APPLICATION OF THE INDICATORS OF HYDROLOGICALTERATION SOFTWARE IN ENVIRONMENTAL FLOW SETTING1

Ruth Mathews and Brian D. Richter2

ABSTRACT: River scientists are increasingly asked to provide recommendations about the amount and timingof water flows needed to support ecosystem health. The need for scientifically credible environmental flowassessments and the limited availability of resources to conduct in-depth studies place a premium on methodsthat can be readily applied at low cost. The Indicators of Hydrologic Alteration (IHA) software program was ori-ginally developed by The Nature Conservancy in the 1990s to quickly process daily hydrologic records to enablecharacterization of natural water conditions and facilitate evaluations of human-induced changes to flowregimes. The evolution of the IHA software is discussed, including recent revisions and additions to the IHAthat have improved its utility in environmental flow-setting processes. Drawing from holistic methodologiesdeveloped around the world, the ability to calculate characteristics of five components of flow important to riverecosystem health – extreme low flows, low flows, high-flow pulses, small floods and large floods – has been addedto the IHA. A practical advantage of these environmental flow components is that an environmental flow pre-scription based upon them can be readily implemented in most water management settings. Frequently used asa one-time generator of flow statistics, the value of the IHA increases when used interactively with ecologicalmodels. A process for linking IHA output to an ecological model to explore flow-ecology relationships within anadaptive management context is presented and demonstrated through its application on the Green River, Wash-ington. The IHA used in conjunction with ecological models facilitates the creation and testing of flow-ecologyhypotheses, formulation of water and land protection or restoration goals, and development of a focused researchand monitoring program, all important components of environmental flow-setting processes.

(KEY TERMS: environmental flows; hydrologic statistics; indicators of hydrologic alteration; adaptive manage-ment; ecological models; instream flow; water resources management; hydrologic alteration; rivers ⁄ streams;surface water hydrology; sustainability.)

Mathews, Ruth, and Brian D. Richter, 2007. Application of the Indicators of Hydrologic Alteration Software inEnvironmental Flow Setting. Journal of the American Water Resources Association (JAWRA) 43(6):1400-1413.DOI: 10.1111/j.1752-1688.2007.00099.x

INTRODUCTION

Around the world, water managers, naturalresource agencies, governments, and water users are

being asked to consider the water flows needed tosupport riverine species and maintain essential eco-system processes, and then reserve or allocate waterto meet these needs. As river flows are depleted orotherwise altered, ecological degradation results and

1Paper No. J06030 of the Journal of the American Water Resources Association (JAWRA). Received February 15, 2006; accepted March 7,2007. ª 2007 American Water Resources Association. Discussions are open until April 1, 2008.

2Respectively, CEO, River Matters, 716 14th Street, Bellingham, Washington 98225; and Director, Global Freshwater Initiative, The Nat-ure Conservancy, 490 Westfield Road, Charlottesville, Virgina 22901 (E-mail ⁄ Mathews: riverspeaker@earthlink.net).

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society loses benefits provided by healthy, functioningecosystems, such as commercial and subsistence fish-eries, water purification, flood storage, recreation andesthetic values (Postel and Richter, 2003; Richteret al., 2003; Annear et al., 2004).

The societal demand for river ecosystem protectionhas led to increasing requests for river scientists toprovide recommendations about the amount and tim-ing of water flows needed to support ecosystemhealth. These requests are emerging from a diversearray of regulatory or policy arenas: dam re-licensingor re-operation, water permit applications, endan-gered species laws, water supply planning, etc. As aresult, the methods and tools – and even theoreticalframeworks – for environmental flow determinationare evolving rapidly.

In recent years, the central role of naturally varyingwater flows in maintaining river, floodplain and estua-rine health has been firmly established (Arthingtonet al., 1992; Sparks, 1995; Walker et al., 1995; Poffet al., 1997; Bunn and Arthington, 2002). River ecolo-gists now understand that the full range of naturalflow variation – ranging from base flows to high-flowpulses and floods – play important ecological roles in ariver ecosystem. This scientific knowledge presents asignificant challenge for parties involved in design-ing and implementing environmental flows, however.While it is widely accepted that a single minimum flowlevel is not sufficient to sustain river health, it has pro-ven quite difficult for scientists to describe the fullerrange of flow variability necessary to support a healthyriver ecosystem, and to do so in a manner that can bepractically implemented by water managers. Addition-ally, the resources available to conduct such analysesfor the many rivers experiencing development pres-sure are limited. The tension between the need for sci-entifically credible environmental flow assessmentsand limited resources places a premium on tools andmethods that can be readily applied at low cost.

A variety of methods have been developed for set-ting environmental flows; each has its strengths andweaknesses and requires varying levels of effort(Tharme, 2003). Some of these methods employ scien-tific expertise from a variety of disciplines and sophis-ticated computational models and tools. However,these approaches tend to be time-consuming andexpensive. They are most appropriate for in-depth,site-specific analysis of environmental flow needs.

On the other end of the spectrum are ‘‘desktop’’ or‘‘standard-setting’’ methods that can be readilyapplied at minimal cost, particularly when adequatehydrologic data are available. Among these are hydro-logically based standard-setting approaches, such asthe Tennant Method, the Aquatic Base Flow Stand-ard, and flow duration curve methods (Tharme, 2003;Annear et al., 2004). Each of these methods uses

hydrologic data to establish a flow rate that should bemet or exceeded, based upon statistical evaluation ofhistorical flows. Using these methods, environmentalflow targets can be developed for the year as a whole,or for individual seasons or months. These methodsare designed to protect some portion of the overallflow in a river (e.g., 30% of mean annual flow). Whileuseful for their ease of application, these methods, aswell as other commonly used standard-setting meth-ods, have been criticized because they do not ade-quately reflect the full range of variability in flowsthat is essential for sustaining river-dependentspecies and ecosystem processes for the long term(Tharme, 1996; Arthington and Zalucki, 1998; Braggand Black, 1999; Railsback, 2001; Annear et al. 2004).

