelson, 1986).
For example, Reiser and Bjornn (1979) and Hall and Baker (1982)
explain in great detail in comprehensive guidance reports on habitat
requirements of anadromous salmonids on Forest Service lands how
physical variables (primarily water velocity and substrate material)
affect spawning and incubation success. However, they provide rela-
tively little information on how physical variables affect juvenile rear-
ing, although rearing habitat was known to affect juvenile salmon
production, at least in artificial channels (Ruggles, 1966).
3.2 |Methodological Evolution: 1956 to 1972
Limited computer capability and availability slowed the evolution of
the HSC curve and the frameworks within which it was used. Without
computers, the tedium and error potential of manually iterating
through depth, velocity, and cover HSC curves for each sample point
of many transects over many discharges and multiple species life
stages is demanding on personnel and time consuming in both the
field and office (Sams & Pearson, 1963). However, this period was
not without a few major contributions. Westgate (1958), working on
the Cosumnes River in California, made several notable contributions
that anticipated the methodological evolution of the 1960s. He was
the first to create the iconic discharge versus cumulative spawning
habitat plot (Figure 2 in his report) by plotting spawning habitat area
versus flow for four discharges. He developed an HSC curve progeni-
tor he termed a “table of standards”to weight the usability for differ-
ent depths, velocities, or gravel compositions for spawning within each
uniform stream subsection (usually, bank to bank parallelograms). All
previous descriptions of physical habitat we could locate were
expressed as either binary (habitat values only of 0.0 or 1.0) or limiting
criteria to simplify mapping and analysis. Limiting criteria (i.e., habitat is
considered acceptable when velocity is less than a limit or depth is
greater than a limit) were developed by the Colorado Division of Wild-
life (Rose & Johnson, 1976) and in Oregon (Hutchison & Aney, 1964;
Thompson, 1972) as well as in other states. Sometimes, a minimum
depth was used with velocity criteria having both a maximum and min-
imum value. For brevity, we lump limiting criteria together with binary
criteria because of their similarity.
Waters (1976) credits Kelley et al. (1960) for developing the basic
framework of an instream flow method using HSC. The proposed
method of Kelley et al. (as cited in Linn, 1961), sometimes referred
to as the “California Method,”is widely cited in early papers on
instream flow methods. The proposed method was first implemented
during a multiagency training session held in October 1960 (Linn,
1961). The training session and resulting documentation were notable
because they captured the prevailing attitude of the time on the state
of the art in habitat assessment. First, they included a review of
existing methods used in California for recommending instream flows
to sustain fish production. The review reflected the conclusions of
many working on environmental flows at the time: There was insuffi-
cient knowledge about the needs of trout and how to measure them,
existing methods were not quantitative and could not be explained or
justified, they produced inconsistent and highly subjective decisions,
and, most importantly, a more objective method was needed to deter-
mine the flow needed by existing fish populations. Second, the class
included an instructor of the United State Geological Survey (USGS)
method for gauging streams, reinforcing the early nexus of habitat
assessment with gauging methods. Third, the influence of the New
Zealand scientists can be seen with the addition of two new binary
habitat criteria (food production area and shelter habitat for adult fish)
in addition to spawning habitat. Importantly, none of the criteria were
based on original data (they were presented as hypotheses), and there
were no recommendations on how shelter habitat for adult fish should
8NESTLER ET AL.
be measured. Fourth, the study stream was gaged at four separate dis-
charges. The areas meeting the criteria for each habitat category (i.e.,
usable area in ft
2
) were plotted for each stream section and for each
of the four discharges to produce the iconic habitat area versus dis-
charge plot that is the primary goal of ecohydraulic modelling
methods. The class concluded that the method provided consistent
results because the general shape of the curves produced by each
class member was similar, although individual habitat area estimates
were variable. They felt that, with refinements, the approach could
be made to be sufficiently objective and an improvement over the
subjective methods used at the time.
Sams and Pearson (1963) were the first to use a complete
ecohydraulic framework for an instream flow study. The Sams and Pear-
son study included the “one‐point method,”which can be considered
the simplest form of a physical habitat calculation. The method yielded
one streamflow value—the desired instream flow—calculated as the
product of the optimal depth times the optimal velocity times the tran-
sect top width. The Sams and Pearson study included four other physi-
cal microhabitat methods where the criteria for establishing instream
flows were based on the velocities and depths preferred by a specific
life stage of fish (in their case, spawning salmonids). They fully inte-
grated USGS gauging methods (Corbett, 1943) with habitat analyses
by separating each transect into cells for measurement of depth and
velocity and description of spawning habitat. Moreover, Sams and Pear-
son did not summarize their biological utilization data as HSC curves for
velocity and depth but instead summarized depth and velocity utiliza-
tion with means and 95% confidence limits (methodologically approxi-
mately equivalent to binary criteria). They collected habitat criteria
data and summarized their results using closely spaced transects instead
of habitat mapping used in other studies in both California and Wash-
ington. They also introduced the “weighted usable width analysis”(i.e.,
spawners select certain hydraulic conditions more than others within
the range of acceptable hydraulic conditions) similar to Westgate
(1958) as an improvement over “usable width analysis”(i.e., a binary
approach where utilization is uniform within the range of acceptable
hydraulic conditions). Although of historical interest, the Sams and Pear-
son (1963) methods were not adopted in subsequent studies along the
Pacific Coast, as evidenced in a compilation of studies by Wesche and
Rechard (1980) in which only the one‐point method is described.
Rantz (1964) was the first to determine the optimum spawning dis-
charges for Chinook salmon in northern California coastal streams
using criteria developed by another researcher (Westgate, 1958)
working in central California. This action may have suggested to future
researchers that habitat criteria could be developed and archived in a
library for later use. This was a significant step because the greatest
expense in conducting microhabitat analyses is the development of
HSC curves. As a historical side note, Baxter (1961) developed a com-
prehensive method that mixed geomorphic and hydrologic methods to
determine environmental flows to protect fisheries in Scotland, similar
to Tennant's (1972, 1976) efforts in Montana, which are based on
judgement and observation. He would have almost certainly influ-
enced the development of environmental flow methods, had scientists
in the USA been aware of his studies.
The decade of the 1960's also saw a shift in the scale of study design
and implementation, from an individual researcher or small, focused
team, to a large program perspective that typically included one or more
state agencies with support from selected federal agencies. The
instream flow issue continued to grow in importance and transitioned
from being an issue important to a few individual western USA states
or a utility to being recognized at regional and national scales.
3.2.1 |Legacies of the methodological evolution
period: 1956 to 1972
Origin of the HSC curve was lost
The foundational studies from the incubation period were published in
relatively obscure agency and utility reports never intended for wide-
spread distribution. We were unable to locate the documentation for
these studies (e.g., Deschamps, Wright, & Magee, 1966; Kelley et al.,
1960) from interlibrary loan, searches of library databases, or requests
to our extensive professional networks.
Poor scientific underpinning for development of HSC curves
The urgent need to determine beneficial use of water for instream
flows forced early workers to relate fish rearing and adult habit to river
flow by simply extending existing methods for relating salmon
spawning and incubation to flows. The necessary supporting science
was never conducted even though it was recommended to do so by
early method developers.
Limited peer review
The early technology legacies identified for the decade of the 1950's
survived this time period with little or no critical evaluation. Scrutiny
through the academic peer review process was largely missing during
this period because most of these studies would have been deemed
“unscientific”by journals, for they were highly procedural, performed
in response to legal requirements and institutional direction, of a prac-
tical nature, and did not involve scientific hypothesis testing. The opti-
mum development, limitations, and assumptions of the HSC curve
were consequently poorly supported by scientific documentation,
opening the door to future criticism, beginning in the 1980's.
3.3 |Development of Analytical Techniques: 1973 to
1976
Stalnaker (1982) characterizes the 1970s as the “coming of age”of
flow assessments. Five factors converged during this time period to
fully enable hydraulic microhabitat flow assessments: (a) Mainframe
computers were common at major universities and at water agencies
such as the Bureau of Reclamation, (b) computer operating systems
like network operating system (NOS) enabled relatively easy remote
access of centrally located Control Data Corporation (CDC) com-
puters, (c) application of computer programs in analysis became a rou-
tine part of the education of professionals involved in water resources
management, (d) the conceptual foundation and methodological
framework for microhabitat assessment were largely developed during
NESTLER ET AL.9
the previous two decades, and (e) instream flow requirements were
legally required and institutionally accepted. For example, Stalnaker
(1982) identified over 20 state and federal laws and major court
decrees that supported or authorized instream flow assessment and
nine major reports that shaped regional and national policy.
3.3.1 |Foundational studies
The accepted dates for publication of the univariate HSC curve and a
complete framework in which it was embedded are generally attrib-
uted to Rantz (1964), Collings (1972) and Collings, Smith, and Higgins
(1972), Hooper (1973), Wesche (1973), White and Cochnauer (1975),
and Waters (1976), with Waters (1976) creating the most complete
framework (Morhardt & Hanson, 1988). The foundational studies
were performed by coordinated interagency teams belonging to large
organizations with internal support to conduct large‐scale studies,
including access to computers and computer programmers. For exam-
ple, Collings et al. (1972) were members of the USGS and performed
their studies under a cooperative agreement between the USGS and
the Washington State Department of Fisheries. Collings et al. com-
bined the methods of Westgate (1958) and Rantz (1964; described
earlier) and Deschamps et al. (1966 as cited in Smith, 1973). The goal
of Collings et al. (1972) was to develop a suitable method to determine
the most desirable streamflow or flows for spawning and rearing of
various salmon species in the major salmon producing streams in
Washington. Their studies on four Washington stream exhibited all
features of an ecohydraulic approach: Sample points along transects
described physical habitat characteristics, use of published binary
criteria (portending the creation of HSC curve libraries) to characterize
spawning habitat, and a wetted perimeter approach to characterize
rearing habitat; analysis output used to build habitat maps for different
discharges, and the creation of habitat versus flow graphs as a final
product that could be used to optimize instream flow requirements
for different species and life stages. In addition, they determined the
repeatability of their methods and were one of the first instream flow
reports to include monthly flow duration information.
Hooper (1973) and Waters (1976) were part of a team working
within PG&E Company tasked to develop a tool that could help deter-
mine release patterns from dams as part of the hydropower licensing
process. Their extensive network of partners and stakeholders (Cali-
fornia Department of Fish and Game, the U.S. Forest Service, the U.
S. Bureau of Sport Fisheries and Wildlife, and the Southern California
Edison Company) would anticipate future interagency studies charac-
terized by cooperation among many or all stakeholders.
Although they did not contribute to the development of HSC
curves directly, studies by White and Cochnauer (1975) were notable
for several reasons. First, they studied several species of warm water
fishes in large Idaho Rivers instead of salmon or trout in small‐to
medium‐sized streams studied by most early workers. They were also
one of the first users of a hydraulic model, the Water Surface Profile
model of the U.S. Bureau of Reclamation, to simulate water surface
elevations and depths at unmeasured discharges as part of an instream
flow study. The Water Surface Profile model was later incorporated
within the PHABSIM system of models to simulate hydraulic informa-
tion that could be integrated with HSC curves to describe habitat con-
ditions at unmeasured discharges.
Through the middle of the 1970s, evaluation of instream flow needs
used binary HSC curves. Binary HSC were applied both to transect
samples (Kelley et al., 1960, as cited in Linn, 1961; Hunter, 1973; and
Smith, 1973) and to area samples (Collings et al., 1972) to compute hab-
itat indexes that could be related to flow. Waters (1976, p. 2) expanded
his definition of a comprehensive aquatic assessment tool based on
HSC curves that later became institutionalized within the PHABSIM
system as “… there are microhabitat requirements for upstream and
downstream passage, reproduction, egg and larvae rearing, resting,
feeding, and cover. Each microhabitat requirement can be defined by
water depth, water velocity, and bottom substrate. Hence we have the
basis for a model”(italics added for emphasis). He credits Kelley et al.
(1960) as implemented by Linn (1961) as the inspiration and source of
his method. Like Kelley (1960), Linn's report does not present data
nor describe a study with methods and results but only describes proce-
dures based on methods used in California.
Waters (1976) initiated a change in the form of HSC, using his
background in multivariate analysis, the literature review paper by
Hooper (1973) of which he was an uncredited coauthor, and his
access to PG&E Company computers and programmers. His step‐wise
HSC functions allowed for contiguous ranges of physical habitat suit-
ability to be expressed for aquatic organisms, which more closely
followed the frequency histograms resulting from field observations
(Figure 1 of Waters, 1976). He presented habitat criteria in a form that
are clearly HSC curves for three life stages/habitat needs: spawning,
resting microhabitat, and food production. He presented criteria
termed “weighting factors”for depth, velocity at 0.2 ft from the bot-
tom, and bottom substrate based on a sediment categorization
scheme. His report is the first document that could be characterized
as a user's manual for conducting an instream flow study. Access to
the PG&E Company computers by Waters and his team allowed the
more complex HSC to be integrated with hydraulic data from sample
points across many transects over a range of simulated flows and to
calculate a more nuanced habitat index function.
3.3.2 |Two foundational events
In addition to the reports by Hooper (1973) and Waters (1976), two
events occurred in 1976 that were pivotal for the maturation of the
HSC curve and the framework within which it is embedded (Wesche
& Rechard, 1980). The first was the “Symposium and Specialty Confer-
ence on Instream Flow Needs”held in Boise, Idaho, in May of 1976
documented in a two‐volume conference proceedings (Orsborn &
Allman, 1976). The widely attended conference included leaders
working on all aspects of the instream flow issue, including aquatic
biologists, fishery experts, engineers, economists, and hydrologists,
as well as social scientists, lawyers, and political scientists. The
attendees represented state and federal agencies as well as nongov-
ernmental organizations having an interest in water and related
resources management. The conference had the goals to “… emphasize
10 NESTLER ET AL.
the interdisciplinary aspects of current problems, namely communica-
tion and the awareness of legal, social, and technical aspects of preserv-
ing instream values and diversionary necessities …” (italics added for
emphasis) and give legal and social topics equal priority with technical
topics. Before 1976, the technology to address instream flow needs
was initially developed by individuals or small teams associated with
state resource agencies and later by teams associated with larger
agencies. After 1976, technology development to address instream
flow needs was consolidated and advanced at a coordinated, regional,
or national level.