Regardless of the environmental flow method selec-ted, it is very useful to characterize the naturallyvarying flow that existed in a river prior to substan-tial human influence; such characterizations provideinsight into the flow regimes to which native speciesand ecosystems have adapted. Comparisons of thenatural flow regime with current or projected condi-tions can shed light on the degree of departure fromnatural flow conditions that has already taken placeor is expected in the future. This can focus flow pro-tection and restoration activities, provide directionfor ecological research and monitoring, and identifypriority management actions.

In this paper, we discuss a new version of a hydro-logic assessment tool that can be used in conjunctionwith a variety of approaches for setting environmen-tal flows. In the early 1990s, The Nature Conser-vancy (TNC) developed a software program, calledthe Indicators of Hydrologic Alteration (IHA), to sup-port hydrologic evaluations (Richter et al., 1996,1997). This program, available at no cost from TNC,has been used by scientists and water managers inriver basins throughout the United States (U.S.) andworldwide (TNC, 2005, 2007). TNC recently madesubstantive enhancements to the IHA to include newcapabilities to support environmental flow assess-ments. In this paper, we discuss recent enhance-ments to the IHA, describe how to use the IHAinteractively with other ecological information toevaluate environmental flow needs, and demonstrateits application in the Green River in Washingtonstate, USA.

EVOLUTION OF THE INDICATORSOF HYDROLOGIC ALTERATION

The IHA was originally developed to enable rapidprocessing of daily hydrologic records to characterize

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natural flow conditions and facilitate evaluations ofhuman-induced changes to flow regimes. The pro-gram was designed to calculate the values of 33hydrologic parameters that characterize the intra-and inter-annual variability in water conditions,including the magnitude, frequency, duration, timingand rate of change of flows or water levels (Richteret al., 1996). Two primary criteria were used inselecting the original suite of 33 hydrologic parame-ters: their ecological relevance, and particularly, theiruse in published ecological studies; and their abilityto reflect human-induced changes in flow regimesacross a broad range of influences including damoperations, water diversions, ground-water pumping,and landscape (catchment) modification.

Values are computed for each of the 33 parametersfor each year of record, enabling users to assess theinter-annual variability and changes in each hydro-logic parameter for selected time periods or for theentire period of record. Users can conduct an impactanalysis using data from before and after an impact,such as dam construction (Figure 1), or perform atrend assessment of more gradual changes in hydro-logic conditions, such as those attributable to conver-sion of a landscape from forest to agricultural use.While less dramatic than the alterations evident inthe hydrograph after construction of a dam, trendassessments can identify parameters that have chan-ged over time. An IHA analysis can also be applied tooutput from hydrologic models that are used intesting future water management scenarios, or incomparing model-simulated naturalized flows withcurrent conditions. The IHA software facilitatesanalysis of variability and change in hydrologicparameter values over time by producing tabularsummaries and graphical output.

The value of the IHA is in its ease of operation, itsanalysis of the full range of variability in the hydro-graph, and its speed in producing visual summariesthat allow a rapid assessment of the natural orunregulated hydrograph and existing or proposedalterations. This can be quite useful when evaluatingmultiple sites within a basin or modeling numerouswater management scenarios. One must be careful touse the full range of output and analysis options toensure a comprehensive and accurate picture of thenatural hydrograph and flow alterations. User-defined seasons of analysis can be used to describeflow events that occur at different times of the yearin the same river basin and have distinct characteris-tics (e.g., floods resulting from rain or rain on snowand floods due to snowmelt). With repeated use of thesoftware in a variety of river basins, a user will beginto recognize ‘‘signatures’’ of particular climatic influ-ences (e.g., snowmelt or monsoons) and types of flowalteration (e.g., flood control, hydropower peaking,water withdrawal, etc).

On the other hand, the 33 hydrologic parameterscomputed by the IHA can be calculated using othersoftware programs. First time users may be over-whelmed with the number of analysis options and thevariety of output and may initially have difficultyinterpreting the results. Also, the IHA does not sup-ply all the statistics that could be useful in analyzingnatural hydrographs and flow alteration or in devel-oping environmental flow recommendations. It is bestwhen used in conjunction with other statistics (e.g.,flow duration curves, analysis of intra-daily flows, orcustomized parameters specific to a particular ecolog-ical setting, such as the ratio of spawning to incuba-tion flows for Chinook salmon).

Use of the IHA software spread very rapidly fol-lowing publication in 1996 of a scientific paper descri-bing its use and availability, and the program hasbeen widely used for evaluating current or proposedfuture conditions relative to the natural flow regime(Richter et al., 1996). Almost immediately, users ofthe software began asking the IHA’s architects fortheir recommendations on its utility for determiningenvironmental flow needs. While the IHA softwarehad proven useful in describing human-induced chan-ges in flows, users wanted to know how to determinehow much flow alteration was ‘‘too much.’’

In response, Richter et al. (1997) published a paperdescribing a ‘‘Range of Variability Approach’’ (RVA)for setting environmental flow targets based upon ariver’s natural flow variability. The RVA was incor-porated into the IHA software at the same time. TheRVA was expressly designed for application in situa-tions in which very little or no ecological informationis available to support environmental flow determin-ation; the RVA was intended for use in developing

FIGURE 1. An Example of Graphical Output From the Indicatorsof Hydrologic Alteration Software Showing Changes in MonthlyMedian Flows After Construction of Ross Dam on the Skagit River,Washington. Computed using daily flow data from USGS gauge12178000 at Newhalem, Washington.