History often repeats itself. In 1888, nearly 100 years prior to the
conference, the USGS created the Irrigation Survey to develop the
technology necessary to measure and inventory water resources in
the arid west of the USA. The resulting technology led to the creation
of new institutions for the equitable distribution of water. However,
these actions created a new issue—the recognition that new technol-
ogies must also be developed to preserve or restore the natural values
of instream flows once these too were included in the new definition
of beneficial use of water. Just like the pressures that resulted in the
creation of the Irrigation Survey, the social, political, legal, and techno-
logical pressures that led to the organization of the conference also
set the stage for the creation of the Cooperative Instream Flow Ser-
vice Group (CIFSG—for brevity, we retain the original name of this
group although it transitioned through several name changes).
The multiagency, interdisciplinary CIFSG was charged with three
objectives; one of which applies to understanding the development
of HSC curves: “identify instream flow requirements using improved
methodologies”(CIFSG, 1977). The objective was to be implemented
in three steps (italics of key words added for emphasis):
1. synthesize and transfer techniques for immediate application to cur-
rent problems,
2. promote research and development of data collection methods, and
3. update and improve operational techniques with new technology.
Conspicuous by their absence, there were no objectives for (a)
developing a scientific foundation for determining instream flows,
(b) conduct research to determine how best to integrate hydraulic
and geomorphic variables to estimate habitat value, or (c) studies
to evaluate, compare, or test different or new methods of quantify-
ing habitat–flow relationships. These omissions were made in spite
of recommendations by a number of early workers on instream flow
methods that additional research was needed. Instead, the CIFSG
focused on integrating existing technologies. For example, each of
the basic building blocks of what was later to become known as
the PHABSIM system was already in existence, having been created
earlier either by one of the western state fishery agencies or by one
of their partners as part of efforts to quantify the beneficial use of
instream flows. For example, the PHABSIM system used the ideas
of velocity and depth criteria and substrate similar to Waters
(1976). The ideas of simulating velocities and depths for locations
in a cross section using the U.S. Bureau of Reclamation water sur-
face program (WSP) and calculating width and wetted perimeter as
a function of discharge were based on precedents from the Montana
Fish and Game (Spence, 1975) and from Idaho Department of Fish
and Game (White, 1976; White & Cochnauer, 1975) and in a report
on instream values in the Northern Great Plains (Bovee, Gore, &
Silverman, 1978).
From our research, we conclude that the CIFSG probably avoided
goals to create novel new methods or conduct scientific studies on
environmental flow methods because such goals would put them in
potential technology conflict with the very entities whom they were
charged to support via collaboration and cooperation. As this review
has shown, the tools and perspectives of western water management
agencies were developed over decades of methodological evolution of
approaches for dealing with water distribution issues. The introduc-
tion of completely new approaches not tied to the slow evolution of
existing methods had the potential to disrupt water resources
decision‐making. It would also throw into disarray past and existing
water resources distribution decisions based on the individual
methods used in each state. However, improvement of existing
methods protects the integrity of past decisions with the promise of
faster, more efficient, and more scientifically robust future methods.
Unfortunately, the decision by the CIFSG to eschew development of
new methods also perpetuated the legacies upon which the PHABSIM
system was constructed.
3.3.3 |Legacies of the development of analytical
techniques period
The importance of the beneficial use concept is reduced as a
technology driver in regions that do not employ prior appropri-
ation water law
For example, most of the attendees of the “Symposium and Specialty
Conference on Instream Flow Needs”in 1976 (Orsborn & Allman,
1976) and the director and most of the initial staff of the nascent
CIFSG were from western USA states. Therefore, the HSC curve as
a core concept within PHABSIM system and the PHABSIM system
itself would be met by skepticism or confusion as the instream flow
issue expanded from a regional biological, geomorphic, legal, institu-
tional, and social setting to national and international settings without
the same traditions and history as the western USA.
Regional tools and perspectives expand nationally
Several relatively little‐known studies become the template for
approaches to quantify beneficial uses of instream flows that would
be later used to organize major components of the PHABSIM system.
Therefore, the scientific pedigree of the HSC curve and the PHABSIM
system were never established in academic peer‐reviewed, scientific
journals.
3.4 |Golden Age: 1977 to 1985
Prior to the creation of the CIFSG, HSC curves for juvenile and
nonanadromous adult fishes were simple extensions of the methods
used to create HSC curves for early life stages of salmonids. There
NESTLER ET AL.11
was little guidance or uniformity on how HSC curves should be devel-
oped, categorized, evaluated, or utilized. Waters' (1976) HSC format
and overall methodology came to the attention of the CIFSG, which
was embarking on a similar integration of HSC and riverine hydraulics
for instream flow evaluations. Waters' approach became the concep-
tual basis for much of the HSC fish habitat portion of the PHABSIM
system. Bovee et al. (1978) and the Washington Department of Ecol-
ogy contributed the foundations for water quality, ice dynamics, water
balance (e.g., ground water and evapotranspiration), and geomorphol-
ogy components.
3.4.1 |Refining of HSC by the CIFSG
Bovee and Cochnauer (1977) advanced HSC development by replac-
ing the step function of Waters (1976) with a full curvilinear format.
This format provided additional subtlety to the expression of habitat
use by organisms and allowed the application of curve‐fitting tech-
niques to frequency histograms. Curve fitting, whether manual, poly-
nomial, Poisson, or variants of these methods, smoothed out the
gaps and peaks, which are typical of histograms derived from field
data, especially those derived from smaller sample sizes. The Bovee
and Cochnauer (1977) definition effectively ended the application of
binary criteria to most instream flow analysis because multivariate
HSC curves can describe increments of suboptimum habitat condi-
tions and be inherently more accurate in describing fish habitat use.
In addition, Bovee and Cochnauer (1977) expanded the use of HSC
beyond traditional spawning habitat studies to nonsalmonid species
and for multiple life stages. Within 2 years of this expansion, HSC
were developed for dozens of species and life stages, aquatic macroin-
vertebrates, and recreational activities such as fishing and canoeing
(FISHFIL, 1979; Mosley, 1985). Subsequent publications by the CIFSG,
particularly Bovee (1982), Bovee (1986), and Bovee and Zuboy (1988),
provided expanded guidelines for categorizing types of HSC, designing
data collection efforts, describing field methods, mathematically
correcting for sampling bias, and fitting curves to data. Below, we
describe a few of the more notable contributions of the CIFSG
towards HSC curve development.
3.4.2 |Categories of HSC curves
Bovee (1986) identifies three categories of HSC curves (Figure 4):
binary, univariate, and multivariate response surfaces (Hardy, Prewitt,
FIGURE 4 Examples of the three possible
formats for HSC curves for depth (Figure 3 in
Bovee, 1986): (a) binary, (b) univariate, and (c)
multivariate response surfaces. Panel b can be
presented as a histogram where each vertical
bar represents the proportion of the
population using each depth category or
smoothed either by eye (as shown) or using
statistical curve smoothing
12 NESTLER ET AL.
& Voos, 1982; Voos, 1981). He also points out that a conditional HSC
curve can be developed for a life stage (e.g., spawning, juvenile, or
adult), behaviour (e.g., resting/holding, feeding, maintaining position,
random movement, staging, nest guarding, hibernating, migrating, or
escape), or situation (e.g., selection of a certain depth or velocity range
is contingent on the availability of overhead cover). Bovee (1986) also
identifies three methods of creating HSC curves based on data quality
(Table 1). Category I curves are derived from literature sources, profes-
sional judgement, or some combination of the two. Professional judge-
ment includes round‐table discussions, the Delphi Technique (i.e.,
questionnaire based; Crance, 1987), and habitat recognition. In habitat
recognition, experts point to specific stream areas where, in their opin-
ion, target species can be expected to be found. The physical conditions
may be measured at these locations by a gauging crew and summarized
using methods similar to developing HSC curves from measured fish
position data. Developing HSC curves from professional judgement
may not seem scientifically robust (objective #5); however, importantly,
it meets objectives #1–#4, which are required of a methodology to
quantify beneficial uses of instream flows. Recently, methods have
been developed to better analyse and use expert opinion as part of
environmental flow determinations (de Little et al., 2018). Experience
with Category III HSC has shown that strict utilization mathematically
corrected by availability can introduce additional bias into creation of
HSC (Morhardt & Hanson, 1988) and the CIFSG subsequently ceased
recommending the approach. Currently, the biasing effect of habitat
availability is addressed through study design by sampling all available
habitat strata with equal effort (Bovee et al., 1998).
Cover and substrate codes
Bovee (1986) describes the importance of cover and substrate and
offers a number of suggestions for coding nonhydraulic variables
important to stream ecology as either substrate or cover. Unlike depth
and velocity (which change with discharge), substrate/cover is a
nonhydraulic variable that can represent either the composition of
the stream bottom (substrate) or describes features important to
aquatic biota in or near the stream that cannot be coded as substrate
(e.g., cover). The importance of substrate in a habitat analysis can be
traced to its role in selection of salmon spawning sites and its associ-
ation with benthic macroinvertebrate production (i.e., fish food). Cover
is a catchall term that can describe a variety of nonhydraulic and
nonsubstrate stream features (e.g., dense overhead vegetation, under-
cut banks, shading, velocity shelter, or presence of instream objects).
Cover/substrate is important in understanding habitat utilization
during development of HSC curves. However, use of the cover/
substrate code does not typically change the ranking of alternative
flow regimes because cover/substrate at sample points does not
change with discharge and, therefore, is a constant in the analysis
unless (a) points are dewatered over the range of flows used in the
analysis or (b) the distribution of cover/substrate is stratified within
the channel. Importantly, the PHABSIM system only assigns a single
variable to capture cover and substrate effects on habitat, thus limiting
the capability to describe complex nonhydraulic channel and riparian
features. It also limits the ability to capture a mix of substrate where
sizes are noncontiguous on the Wentworth scale, for example, 50%
sand and 50% boulder. An exception was the HABTAE program of
the PHABSIM system, but it was a seldom used subroutine.
Weighted usable area
HSC curves applied to individual sample points must be integrated
into a summary variable to create actionable information for manage-
ment at larger temporal or spatial scales. To perform this function, the
CISFSG introduced a new summary variable termed weighted usable
area (WUA) calculated from the recursive application of HSC curves
to the hydraulic information associated with each point along one or
more transects for a given discharge. They defined WUA as follows:
…the total surface area having a certain combination of
hydraulic conditions, multiplied by the composite
probability‐of‐use curves for that combination of
conditions …This procedure roughly equates a larger
area of marginal habitat to an equivalent smaller area of
optimal habitat. (from Bovee & Cochnauer, 1977, p. 34)
WUA has become one of the most controversial topics in applied
aquatic ecology and is the direct or implied target of numerous
reviews and critiques.
This definition continued the historical practice of describing hab-
itat suitability by starting with physical area and assigning to that area
a habitat value using composite HSC (e.g., Collings et al., 1972). How-
ever, users of PHABSIM often made the assumption that area is rep-
resented by cells that physically extend in between measurement
points (per standard discharge computation) but also in between tran-
sects that were typically placed in contiguous mesohabitat units within
“representative”reaches, often at some distance from each other. This
assumption prompted much of the criticism the CIFSG subsequently
received (see Section on Reviews), principally because it is immedi-
ately apparent through observing the complex habitat of most streams
TABLE 1 Categories of HSC curves from Bovee (1986)
Category Data base Quality statement
I—Judgement Expert opinion Not supported by field data collected as part of study
II—Utilization Where target organisms are collected
or observed
Biased by environmental conditions available at time‐of‐collection
or observation
III—Preference Utilization corrected by availability (a) More transferrable (Thomas & Bovee, 1993) than Category II
criteria
(b) Most scientifically defensible and
(c) Most expensive to create
NESTLER ET AL.13
that physical conditions present at any given point actually do not
extend very far from that point (Payne, 2003). An alternative assump-
tion could have been made that defined WUA as the sum of the com-
posite suitabilities of individual measurement cross sections multiplied
by the area each cross section represented (area weighted suitability).
This is, in fact, how the computations are performed within the CIFSG
software. It also “roughly equates a larger area of marginal habitat to
an equivalent smaller area of optimal habitat”and has no effect on
the utility of the WUA concept itself.
The basic building block of an instream flow analysis is the WUA
versus discharge plot. However, by itself, this plot is not useful for
assessing the impacts of alternative flow regimes. Bovee (1986)
describes alternative methods based on a foundation of HSC curves
for deriving flow recommendations or assessment of flow‐related pro-
ject impacts useful in instream flow negotiation (Table 2).
3.4.3 |Problems and legacies of HSC curve and
hydraulic habitat modelling
Picking the peak of the curve as the instream flow
recommendation
It was not uncommon for natural resource agencies to use only habitat
optimization (pick highest point on WUA vs. discharge plot) to select
an instream flow requirement without consideration of naturally avail-
able streamflow. This action undermines the very purpose of an
instream flow analysis because all flows other than the optimum
would be eliminated from consideration during the determination of
an acceptable environmental flow, and no evaluation of alternatives
is possible.
Depth, mean velocity (in the vertical), and substrate are the
focus of modelling
From the beginning, these three variables were considered to be the
determinants of WUA values for each target species. Subjective assess-
ment of cover types has been added as a variable in numerous
instances, either alone or in combination with substrate, in the judge-
ment of the modelers. However, different species of fish (and other
taxa) may cue on different hydraulic variables depending upon their
swimming ability, position in the water column, body form, size, and
behaviour. Shear, turbulence, velocity gradient, water acceleration, or
any of a large number of different hydraulic variables could have been
considered in the development of HSC curves to better explain the dis-
tribution of aquatic organisms. For example, velocity gradient was
found to be a critical hydraulic variable for understanding the swim path
selection of emigrating juvenile salmon at dams (Goodwin et al., 2014;
Haro, Odeh, Noreika, & Castro‐Santos, 1998; Kemp, Gessel, & Williams,
2005; Nestler, Goodwin, Smith, Anderson, & Li, 2008) and may also be
important for habitat selection. This omission is understandable during
the initial development of HSC curves because neither measurement
nor simulation tools were sufficiently sophisticated for use in more
comprehensive habitat studies. By 1979, computational fluid dynamics
(CFD) models were available that could support more comprehensive
hydraulic modelling of streams (including velocity gradients; Lynch,
1983). By the early 1990s, very dense measurements of water flow pat-
terns were possible using acoustic Doppler equipment, thus negating
the need to use stream gauging methods for large rivers (Gordon,
1989). However, the field equipment required to record these parame-
ters and then use that information to construct HSC were expensive
and not widely available within many government agencies applying
these methods at that time.