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initial flow management targets to jump-start anadaptive flow management program in which know-ledge about flow-ecology linkages would accrue overtime and flow targets could be refined accordingly(Figure 2) (Richter et al., 1997; Richter and Richter,2000).

When applying the RVA, targeted ranges of valuesfor any or all of the 33 IHA parameters can bedefined based on analysis of daily flow records from atime period representing the natural flow regime (i.e.,negligible human impacts). When the RVA was firstincorporated into the IHA software, the programwould compute statistics describing the dispersion(i.e., one standard deviation, or the 25th and 75thpercentiles) of annual parameter values around theircentral tendency (means, medians), for each of the 33parameters, during the pre-impact or reference per-iod. Users could then adopt a targeted range of val-ues for each parameter, such as the 25th and 75thpercentiles, as the basis for an environmental flowtarget. For example, environmental flow targets canbe defined as maintaining parameter values withinthe 25th and 75th percentiles for 50% of the time.

As the RVA began to be used in many differentsettings, it was recognized that its application couldlead to unintended consequences in which the mid-range of the variability in hydrologic conditions couldbe preserved at the expense of extreme values (e.g.,the entire distribution of a hydrologic parametercould be shifted up or down while preserving theoccurrences within the RVA targeted range, therebylosing the naturally occurring low or high occurrencesof each parameter) (Figure 3). This realization led to

a revision of the RVA default. In its current version,the program parses the full range of natural vari-ation for each hydrologic parameter (based upon thepre-impact or reference period) into equal thirds or‘‘bins.’’ This capability strengthens the IHA’s abilityto assess alteration across the full spectrum of flowvariability. The default environmental flow target isthen expressed as managing flows such that theannual values for a parameter fall into each of thethree bins one-third of the time.

A number of factors have limited the utility of theRVA in developing environmental flow targets, how-ever. First and foremost is the issue of complexity.Water managers are quickly overwhelmed when pre-sented with 33 different environmental flow targets.In some cases, this problem has been overcome byusing only a subset of the 33 parameters deemed tobe most important in particular rivers. Secondly, it isvery difficult to manage a dam or other water infra-structure to attain some of the targets derived in theRVA. For example, water managers find it very diffi-cult to manage for a 30- or 90-day flow target whennatural climatic variation is generating widely vary-ing flows in the system. Finally, compliance with theRVA targets is determined retroactively instead of inreal time. In other words, the successful attainmentof the flow targets is not measured until the end ofeach year of implementation. This can create anuneasy situation for agencies and others who haveresponsibility for protecting natural resources, andcan raise significant concern when endangered spe-cies are involved and legal mandates require highlevels of certainty.

Step 4. Work with watermanagers, dam operators, orwater rights holders to identifyopportunities and approaches

for modifying water or landmanagement to restore or

protect natural flow conditions.

Step 6. Evaluate datacollected in step 5, and revisit

Steps 3-5 as feasible andnecessary.

Step 3. Develop hypothesesabout impacts to species or

ecosystem processesassociated with hydrologicalterations. Models can beused to depict and refine

these hypotheses.

Step 5. Design and implementan ecosystem monitoring

program capable of trackingbiotic responses to flow

restoration or degradation.Implement flow management

strategies.

Step 2. Quantify the degree towhich hydrologic conditions

have been (or will be) altered by human influences.

Step 1. Characterize natural range of variability inhydrologic conditions

(magnitude, frequency, timing,duration, rate of change).

FIGURE 2. The Six Steps in the Range-of-Variability Approach (RVA) Adapted From Richter and Richter (2000).

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These difficulties in applying the RVA in environ-mental flow assessment caused the IHA developers tobegin evaluating other ways of characterizing flowconditions that could be more easily translated intoenvironmental flow recommendations. In 2005, 34 newparameters called ‘‘environmental flow components’’(EFCs) were added to the IHA software. These newparameters were added to complement the original 33parameters and characterize the hydrograph in amanner that is representative of key flow-ecology rela-tionships. The theoretical basis for these new parame-ters, and their utility in setting environmental flowtargets, is discussed in the remainder of this paper.

ENVIRONMENTAL FLOW COMPONENTS

In recent years, river scientists in countriesaround the world have been forging new methodolo-gies for establishing environmental flows. Holisticapproaches, such as Benchmarking and the HolisticMethod in Australia (Arthington et al., 1992; Brizga

et al., 2002), and the Building Block Methodology andDRIFT in South Africa (King and Louw, 1998)address the complex nature of riverine ecosystems byevaluating the flows necessary to maintain the geo-morphic, hydraulic, biotic, and water chemistryaspects of a healthy river and floodplain ecosystem.These holistic methodologies each rely upon statisticalcharacterizations of key aspects of flow regimes. Thehydrological characterizations differ to some degree,but five major components of flow have repeatedlybeen considered as being ecologically important in abroad spectrum of hydro-climatic regions: extreme lowflows, low flows, high flow pulses, small floods, andlarge floods. These five components of flow regimeshave been incorporated into the new version of theIHA software as ‘‘environmental flow components.’’

The five flow components provide a heuristic frame-work for describing the ways in which an organismexperiences river flow variability. For instance, lowflows determine the amount and characteristics(e.g., temperature, flow velocity, connectivity, etc.) ofaquatic habitat that is available for most of the year.From this seasonally varying backdrop of low flows,several kinds of flow ‘‘events’’ may occur that typically

Roanoke River, NCBaseflow

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DAM CONSTRUCTION

FIGURE 3. Comparison of Pre-Dam (1912-55) Values for a ‘‘Base Flow Index’’ (7-day minimum flow ⁄ mean annual flow) With Post-Dam Val-ues (1957-2000) for the Roanoke River in North Carolina. Kerr Dam was constructed in 1956. Graph shows that while the number of occur-rences in the 25th-75th percentile range has remained fairly consistent, the number of occurrences greater than the 75th percentile valuehas increased from 11 prior to dam construction to 20 after dam construction. A comparable decline in occurrences in the lower values forbase flow can be seen as well. This IHA analysis is based on daily flow data from USGS gauge 02080500 at Roanoke Rapids, North Carolina.