In addition to hydraulic elements, numerous other habitat compo-
nents have not been incorporated into WUA computations. To name a
few, they could include the influences of competition, predation,
experience, territoriality, feeding behaviour, turbidity, schooling ten-
dencies, and many others (some of which were included in the
seldom‐used HABEF program of the PHABSIM system). Scale effects
in habitat analyses are also known to be important but have been gen-
erally ignored (Gaillard et al., 2010). For example, the same procedures
are used for creating and applying HSC for fishes of vastly different
sizes inhabiting the same river (e.g., minnows vs. sturgeon). Early stud-
ies on habitat description in wadeable streams serendipitously cor-
rectly approximated the scale of data collection to the likely scale of
habitat utilization. Sampling stations separated by about a metre or
less and with depths of a metre or less probably produce hydraulic
data that influence the behaviour of target fish about the size of adult
trout. The range of the fish mechanosensory system (used to acquire
hydrodynamic signals) varies with the source but is about one to
two body lengths for small vibrating spheres used to emulate prey
items and is greater for larger scale disturbances in the flow field
(Coombs & Montgomery, 2014). Without a measure of scale in habitat
TABLE 2 Definition and use of alternative methods for summarizing the application of HSC to obtain flow‐related project impacts (summarized
from Bovee, 1982, 1986)
Habitat type Definition/how created Use
Habitat optimization Mean monthly flow minimizing habitat reductions for all
species and life stages
Derive optimum instream flow recommendation
Habitat time series Integrate habitat vs. discharge function with time series
of discharge
Display & quantify impact by subtracting without‐project
from with‐project habitat time series
Effective habitat
time series
Compute habitat requirements for each life stage at
discrete times (called habitat ratios) to compile a
life table comparing required to available habitats
(a) Relate flow requirement for one month with all other months
(b) Estimate adult habitat over time
(c) Incorporate habitat utilization lags from extreme events
Habitat duration
curve
The percent of time a certain amount of habitat is
equalled or exceeded
Express impact as frequency rather than amount
14 NESTLER ET AL.
assessments, there is the potential for inconsistency between the
scales at which fish respond to physical habitat variables with the den-
sity of hydraulic information obtained from supporting hydraulic simu-
lation or field measurements.
Continuing the trend towards reductionism
The assumption that “all problems of increasing biocomplexity in
streams can be addressed by constructing more conditional HSC
curves”led to data intensive and cumbersome analyses with
difficult‐to‐interpret results. It also maintains the legacy assumptions
and procedures of earlier phases of development of the HSC curve.
The original assumptions and uncertainties made in the 1950s about
how to optimally describe rearing and nonanadromous adult habitats
were rarely tested, improved, or confirmed. Notable exceptions
include studies on rock bass and smallmouth bass (Bovee, Newcomb,
& Coon, 1994) and on macroinvertebrates (Jowett, Richardson, Biggs,
Hickey, & Quinn, 1991). No documentation of efforts was located that
evaluated the efficacy of using depth–velocity–substrate/cover HSC
curves to describe the habitat requirements of nonsalmonid species
in other parts of the USA or the world.
Treating habitat like a commodity
Habitat had to be considered as a commodity like bushels of corn or
tons of mine ore to support trade‐off analysis among competing uses
of water as part of equitable water resource allocation. However, the
original assumption that habitat could realistically be treated as a
tradeable commodity was never explored or tested. For example, if
habitat is doubled, then does that mean that there is a doubling in aes-
thetic value, fish production, river‐borne recreation, or any other
instream use?
3.5 |Period of Reflection and Criticism: 1987–2010
A number of documents were published during this time period that
review or critique the PHABSIM system either by itself or in compar-
ison to other environmental flow assessment methods. These docu-
ments are also statements about the HSC curve because it is at the
core of all of the microhabitat assessment methodologies.
3.5.1 |State, provincial (Canada), and country (New
Zealand) reviews
Early proliferation of methods with relatively little scientific review
created confusion in the minds of state and provincial resource man-
agers about assumptions, accuracies, costs, and complexities in each
method that are part of the IFIM. For the sake of completeness, we list
the following reviews and critiques (some apply to all methods includ-
ing PHABSIM, and some only to PHABSIM) performed for Newfound-
land (Bietz & Kiell, 1982), Oklahoma (CH2M Hill, 2013), South
Carolina (de Kozlowski, 1988), Georgia (Evans & England, 1995),
Nebraska (Hilgert, 1982), New Zealand (Hudson, Byrom, &
Chadderton, 2003; Irvine, Jowett, & Scott, 1987; Scott & Shirvell,
1987), prairie provinces of Canada (Instream Flow Needs Committee,
1998), British Columbia (Lewis, Hatfield, Chilibeck, & Roberts, 2004;
Neuman & Newcombe, 1977), Texas (Mallard et al., 2005), California
(Moyle, Williams, & Kiernan, 2011), Colorado (Nehring, 1979), Mon-
tana (Nelson, 1977), Nova Scotia (Shirvell & Morantz, 1983), Okla-
homa (Orth & Maughan, 1981), and Virginia (Vadas & Weigmann,
1993). Of the 18 reviews in this category, 13 were published by a
state or provincial agency, three were published in a peer‐reviewed
journals (Irvine et al., 1987; Orth & Maughan, 1981; Scott & Shirvell,
1987), another in Transactions of the Canadian Electrical Association,
Engineering, and Operating Division (a decidedly odd journal to pub-
lish a topic in applied aquatic ecology; Shirvell & Morantz, 1983),
and another published by the U.S. National Academies (Mallard
et al., 2005). As a group, these reviews are remarkably divergent in
their opinions about the scientific rigour of the PHABSIM system
(and, by extension, of the HSC curve). For example, Shirvell and
Morantz (1983, p. 11) identified a number of shortcomings and recom-
mended that “Practitioners should discontinue the uncritical applica-
tion of the incremental methodology for determining minimum
streamflows in rivers where the populations are not limited or regu-
lated by the availability of suitable habitat”but without describing
how a researcher would make such a determination. In contrast,
Hilgert (1982, p. 8) provided a generally useful and accurate review
concluding that “Although requiring a greater time and financial com-
mitment, the Incremental method provides the information needed for
responsibly resolving conflicts.”(Note that the author writes “Incre-
mental method”but means PHABSIM system).
3.5.2 |Reviews in peer journals and agency reports
Armour and Taylor (1991) published the results of surveys of IFIM
users in U.S. Fish and Wildlife Service Field Offices. The survey
responders and the authors (who were employed by the U.S. Fish
and Wildlife Service) agreed that HSC curves and computer models
should be improved and that the linkage between WUA and fish
response should be better described. The adequacy of the WUA index
as presently formulated to optimally predict aquatic habitat is unclear.