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trigger responses in aquatic organisms. Depending onthe species, each of these events can be a stressfultime during which high rates of mortality may occur,or alternatively, may provide conditions essential forspecies persistence, such as habitat conditions neces-sary for reproduction. These events are further des-cribed as follows:

(1) Extreme low flows that occur during times ofdrought may alter water chemistry, concentrateprey species, dry out low-lying areas in thefloodplain, and are often associated with higherwater temperature and lower dissolved oxygenconditions. These very low flows can reduceconnectivity, thereby restricting movement ofsome aquatic organisms.

(2) High flow pulses occur during rainstorms orbrief periods of snowmelt. Water levels riseabove low flow levels but do not overtop thechannel banks. For many organisms, theseshort-term changes in flow may provide neces-sary respite from stressful low-flow conditions.These pulses of freshwater may relieve higherwater temperatures and low dissolved oxygenavailability typical of low flow conditions, flushwastes, and deliver organic matter that nour-ishes the aquatic food web. High flow pulses typ-ically facilitate improved access to upstream ordownstream areas for mobile organisms.

(3) Small floods are river flows that overtop the mainchannel banks and occur frequently (e.g., every2-10 years). These floods allow fish and othermobile organisms to access floodplains and habi-tats, such as secondary channels, backwaters,sloughs, and wetlands. These areas can providesignificant food resources allowing for fastgrowth, offer refuge from high-velocity, lower-temperature water in the main channel, or beused for spawning and rearing. Small floods mayrecharge shallow aquifers and the hyporheiczone, which are important to macroinvertebratesand riparian plants.

(4) Large floods occur rarely but play a critical rolein a river ecosystem. They can move significantamounts of sediment, large woody debris andother organic matter, form new habitats, andrefresh water quality conditions in both themain channel and floodplain water bodies. Thesefloods can scour spawning beds, transport organ-isms downstream, and remove vegetation frombars, islands and banks, all of which can beadvantageous for some species and problematicfor others.

Each of these flow conditions plays an importantrole in the long-term health of a river ecosystem.

Native organisms’ life histories are tied to the timing,magnitude, duration, and frequency of these events,and the rate of change between these events and thelow-flow baseline.

Importantly, an environmental flow prescriptionbased upon these EFCs reflects the full range of nat-urally varying river flows and can be readily imple-mented in most water management settings. Inunregulated systems, water managers may need topay attention only to the low-flow or extreme low-flowparameters, which are most vulnerable to increasinglevels of water extraction from rivers. In regulatedsystems, water managers are able to release waterfrom dams to attain targeted high-flow pulses andfloods at the desired magnitude, frequency, timing,duration, and rate-of-change.

The capability to assess the natural variability ofthe EFC characteristics, or changes or trends in theircharacteristics, has been added to the IHA software(Figure 4). The program now categorizes each dailyflow value, or strings of daily flows, into one of the fiveEFC categories, and then describes their magnitude,frequency, duration, timing and rate of rise and fall,as applicable (Table 1, Figure 5). Daily flows eithermeet the user-defined criteria for an event and arecategorized as a high flow pulse, small flood, largeflood or extreme low flow or they are categorized aslow flows (see Indicators of Hydrologic AlterationUsers Manual for calculation algorithm). The programsummarizes the central tendency and variability oflow flows on a month-by-month basis. A number offlexible user options exist for defining the EFCs, suchas defining the return intervals of ‘‘small’’ vs. ‘‘large’’floods, or defining the threshold percentile level (e.g.,10th percentile) of low flows that are categorized as‘‘extreme low flows.’’ If desired, user options can beemployed in the IHA software to focus on critical sea-sons, such as a known spawning period, to provide aseason-specific characterization of the EFCs. The IHAsoftware retains the capability to analyze trends overtime or changes between user-defined ‘‘pre-impact’’and ‘‘post-impact’’ time periods for each of the EFCs.In addition to expanding the IHA software to includecomputation of the EFCs, graphical and tabular out-put capabilities have been improved considerably.

USING EFCs IN DEVELOPING ENVIRONMENTALFLOW RECOMMENDATIONS

The IHA supplies a wealth of information that canbe used in environmental flow-setting processes,whether there is scant ecological knowledge or thecatchment has been well studied. Adherence to the

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adaptive management cycle articulated in the RVAremains important when using the EFCs to developenvironmental flow recommendations. To make full

use of the IHA it should be used in conjunction withconceptual or quantitative ecological models to createand test ecological hypotheses, identify flow protec-tion and restoration goals, focus research and monit-oring activities and prioritize land and watermanagement actions (Figure 2). This becomes an iter-ative process as lines of inquiry are explored, monit-oring results are assessed, and new information isused to refine the criteria defining the EFCs in theIHA.

The IHA has often been used as a one-time gener-ator of flow statistics, but its value increases signifi-cantly when it is used as an interactive tool inexplorations into flow-ecology relationships. The abil-ity for the IHA to quickly process flow data, com-bined with the range and flexibility of user-definedcontrols, enables IHA users to formulate testablehypotheses about flow-ecology relationships, identifydata gaps, and refine the description of flow-ecologylinkages. During the initial phases of an environ-mental flow-setting process, an IHA analysis ofunregulated or naturalized flows can be used tocharacterize the flow conditions and habitats towhich species have adapted (RVA Step 1). Reviewingthe graphic and tabular results for each of the fiveenvironmental flow components produces a picture ofthe general hydrologic character of a river andreveals important qualities of flow events, such asfloods or extreme low flows (Table 1). Much can belearned about the climatic influences on a river’shydrologic regime from this analysis. This hydrologiccharacterization establishes a foundational baselinefor developing initial hypotheses of flow-ecology link-ages (e.g., repeated flow events may trigger orenable certain activities, such as spawning or feed-ing). If human-induced hydrologic alteration hasoccurred, the IHA can compare regulated or altered

Environmental Flow Components

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FIGURE 4. The New Version of the IHA Software Categorizes Each DailyFlow Value Into One of Five ‘‘Environmental Flow Components’’ (EFCs).

TABLE 1. List of Environmental Flow Components (EFCs)That Can Be Used in Developing Environmental Flow

Recommendations, as Calculated by the IHA Software (TNC, 2007).

EnvironmentalFlow Component Definition IHA Statistics

Extreme low flows 10th percentileof all low flows

Mean or median values for:MagnitudeFrequencyDurationTiming(subtotal 4 parameters)

Low flow Low flow (baseflow) in eachmonth

Mean or median values for:Monthly low flows(subtotal 12 parameters)

High flow pulses Flows greaterthan low flowsbut less thanbankfull

Mean or median values for:MagnitudeFrequencyDurationTimingRate of rise and fall(subtotal 6 parameters)

Small floods Flows equal toor greater thanbankfull flowsbut less thanthe 10-yearflood

Mean or median values for:MagnitudeFrequencyDurationTimingRate of rise and fall(subtotal 6 parameters)

Large floods Flows equal toor greater thanthe 10-yearflood

Mean or median values for:MagnitudeFrequencyDurationTimingRate of rise and fall(subtotal 6 parameters)

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flows to the unregulated or naturalized flows toestablish the specific types of flow alteration thathave transpired and the extent of this alteration(RVA Step 2). This can further elucidate or under-score initial hypotheses if negative impacts to spe-cies have been observed.

The next step is to construct an ecological modelthat links life history traits and habitat needs for

species known to inhabit the river or riparian ecosys-tem to the EFCs (RVA Step 3). This is an importantstep at the beginning of an adaptive management pro-cess (Richter et al., 2006). The ecological model shouldreflect current ecological knowledge and document theknown or hypothesized relationships between species’life stages and the EFCs. This model should, to theextent possible, indicate the characteristics of the

FIGURE 5. Upper Graph: The Annual Frequency of High Flow Pulses (one of the five EFCs) in the Green River, Utah, Is Calculated for theYears Prior to Construction of Flaming Gorge Dam in 1963 and Compared to the Annual Frequency of High Flow Pulses After Dam Con-struction. Flaming Gorge Dam is operated for hydropower production. Computed using daily flow data from USGS gauge 09234500 at Green-dale, Utah. While not shown here, the IHA also calculates the magnitude, duration, timing and rate of rise and fall of the high flow pulses.Lower Graph: The Timing of Extreme Low Flows in the Colorado River in Austin, Texas Prior to the Construction of Mansfield Dam in 1942is Compared With Post-Dam Values Construction. Computed using daily data from USGS gauge 08158000 at Austin, Texas. While not shownhere, the IHA also computes the values for the magnitude, frequency and duration of extreme low flows.

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magnitude, timing, frequency, duration, and rate ofchange of particular flow components that are import-ant; doing so can help structure the initial formula-tion of an environmental flow recommendation(Figure 6). The model can guide the exploration offlow-ecology relationships through monitoring andresearch by making explicit existing knowledge anddata gaps (RVA Step 5). This also clarifies whichwater management targets may need to be revised inthe future as a result of the monitoring and researchprogram (RVA Step 6).

The output from the IHA, the ecological model,and the specific requirements of target species or eco-system processes become the building blocks fordeveloping an environmental flow recommendation. Itis fairly straightforward to translate the EFC outputfrom the IHA software into first-approximation watermanagement targets (Figure 7) (Richter et al., 2006).The IHA in itself does not provide an answer to thequestion of how much hydrologic alteration of anyone or combination of EFCs is too much. However, itis possible to develop statistical correlations betweenspecific values of the EFCs and ecological indicators,if the requisite ecological data are available.Otherwise, the determination of specific thresholdrequirements for flows should be based upon the IHAanalysis of unregulated or naturalized flows andavailable ecological information, supplemented withprofessional judgment. Flow protection and restor-ation goals can be set to maintain or restore criticalaspects of the flow regime as determined by the eco-logical model. In addition, by characterizing the typesand degree of hydrologic alteration, the results of theIHA are useful in setting water or land managementpriorities.

Regardless of the level of certainty about specificenvironmental flow requirements, these valuesshould always be refined through an adaptive man-agement program (Richter et al., 1997, 2006). Thelinkage between the IHA output and ecological mod-els can focus the development of the research andmonitoring component of an adaptive managementprogram. Studies and monitoring can be structuredto strengthen understanding of the relationshipsbetween individual flow components and their char-acteristics (i.e., their magnitude, timing, frequency,duration and rate of change, and species’ life histor-ies and habitat needs). As new learning occurs, theecological models and the environmental flowsshould be revisited and refined. This informationcan also be used to customize the IHA settings,keeping the software a part of the dynamic learningprocess.

GREEN RIVER, WASHINGTON: A CASE STUDY

The Green River flows 82 miles from the CascadeMountains to its historical confluence with the BlackRiver, where it becomes the Duwamish River andenters Elliot Bay in Puget Sound. Along its path, theGreen River travels through steeply sloped, forestedvalley into a wide, low-gradient alluvial valley with abroad floodplain and wetlands. Currently, land use inthe upper watershed is primarily forestry, the middleriver is flanked by a mix of residential, forestry, andagricultural land uses, and the lower watershed ishighly urbanized with residential, commercial andindustrial uses. In addition to land use influences onthe hydrologic system, other hydrologic alterationsinclude withdrawals for water supply, the completererouting of two tributaries away from the GreenRiver in the lower watershed and the construction ofHoward Hanson Dam in 1962. Howard Hanson Damis operated by the U.S. Army Corps of Engineers forflood control and augmentation of late summer lowflows. Much of the unconfined portion of the river hasrevetments or levees for bank stabilization and floodcontrol.