Mathur, Bason, Purdy, and Silver (1985), Conder and Annear (1987),
and Shirvell (1989) point out the inconsistent correlation between
WUA and standing crops of fish, although PHABSIM guidance docu-
ments all have the disclaimer that this correlation should only occur
if habitat is limiting. In contrast, Nickelson, Beidler, and Willis (1979),
Stalnaker (1979), and Wesche (1980) found positive correlations
between WUA and fish biomass, and Bourgeois, Cunjak, Caissie, and
El‐Jabi (1996) found a complex relationship dependent upon how
WUA was integrated over time and space. The question of how best
to accumulate microhabitat measurements into a robust and interpret-
able summary index that correlates to fish population abundance
remains to this day. The manipulative experiments required to link
WUA habitat to standing crop were never conducted during the
development of methods to create HSC or during the creation of
the PHABSIM system (Fausch et al., 1988) nor were methods other
than utilization considered in HSC development (e.g., foraging‐model
HSC, see Baker & Coon, 1997). These same questions of scientific
NESTLER ET AL.15
rigour still concern more modern reviewers of microhabitat assess-
ments including Arthington and Pusey (1993), King and Tharme
(1994), Tharme (1996), and Pusey (1998). Consistently, reviewers rec-
ommend that science, and not assumption, must be the basis for relat-
ing habitat to animal behaviour (Lancaster & Downes, 2010). Reviews
by Patten et al. (1979), Gore and Nestler (1988), Bain and Boltz (1989),
EPRI (2000), and Moyle et al. (2011) suggest a number of recommen-
dations to enhance the scientific underpinning of methods that use
HSCs as their core.
4|DISCUSSION
Hydraulic microhabitat assessment, particularly use of the PHABSIM
system, became commonplace throughout the world with the easy
availability of software and training. However, relatively little advance-
ments in the development and use of HSC occurred after 1985 as
HSC‐based assessment became institutionalized in many regions of
the world. Few, if any, of the issues and uncertainties raised by
reviewers and practitioners, were ever addressed. Below, we answer
the five questions posed at the beginning of this review to focus the
large amount of information available on the development and use
of HSC. Generally, we conclude that the scientific foundation for
HSC is insufficient. However, we believe that HSC‐based aquatic
assessments can still be a valuable tool to instream flow needs. We
end the discussion with four recommendations for studies to address
the science gaps that our review exposed.
4.1 |Question 1: What were the historical
motivations behind the HSC curve, and how did the
motivations influence its form and development?
We document an inexorable coupling of the development of the
HSC curve to the development and application of western USA
water law. The extension of beneficial use to include instream values
was the single most critical component of the western water law
that guided the acceptance of the HSC curve by parties involved
in water use negotiations. The outputs of flow assessment methods
based on the HSC curve allowed instream values to be negotiated
versus other beneficial uses of water. It even appears that research
into instream flow methods ceased once it became apparent that
HSC‐based methodologies could be used to estimate beneficial use
of instream flows.
The HSC curve was the winner among other methods that were
developed or considered during the 1960s and 1970s because of its
ability to quantify the beneficial use of instream flows. Initial compet-
ing methods to the HSC curve could be loosely categorized as hydro-
logic (e.g., the Tennant method), geomorphic (e.g., wetted perimeter;
reviewed in Mackey, Barlow, & Kernell, 1998), and statistical methods
in which a range of hydrologic and landscape metrics were evaluated
to explain fish abundance (e.g., Binns & Eiserman, 1979; Burton &
Wesche, 1974). The wetted perimeter method generally returns only
a single value or a few values and, consequently, is not incremental
over the full range of flows that have to be considered by a water
management agency (Estes & Orsborn, 1986). The Tennant method
(Tennant, 1976) and other hydrologic methods (Hoppe, 1975; Hoppe
& Finnell, 1970) had strong elements of regional application and pro-
fessional judgement and, therefore, were generally not considered to
be of equal rigour as methods used to estimate water needs for con-
sumptive uses (Stalnaker, 1982). Statistical methods could not esti-
mate “beneficial use”when flow was not identified as the most
important variable in determining standing crop of target fish. Impor-
tantly, regions having different water laws, social attitudes, or cultural
traditions from those of the western USA may not have developed an
instream flow assessment method based on the HSC curve.
4.2 |Question 2: What were the origin and stages of
development of HSC, and how well is the evolution of
HSC documented in the scientific literature?
Methodologically, the evolution of HSC can be separated into its
hydraulic and biological components. The hydraulic component of
HSC clearly originated from gauging methods developed by the Irriga-
tion Survey of the early U.S. Geological Survey. The origin and devel-
opment of the biological component of HSC are less clear because
much of the original documents were published in relatively obscure
sources that are no longer available. What is clear, however, is that
the progenitors of the HSC were developed for salmon spawning
and egg incubation because early workers thought that these life
stages limited salmon abundance. There was considerable scientific
work to document the effect of hydraulics on the success of these
two salmon life stages. The addition of HSC for juvenile and resident
adult salmon habitat at the beginning of HSC development appears
to be a simple extension of the methods used to quantify habitat
needs for salmon spawning and egg incubation and supported by rel-
atively few scientific studies.
The low priority given to further scientific development of HSC for
life stages other than salmon spawning and egg incubation may seem
odd from a scientific perspective but is understandable from a legal
and institutional perspective. The pressure to conduct additional sci-
entific studies to further develop HSC was reduced or eliminated once
it became clear that a methodology based on HSC could generate an
estimate of beneficial use of instream flows acceptable to the parties
involved in equitable water resources distribution. In addition, two
early technology developments by the CIFSG limited the further evo-
lution of HSC. The publication of guidelines for constructing HSC
curves (Bovee, 1978b) and creation of FISHFIL (FISHFIL, 1979), a
library of HSC curves for different species, life stages, and aquatic
activities, froze the format of HSC curves to the template used in
the early 1980s. The release of the PHABSIM system generally
retarded the evolution of HSC to form used in the early 1980s. In ret-
rospect, the CIFSG should have continued to pursue the scientific
evolution of HSC as a concept, even at the risk of jeopardizing existing
(at the time) institutional arrangements and legal requirements in the
western USA states.
16 NESTLER ET AL.
4.3 |Question 3: How should the output of an
analysis using HSC curves be interpreted? Can it really
be used to predict fish standing crop?
The WUA concept can be understood, and the many conflicting
reviews of the PHABSIM system can be partially reconciled by view-
ing HSC through the lens of Levins' thesis (Levins, 1966) developed
for biological population modelling. Habitat can be considered to be
one of many factors affecting population abundance that could poten-
tially be simulated in support of a comprehensive population model.
The process of population modelling (but maybe not population
models; Odenbaugh, 2003) can be categorized by how three critical
model desiderata (Table 3) are treated during development of a popu-
lation model (originally presented by Levens, 1966, and further
explained by Odenbaugh, 2003):
Levins argues that only two of the three attributes can be maxi-
mized for any specific model application to derive three model types:
1. Type I models maximize precision and realism at the expense of
generality,
2. Type II models maximize generality and precision at the expense of
realism, and
3. Type III models maximize generality and realism at the expense of
precision.
This inherent limitation in model formulation is tied to natural com-
plexity at the population level, wide array of interspecies interactions
in diverse biotic communities, and interconnected physico‐chemical
dynamics of the system in which populations occur. For example, a
mathematical model that attempts to exactly duplicate population
dynamics, and, therefore, maximize the three model attributes, would
necessarily require an enormous set of coupled differential equations
along with hundreds of model parameters; many of which would be
impossible to estimate. The immensity of the model would make it
computationally impractical, and the voluminous output would likely
be uninterpretable. Consequently, Levins argues that a single, best
all‐purpose model cannot exist for every practical application, so that
population models can only be improved as a pair‐wise progression.
Legal and institutional requirements derived from Western USA
water laws heavily influenced the selection of a Type II model
approach to determine instream flows. Model generality is important
because potential application sites may include every river in a state
or region (for federal agencies whose boundaries cover many states
like the US Forest Service) and must be understood by agency person-
nel. Similarly, model precision is required to make the model output
comparable with assessments for other beneficial uses of water as
part of effective trade‐off analysis among competing water uses (i.e.,
scenario planning; Rowland, Cross, & Hartmann, 2014). Model realism
is the least critical of the model desiderata in the legal‐institutional
framework within which the HSC curve was developed and initially
applied, even though later research showed that different model vari-
ables may be important in different regions (Shirvell, 1989).
Understanding the constraints of Levens' thesis leads to compre-
hension of Bovee's (1977, p. 29) restrictive definition of WUA as
“roughly a habitat's carrying capacity based on physical conditions
alone”and why influences on population processes other than habitat
were ignored. To predict fish biomass requires that many population
processes (e.g., those proposed by Railsback, 2016) be included in a
population model, in addition to physical habitat. However, to include
these additional nonphysical variables to increase the realism would
then violate Levins' model desideratum for generality and result in
what Levins derisively called “Fortran ecology.”A single approach that
included all populations could neither be used for many different sys-
tems nor be routinely understood by agency representatives who do
not have advanced training in population modelling.
4.4 |Question 4: How scientifically robust is the
HSC curve as a concept, and what are the new
research directions and applications?
The scientific robustness of HSC as the core of ecohydraulic flow
assessment or restoration tools is still an open question. The scientific
studies to establish HSC curves as a robust method for characterizing
aquatic habitat for all aquatic species, and all flowing aquatic systems
were never conducted. However, from the perspective of agencies
involved in the equitable distribution of water resources, there will
always be a need for a relatively simple tool, understandable and
acceptable to a range of stakeholders that can assess the value of
alternative instream flows by maximizing the generality and precision
model desiderata. The use of readily available gauging tools seems like
a reasonable approach for quantifying physical conditions in streams
because it is well accepted and many water resource professionals
are knowledgeable about the methods. Alternatively, many water
resources professionals are sufficiently knowledgeable with 2‐D CFD
TABLE 3 Comparison of model desiderata, their definitions, and the constraints imposed on each desideratum by western water law and
institutional constraints
Desiderata Definition Institutional & water law requirements
Generality Applies to many systems and species (a) Must apply to all rivers in a state or region
(b) Must be understood by agency representatives
Realism Has mechanistic fidelity to simulated
processes
Degree of required realism determined by representative(s) of a natural
resource agency
Precision Yields an exact answer Required for trade‐off analysis with other “beneficial uses”of water
NESTLER ET AL.17
modelling that numerical simulation can be substituted for field mea-
surement of hydraulic variables. However, we believe that HSC in
their present form can be substantially improved to capture physical
habitat requirements of a wide range of aquatic species more realisti-
cally. We identify three major factors that hindered the scientific
development of HSC.
1. Although methodological advances were made in the creation and
use of HSC, the foundational, detailed scientific studies to system-
atically evaluate the adequacy of traditional HSC for aquatic biota
in stream systems outside the western USA and for life stages
beyond salmon spawning and egg incubation were not performed.
The many research questions associated with development and
use of HSC identified by previous reviewers largely remain to be
addressed.
2. The HSC curve as a concept is frozen in time to the technologies
available in the early 1980s with a few notable exceptions (Miller,
2001; Hardy, Addley, & Saraeva, 2006; Miller, 2006). Technology
advancements since the early 1980s such as high‐resolution multi-
dimensional computational fluid dynamics modelling (other than 2‐
D vertically averaged codes; Leclerc, Boudreault, Bechara, & Corfa,
1995), acoustic Doppler current profilers to measure flow patterns
(first appearing in the early 1990's; Gordon, 1989; Simpson &
Oltmann, 1990), and advanced fish tagging methods have never
been used to investigate the optimum form of HSC for different
species. The development and use of HSC must be updated using
technologies developed since the 1980s. For example, formats
used to generate HSC that incorporate scale effects (e.g., Nestler
& Sutton, 2000), flexible weights for hydraulic variables and cover,
and additional hydraulic variables beyond depth and velocity were
seldom considered.
3. The history of HSC curves is closely tied to the social values and
cultural norms that eventually resulted in the passage of western
U.S. water laws. How well HSC would have been accepted in other
regions for these nonscientific reasons is unknown. HSC may not
have been the technology winner in competition with other
approaches in other regions of the world with different hydrology,
geomorphology, biota, cultural norms, social values, and legal
frameworks. We thus conclude that the assumption that one
method will meet all technology needs to estimate environmental
flows is not valid. However, the ideas associated with HSC curves
may be a useful starting point in other regions.
4.5 |Question 5: Can lessons learned from the
history of HSC development help guide the
development of future environmental flow methods?
The review by Bain and Boltz (1989) was particularly useful for iden-
tifying research needs to develop habitat assessment tools useful for
application in warm water streams. However, most of the reviews
did not consider program‐level recommendations that could only
emerge from an in‐depth review of the history of HSC. We believe
that it is time for a partnership of agencies, academics, and water
users to conduct research to develop environmental flow methods
that take advantage of the technological advancements made since
the early 1980s. Using the historical development of HSC described
in this paper as a guide, we make four major recommendations at a
program level for the development of microhabitat models that are
loosely based on HSC. These four recommendations can apply to
the development of an entirely new microhabitat assessment
method as well.
4.5.1 |Recommendation 1
Develop a set of objectives to guide methodology development,
modelled after the objectives listed earlier in this paper that resulted
in the selection of an HSC‐based approach in the western USA. The
objectives must
1. Identify the legal requirements and social and cultural values that
an environmental flow methodology must meet. As learned from
this review, social and institutional acceptance and compatibility
with legal requirements may outweigh the need for scientific
rigour.
2. Address the level of precision needed for trade‐off analysis among
environmental flows and other uses of water.
3. Develop a practical and usable approach to flow assessment by
identifying which two of the three of Levins' (1966) model
desiderata should be maximized. It may be wise to consider a
suite of models, with each model maximizing different model
desiderata.
4. Identify the time frame within which a methodology is needed
for decision‐making. A time frame of 1 year will require use of
an off‐the‐shelf tool with the possibility of making minor adjust-
ments. A time frame of five or more years allows sufficient time
for data collection and model development, testing, calibration,
and validation.
5. Describe the level of scientific rigour expected by the partners,
stakeholders, and regulators. For example, estimating expected
fish abundance or biomass associated with each of many alterna-
tives is a considerably more difficult task than ranking alternatives
by anticipated habitat loss. Although the former approach may be
plausible for a large system supporting high‐value aquatic biota, it
may not be feasible for application to a large number of smaller
individual streams because of funding constraints.
Application of the objectives may lead methodology developers to
select a method other than one that uses HSC at its core. For example,
there are statistical methods (e.g., IHA—Richter, Baumgartner, Powell,
& Braun, 1996) based on hydrologic concepts (Poff et al., 1997), which
may be more useful than an HSC approach in settings that do not
require demonstration of beneficial use.
18 NESTLER ET AL.
4.5.2 |Recommendation 2
Update HSC as both a tool and a concept by incorporating technolo-
gies not available in the early 1980s. As an example, position and
movement of fish can be monitored using high resolution acoustic tags
and overlaid onto the output of a high resolution computational fluid
dynamics model using the methods of Goodwin, Nestler, Anderson,
and Loucks (2006). Addressing key methodological questions should
be a primary goal of future environmental flow methods.