Historically, the mix of habitats available in theGreen River watershed, both in the main channeland its tributaries, supported populations of springand summer ⁄ fall Chinook, chum, pink, and cohosalmon, steelhead, bull and cutthroat trout, and poss-ibly sockeye salmon (King County et al., 2004). Theabundance and distribution of salmonids once foundin the Green River, has been greatly diminished. Pu-get Sound Chinook and bull trout are federally listedas threatened species under the Endangered Species

Spawning occurs inlate summer

Successful spawningrequires 2 weeks ofmoderate low flows inlate summer at leastevery two years

Small-bodied speciesneeds stable flows tomaintain position

Individual lifeexpectancy is 2 years

Spawning behaviorlasts two weeks

Spawning occurs inriffle habitat

TIMING

FREQUENCY

DURATION

MAG

NITUDE

RATE OF CHANGE

FIGURE 6. A Conceptual Ecological Model for aHypothetical Species Demonstrates That the Magnitude,

Duration, Frequency, Timing and Rate of Change of RiverFlows Can All Be Influential in a Single Life Stage.

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JAWRA 1408 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION

Act. In light of these trends in salmonid populations,and with the understanding that altered flow regimeshave impacted both the quality and quantity of phys-ical habitat essential to salmon recovery, KingCounty (Washington) initiated the Normative FlowStudies Project. This project is aimed toward develop-ing ecological indicators that will assist the County indesigning management activities that will supportnative, diverse, self-sustaining salmonid populations(http://dnr.metrokc.gov/wlr/BASINS/flows/).

As part of this project, King County, the U.S. Geolo-gical Survey and the U.S. Army Corps of Engineersare collaborating with other agencies, tribes and non-governmental organizations on a Middle Green FlowInvestigation Study. The purpose of this study is todevelop ecological indicators for the portion of theGreen River downstream of Howard Hanson Dam.A technical group began meeting in 2003 and isguiding the development of the study plan and its

implementation. The IHA is being used to characterizethe flow regime as part of a retrospective study thatwill establish the baseline from which to compare therates of change and spatial distribution of particulargeomorphic and biologic processes and structure asthey are influenced by the distinct flow regimes of thepre-Dam and post-Dam river. The IHA also will beused to design monitoring and research activities thatcan inform water and land management decisions.

LINKING ENVIRONMENTAL FLOWCOMPONENTS TO A CONCEPTUAL

ECOLOGICAL MODEL

Characterizing river ecosystems in an ecologicalmodel can quickly reach a level of complexity that

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

Ecosystem Flow RecommendationsSavannah River, below Thurmond Dam (River-Floodplain Segment)

Low Flows

High FlowPulses

Floods

3,000 cfs; 3 successive years every 10-20 years• Floodplain tree recruitment

<5,000 cfs• Adequate floodplain drainage

• Create shallow water habitat for small-bodied fish

>8,000 cfs• Larval drift for pelagic spawners

50,000-70,000 cfs; 2 weeks, avg every 2 yrs• Maintain channel habitats

• Create floodplain topographic relief• Provide fish access to the floodplain

• control invasive species• Maintain wetlands and fill oxbows and sloughs

• Enhance nutrient cycling & improve water clarity• Disperse tree seeds

<13,000 cfs; 3 successive years, every 10-20 years• Floodplain tree recruitment

8,000-12,000 cfs;• Exchange water with oxbows

20,000-40,000 cfs; 2-3 days, 1/month• Provide predator-free habitat for birds

• Disperse tree seeds• Transport fish larvae

• Flush woody debris from floodplain to channel• Floodplain access for fish• Fish passage past NSBLD

>30,000 cfs;5 pulses, >2 days with 2 eventsof 2 week duration (March and early April)

Key

Dry Year

Avg Year

Wet Year

FIGURE 7. Environmental Flow Components Were Used in Developing Flow Recommendationsfor the Savannah River in Georgia and South Carolina (from Richter et al., 2006).

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JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 1409 JAWRA

minimizes their utility in examining flow-ecologyrelationships. To understand specific flow-ecologyrelationships, it is often useful to elucidate portionsof the overall ecosystem model in finer detail. Forexample, each phase of the river portion of the lifecycle of Chinook salmon in the Green River can beexamined independently (e.g., adult migration,spawning, egg incubation, etc). By examining individ-ual life stages and their linkages to flows, the specificenvironmental flow components influencing a lifestage can be identified. The IHA can then be used tocharacterize the magnitude, duration, timing, fre-quency and rate of rise and fall of the EFCs for eachlife stage that, in sum, supported healthy populationsof salmon. In the Green River, adult summer ⁄ fallChinook migrate upstream from mid-June to earlyNovember, with the peak occurring during August. Ithas been observed that migration is initiated inresponse to high flow pulses (Figure 8) (Ruggeroneet al., 2004). Using data from the pre-Dam period ofrecord and flexible user options in the IHA softwarethat limit the season for analysis, high-flow pulsesduring the adult migration period of mid-June toearly November were characterized (Figure 9). Thisoutput from the IHA describes the characteristics offlow that adult Chinook salmon may be responding toas they initiate their upstream migration (RVA Step1).

Another use of the IHA is to compare pre-impactflow records with post-impact flow records to deter-mine which EFCs have been altered and in whatmanner or degree (RVA Step 2). This will help

identify flow-ecology relationships, which need fur-ther investigation by identifying specific hydrologicalterations that may impact particular species orecological processes. For example, the IHA analysisof the Green River indicates that the flood regime,including both small and large floods, has beenaltered (Figure 10). This occurs because HowardHanson Dam is operated for flood control and theCorps of Engineers is managing water flows fromthe dam to stay below a 340 cubic meter per secondthreshold at the Auburn gauge. Returning to thelife stages of Chinook salmon, the next step is toask which of these life stages are affected by ordependent upon the flood regime. This may revealmyriad ways that the success of a species could beinfluenced by a specific EFC (RVA Step 3). Forexample, juvenile salmon can be found in the riverduring the spring flood (snowmelt) period. Thesefloods influence the young salmon in at least twoways, by transporting juveniles downstream and byproviding access to floodplain, off-channel, or back-water habitats (Ruggerone et al., 2004). During thisperiod, juveniles are in search of prey to build bodyweight before entering the ocean environment.Depicting the various linkages between the floodregime and access to and abundance of prey speciesfor juveniles in an ecological model will clarifypathways to investigate through monitoring andresearch activities.

In the Green River, as in many other rivers,there are many influences affecting the flow regimeas well as other essential habitat characteristics(e.g., size and quantity of sediment). These influen-ces, including dam operations, water withdrawals,and a variety of land uses, benefit human popula-tions, which may limit the feasibility or desirabilityof recovering the full range of naturally varyingriver flows. However, the depiction of naturallyvarying river flows and the manner and degree ofhydrologic alteration can establish a foundation fordecisions supporting flow protection or restorationgoals. In some cases, protection or restoration ofthe naturally varying river flows will be possibleand recommended. In other cases, it may benecessary to understand the key characteristics offlow-ecology relationships as they occurred in thepre-impact river and look for ways to mimic theseto the greatest extent possible within the con-straints of human-induced alterations to the river.In either case, the use of the IHA in conjunctionwith ecological models that illustrate the essentialcharacteristics of flow-ecology linkages and examinehow hydrologic alterations may be affecting speciesand ecological processes can assist water and landmanagers in prioritization of restoration and protec-tion activities.

ADULT FISHIN ESTUARY AND LOWER

RIVER

HIGH FLOWPULSE CUESMIGRATION

RAINFALL

ADULTMIGRATION

FIGURE 8. Adult Chinook Salmon in the Green River,Washington, Initiate Upstream Migration During High Flow Pulses.

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JAWRA 1410 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION

SUMMARY

The IHA has already proven useful for characteriz-ing unaltered flow regimes and human-inducedchanges to river flows. Expanding the number ofparameters computed in the new version of the IHAto include the new suite of ‘‘EFCs’’, as well as otherenhancements, significantly increases the utility ofthe IHA in any process to develop environmental flow

recommendations. In addition to simply characteriz-ing hydrologic variability and changes over time, theoutput from the IHA software, in particular the EFCsand their intra- and inter-annual characteristics,become the building blocks of flow-ecology modelsthat lead to environmental flow recommendations,monitoring and research programs, and flow restor-ation and protection activities. New flexibility in theIHA software enables users to explore specific flow-ecology relationships and alterations in the EFCs

Green River, WashingtonHigh Flow Pulse (Peak)

0

20

40

60

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100

120

1936

1938

1940

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1946

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s)

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Green River, WashingtonHigh Flow Pulse (Duration)

0

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Green River, WashingtonHigh Flow Pulse (Timing)

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Green River, WashingtonHigh Flow Pulse (Frequency)

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Green River, WashingtonHigh Flow Pulse (Rise Rate)

0

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)

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Green River, WashingtonHigh Flow Pulse (Fall Rate)

-14

-12

-10

-8

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-4

-2

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e (c

ms/

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)

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FIGURE 9. IHA Analysis of Daily Flow Data From USGS Gauge 12113000 on the Green River in Auburn,Washington, Prior to Construction of Howard Hanson Dam in 1962 Reveals the Historical Character of High Flow

Pulses. This analysis was restricted to the Chinook salmon adult migration season (mid-June-early November).

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JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 1411 JAWRA

help explain changes in ecological conditions or spe-cies populations. Environmental flow-setting proces-ses are greatly enhanced through the iterative use ofthe new version of the IHA in conjunction with eco-logical models. While not replacing in-depth ecologi-cal studies, the IHA used in the manner described inthis paper is both a cost-effective method for deter-mining first iteration environmental flows and afoundation for an adaptive management process thatwill revisit and refine environmental flow recommen-dations over time. Environmental flow recommenda-tions based on the EFCs are in a form that canreadily be implemented by water managers. This

method should help river scientists and water manag-ers to establish river flow protection and restorationgoals that will support healthy rivers and the diversearray of societal benefits they provide.

ACKNOWLEDGMENTS

We would like to express our gratitude for the many years ofhard work and creative energy Chuck Smythe put into the devel-opment of the IHA software. We wish him a long, happy retire-ment. Our thanks go to Gary Brown of Totten Software Designfor writing the code for the new interface. We also commendTom FitzHugh for his dogged pursuit of computational discrepan-cies and for shepherding this version of the IHA through itsdevelopment.

LITERATURE CITED

Annear, T., I. Chisholm, H. Beecher, A. Locke, P. Aarrestad,C. Coomer, C. Estes, J. Hunt, R. Jacobson, G. Jobsis,J. Kauffman, J. Marshall, K. Mayes, G. Smith, R. Wentworth,and C. Stalnaker, 2004. Instream Flows for Riverine ResourceStewardship, Revised Edition. Instream Flow Council, Chey-enne, Wyoming.