4.5.3 |Recommendation 3
Update the way HSC are developed at a program level using a “clean
sheet of paper”approach. The existing format for HSC's using only
two hydraulic variables (average velocity and depth) and one general
variable (cover) equally weighted and multiplied together to form a sin-
gle index per discharge is a very restrictive format for describing a bio-
logical process as complex as aquatic habitat selection. We recommend
a strategy parallel to the successful modelling approach used to develop
design guidelines for downstream passage of emigrating juvenile
salmon in the Pacific Northwest of the USA (Goodwin et al., 2014):
1. Use or develop a modelling framework such as the Eulerian‐
Lagrangian‐Agent Method (ELAM—Goodwin et al., 2006) to effi-
ciently integrate computational fluid dynamics modelling (Eulerian
component), descriptions of the movement paths and locations
of individual tagged fish (Lagrangian component), and a represen-
tation of fish cognition (agent component).
2. Build a strong mathematical and conceptual foundation for the
habitat modelling framework similar to that developed for the
movement model (e.g., Nestler et al., 2012; Nestler, Goodwin,
Smith, & Anderson, 2007).
3. Structure the agent component so that the modelled fish behav-
iour can be related to capabilities of the fish mechanosensory sys-
tem (Nestler et al., 2008) and principles of fluvial geomorphology
(Nestler et al., 2012) to ensure model fidelity to fish biology and
physical habitat structure. For example, Nestler et al. (2008) evalu-
ated 38 candidate hydraulic variables in their creation of defensi-
ble juvenile salmon movement rules.
4. Confirm the performance of habitat models (equivalent to HSC
curves) at different sites, times, and species as were used to con-
firm juvenile salmon movement rules (e.g., Weber, Goodwin, Li, &
Nestler, 2006).
5. Confirm the performance of the habitat summary variable (equiva-
lent to WUA) to forecast population responses to flow at different
sites, times, and species.
4.5.4 |Recommendation 4
Incorporate a holistic perspective into flow assessment by developing
hydraulic criteria for ecological and fluvial geomorphic processes. In
addition to defining physical habitat, hydraulic variables also govern
patterns of sediment deposition and erosion and transport and pro-
cessing of organic matter (Nestler, Baigun, & Maddock, 2016; Nestler,
Stewardson, Gilvear, Webb, & Smith, 2016). For example, the shear
stresses needed to erode or prevent deposition of fine sediments
and sand that may choke the channel or cover the substrate or fill
voids in the substrate can be depicted as a Fluvial Geomorphic Suit-
ability Criteria (FGSC) curve. The shear stresses required to remove
sediment already deposited can be depicted as a second form of the
FGSC curve. Another example is the water velocity or shear stress
required for settling of fine particulate organic carbon in flood plains
where it can contribute to the base of the food chain can be depicted
as an Ecological Process Suitability Criteria (EPSC) curve. Together
with HSC, FGSC, and EPSC can be used to manage physical channel
form and ecological processes.
We recommend that future researchers consider developing suit-
ability criteria for fluvial geomorphic and ecological processes medi-
ated by hydraulic variables. Such an approach could integrate the
flow related disciplines of hydromorphology (Orr, Large, Newson, &
Walsh, 2008), hydroecology (Wood, Hannah, & Sadler, 2007),
ecohydraulics (Nestler et al., 2007), ecogeomorphology (Thoms & Par-
sons, 2002), ecohydrology (Zalewski, Janauer, & Jolankai, 1997), and
ecohydromorphology (Clarke, Bruce‐Burgess, & Wharton, 2003;
Vaughan et al., 2009) to more fully meet the technology needs of river
managers (Gosselin, Ouellet, Harby, & Nestler, 2019). All of these dis-
ciplines are related by hydraulic variables associated with discharge
(depth and velocity), forces acting on the channel bottom (shear),
and consideration of sediment and carbon inputs and the composition,
erodibility, and transport of bottom sediments (Nestler, Stewardson,
et al., 2016). Such an approach is more holistic than traditional HSC
and would allow river scientists to consider fluvial geology and ecolog-
ical functions (Arthington, 2012) as well as habitat effects on individ-
ual species and guilds. Conservation of aquatic life in species rich
temperate and tropical flood plain rivers may be better approached
using relatively few FGSC and EPSC to restore or preserve important
river processes. Such an approach may be more plausible than
attempting to develop HSC for the often hundreds of species that
are known to occur in such systems.
Describing complex processes like channel evolution and biogeo-
chemical cycling into a graphic form similar to a HSC curve may be
overly simplistic from a scientific standpoint. Nonetheless, it may still
be a useful way to communicate scientific issues to nonscientific
stakeholders and partners. It could also be a useful way to develop
or refine conceptual models of the ecological and geomorpholic
impact of environmental flow alternatives.
5|CONCLUSIONS
The history of the development of the HSC curve demonstrates the
powerful influence of the concept of beneficial use of western USA
water law. We believe that technologies based on HSC are still valu-
able because they meet the two model desiderata of Levens' thesis,
NESTLER ET AL.19
generality, and precision that are critical to the development of flow
assessment tools widely useable by regulators and water users for
trade‐off analysis. We propose a number of recommendations to
improve HSC. Major recommendations include updating the way
HSC are developed and used with technologies and methods devel-
oped since the 1980s, creation of a family of models so that users
can match model complexity with funding level and study site com-
plexity, and expansion of the HSC curve to include fluvial geomorphol-
ogy concepts and, thereby, create a more holistic tool. These activities
should be performed under the umbrella of a major national or inter-
national program to minimize the undue influence of a single region
on tool development for, what is, an international challenge.
DATA AVAILABILITY STATEMENT
Data sharing is not applicable to this article as no new data were cre-
ated or analysed in this study. However, a number of references that
were well known during the early history of the HSC curve, but are
presently difficult to acquire, were obtained by the authors during
the preparation of this manuscript. Available references can be
obtained by request to the corresponding author (JMN).
ACKNOWLEDGEMENTS
We gratefully thank Drs. Michael C. Healey, University of British
Columbia, Canada; Marie‐Pierre Gosselin, Tallinn University of Tech-
nology, Estonia; and Mr. Brian Waters, retired from Pacific Gas and
Electric Company, USA, for their thorough reviews of early drafts of
this manuscript. Funding was provided by the U.S. Army Engineer
Research and Development Center by contract to LimnoTech, 501
Avis Drive, Ann Arbor, Michigan, USA 48108.
CONFLICT OF INTEREST
Neither Thomas Payne (fully retired), Robert Milhous (fully retired),
nor David Smith (scientist at the Engineer Research and Develop-
ment Center) have conflicts to declare regarding this manuscript.
John Nestler is partially retired and works part‐time for LimnoTech
assigned to and funded by the U.S. Army Engineer Research and
Development Center. He is also a partner in Fisheries and Environ-
mental Services, Partnership (FESP), where he consults with various
entities on a variety of environmental issues, primarily fish passage
and protection topics. He has no existing or planned work on envi-
ronmental flows.
ORCID
John M. Nestler https://orcid.org/0000-0001-9374-5412
David L. Smith https://orcid.org/0000-0002-3830-8356
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26 NESTLER ET AL.