Arthington, A.H. and M.J. Zalucki, 1998. Comparative Evaluationof Environmental Flow Assessment Techniques: Review ofMethods. Land and Water Resources Research and Develop-ment Corporation, Canberra, Australia.

Arthington, A.H., S.E. Bunn, B.J. Pusey, D.R. Bluhorn, J.M. King,J.A. Day, R.E. Tharme, and J.H. O’Keefe, 1992. Development ofan Holistic Approach for Assessing Environmental FlowRequirements of Riverine Ecosystems. In: Proceedings of anInternational Seminar and Workshop on Water Allocation forthe Environment. John. J. Pigram and Bruce. P. Hooper (Edi-tors). The Centre for Water Policy Research, Armidale, Austra-lia, pp. 69-76.

Bragg, O.M. and A.R. Black, 1999. Anthropogenic Impacts on theHydrology of Rivers and Lochs. Stage 1 Report: LiteratureReview and Proposed Methods. University of Dundee, Scotland.

Brizga, S.O., A.H. Arthington, B.J. Pusey, M.J. Kennard, S.J.Mackay, G.L. Werren, N.M. Craigie, and S.J. Choy 2002. Bench-marking, a ‘Top-Down’ Methodology for Assessing Environmen-tal Flows in Australian Rivers. In: Proceedings of theInternational Conference on Environmental Flows for Rivers.Southern Waters Consulting, Cape Town, South Africa.

Bunn, S.E. and A.H. Arthington, 2002. Basic Principles and Ecolog-ical Consequences of Altered Flow Regimes for Aquatic Biodi-versity. Environmental Management 30:492-507.

King County Water and Land Resources Division [King County],WRIA 9 Watershed Coordination Services and WRIA 9 Techni-cal Committee, 2004. WRIA 9 Strategic Assessment Report –Scientific Foundation for Salmonid Habitat Conservation.Department of Natural Resources and Parks, Seattle, Washing-ton, 153 pp.

King, J.M. and D. Louw, 1998. Instream Flow Assessments forRegulated Rivers in South Africa Using the Building BlockMethodology. Aquatic Ecosystem Health and Management1:109-124.

Poff, N.L., J.D. Allan, M.B. Bain, J.R. Karr, K.L. Prestegaard, B.D.Richter, R.E. Sparks, and J.C. Stromberg, 1997. The NaturalFlow Regime: A Paradigm for River Conservation and Restor-ation. BioScience 47:769-784.

FIGURE 10. Alterations of the Magnitude, Rise Rate, and FallRate of Small Floods After Construction of Howard Hanson Dam in1962. Computed using daily flow data from USGS gauge 12113000in Auburn, Washington. Flows have not reached the pre-Dam10-year flood level since dam construction.

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JAWRA 1412 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION

Postel, S. and B. Richter, 2003. Rivers for Life: Managing Waterfor People and Nature. Island Press, Washington, D.C.

Railsback, S., 2001. Instream Flow Assessment Methods: Guidancefor Assessing Instream Flow Needs in Hydropower Relicensing.Electric Power Research Institute, Palo Alto, California.

Richter, B.D., J.V. Baumgartner, J. Powell, and D.P. Braun, 1996.A Method for Assessing Hydrologic Alteration Within Ecosys-tems. Conservation Biology 10:1163-1174.

Richter, B.D., J.V. Baumgartner, R. Wigington, and D.P. Braun,1997. How Much Water Does a River Need? Freshwater Biology37:231-249.

Richter, B.D., R. Mathews, D.L. Harrison, and R. Wigington, 2003.Ecologically Sustainable Water Management: Managing RiverFlows for Ecological Integrity. Ecological Applications 13:206-224.

Richter, B.D. and H.E. Richter, 2000. Prescribing Flood Regimes toSustain Riparian Ecosystems Along Meandering Rivers. Conser-vation Biology 14:1467-1478.

Richter, B.D., A.T. Warner, J.L. Meyer, and K. Lutz, 2006. A Colla-borative and Adaptive Process for Developing EnvironmentalFlow Recommendations. River Research and Applications22:297-318.

Ruggerone, G.T., D. Weitkamp, and W.R.I.A. 9 Technical Commit-tee. 2004. WRIA 9 Chinook Salmon Research Framework: Iden-tifying Key Research Questions About Chinook Salmon LifeHistories and Habitat Use in Middle and Lower Green River,Duwamish Waterway and Marine Nearshore Areas. Preparedfor WRIA 9 Steering Committee. Prepared by Natural ResourcesInc., Parametrix, Inc., and the WRIA 9 Technical Committee.Seattle, Washington, 118 pp.

Sparks, R.E., 1995. Need for Ecosystem Management of Large Riv-ers and Their Floodplains. BioScience 45:169-182.

Tharme, R.E., 1996. Review of International Methodologies for theQuantification of the Instream Flow Requirements of Rivers.Department of Water Affairs and Forestry, Pretoria, SouthAfrica.

Tharme, R.E., 2003. A Global Perspective on Environmental FlowAssessment: Emerging Trends in the Development and Applica-tion of Environmental Flow Methodologies for Rivers. RiversResearch and Application 19:397-441.

TNC (The Nature Conservancy), 2005. Flow Restoration Database.The Nature Conservancy, Charlottesville, Virginia. http://www.nature.org/initiatives/freshwater/conservationtools/, accessedJanuary 2007.

TNC (The Nature Conservancy), 2007. User’s Manual for the Indi-cators of Hydrologic Alteration (IHA) Software. The NatureConservancy, Charlottesville, Virginia. http://www.nature.org/initiatives/freshwater/conservationtools/art17004.html, accessedJanuary 2007.

Walker, K.F., F. Sheldon, and J.T. Puckridge, 1995. A Perspectiveon Dryland River Ecosystems. Regulated Rivers 11:85-104.

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