how dams vary and why it matters for the emerging science of

August 2002 / Vol. 52 No. 8 BioScience 659

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Dams are structures designed by humans to capturewater and modify the magnitude and timing of itsmovement downstream. The damming of streams and rivershas been integral to human population growth and techno-logical innovation. Among other things, dams have reducedflood hazard and allowed humans to settle and farm pro-ductive alluvial soils on river floodplains; they have harnessedthe power of moving water for commerce and industry; andthey have created reservoirs to augment the supply of waterduring periods of drought. In the 5000 or so years that hu-mans have been building dams, millions have been con-structed globally, especially in the last 100 years (Smith 1971,WCD 2000).

If dams have successfully met so many human needs, whyis there a growing call for their removal? The answers to thisquestion require an appreciation of societys changing needsfor, and concerns about, dams, including the emerging recog-nition that dams can impair river ecosystems (Babbit 2002).But decisions about dam removal are complex, in no small partbecause great scientific uncertainty exists over the potentialenvironmental benefits of dam removal. Certainly, the scarcityof empirical knowledge on environmental responses to damremoval contributes to this uncertainty (Hart et al. 2002). Morefundamentally, however, a scientific framework is lacking forconsidering how the tremendous variation in dam and riverattributes determines the ecological impacts of dams andthe restoration potential following removal. Such an ecolog-ical classification of dams is ultimately needed to support theemerging science of dam removal.

In this article, we develop a conceptual foundation for theemerging science of dam removal by (a) reviewing the waysthat dams impair river ecosystems, (b) examining criteriaused to classify dams and describing how these criteria are oflimited value in evaluating the environmental effects of dams,(c) quantifying patterns of variation in some environmentallyrelevant dam characteristics using governmental databases,

(d) specifying a framework that can guide the developmentof an ecological classification of dams, and (e) evaluatingthe ways that dam characteristics affect removal decisions andthe future of dam removals. We restrict our analysis to theUnited States, where dam removals are currently hotly debated;however, the ecological framework we advocate could also begeneralized to other parts of the world.

How dams impair river ecosystemsAlthough the rationale for dam removal often includes arange of social and economic concerns (RAW/TU 2000), thecentral justification for removing dams from an environ-mental perspective is that they adversely impact the structureand function of river ecosystems. Both individually and cu-mulatively, dams fundamentally transform river ecosystems

N. LeRoy Poff (e-mail: [email protected]) is an associate professor atColorado State University, Fort Collins, CO 80523, where he teaches andconducts research in riverine ecology. David D. Hart (e-mail: [email protected]) is an ecologist, academy vice president, and director of thePatrick Center for Environmental Research at the Academy of Natural Sciences,Philadelphia, PA 19103. 2002 American Institute of Biological Sciences.

How Dams Vary and Why It Matters for the EmergingScience of Dam Removal

N. LEROY POFF AND DAVID D. HART

AN ECOLOGICAL CLASSIFICATION OF DAMS

IS NEEDED TO CHARACTERIZE HOW THE

TREMENDOUS VARIATION IN THE SIZE,

OPERATIONAL MODE, AGE, AND NUMBER

OF DAMS IN A RIVER BASIN INFLUENCES

THE POTENTIAL FOR RESTORING

REGULATED RIVERS VIA DAM REMOVAL

660 BioScience August 2002 / Vol. 52 No. 8

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in several ways: (a) They alter the downstream flux of waterand sediment, which modifies biogeochemical cycles as wellas the structure and dynamics of aquatic and riparian habi-tats. (b) They change water temperatures, which influencesorganismal bioenergetics and vital rates. (c) And they createbarriers to upstreamdownstream movement of organismsand nutrients, which hinders biotic exchange. These funda-mental alterations have significant ecological ramifications ata range of spatial and temporal scales.

Local effects. The local, or site-specific, alterations causedby dams, especially very large dams, have been studied ex-tensively over the last few decades (Ward and Stanford 1979,Petts 1984, Ligon et al. 1995, Collier et al. 1996, Pringle et al.2000). Storage of water and capture of sediment by dams causeprofound downstream changes in the natural patterns ofhydrologic variation and sediment transport. Numerous eco-logical adjustments follow. For example, reduction in themagnitude of downstream peak flows typically isolates themain channel from the floodplain, resulting in reduced re-cruitment of riparian species (Scott et al. 1996) and reducedaccess to floodplain habitats for fishes (Bayley 1995). Long-term storage and nonseasonal release of floodwaters can se-verely alter downstream food webs and aquatic productivity(Wootton et al. 1996). Many hydropower dams operate to pro-duce dramatic daily flow variation that effectively reducesdownstream habitat and aquatic productivity (see Poff et al.1997 for examples). Water released from the reservoir maycarve into the downstream river channel as it reestablishes itstransport capacity, causing channel incision and isolating itfrom adjacent floodplains or tributary outlets (Petts 1984, Col-lier et al. 1996). Fine sediments are preferentially transported,often resulting in an excessive coarsening and armoring of theriverbed and a reduction in habitat quality for bottom-dwelling organisms.

If reservoirs exceed a certain depth and flows are slowenough, thermal stratification can occur. Deep waters can havevery different temperatures than those on the surface, oftenmaintaining temperatures near 4oC. Thus, downstream fromreservoirs that release this deep water, the thermal regime ischaracteristically summer cool, winter warm. Because tem-perature directly affects the growth and developmental ratesof aquatic organisms, such altered thermal regimes greatlymodify the densities and kinds of species present. This newdownstream regime is favorable for cold-adapted species liketrout, and warm-adapted species often diminish in abundanceor are lost (Ward and Stanford 1979). Thermal alterationand biological disruption can persist for tens of kilometers(km) downstream, depending on downstream tributary in-flows (Muth et al. 2000).

Landscape effects. Dams occur so frequently in manywatersheds that their cumulative ecological effects are likelyto be profound, although this idea has received less attentionthan studies of individual dams. For example, Benke (1990)reported that there are only 42 high-quality, undammed

rivers longer than 200 km remaining in the continentalUnited States, and Wisconsin has an average of one dam forevery 14 km of river (WDNR 1995). The extensive frag-mentation of free-flowing rivers promotes ecosystem isola-tion. The imperiled status of many salmon stocks in the Pa-cific Northwest is in part attributable to the gauntlet of damsthese fish encounter in their migrations to and from theocean (NRC 1996). Fragmentation also prevents the disper-sal and persistence of inland species. For example, the diver-sity of European riparian communities is probably reducedbecause of the interruption by multiple dams of the down-stream transport of water-dispersed seeds (Nilsson andBerggren 2000). Prevention of exchange among isolated pop-ulations may also imperil inland fish populations and otherspecies such as mussels (Pringle et al. 2000, Fausch et al.2002).

Water storage and sediment capture by thousands of damshas also measurably altered earth surface processes at re-gional and global scales (Graf 1999, Rosenberg et al. 2000).For example, the suspended-sediment loads carried by theMississippi River to the Gulf of Mexico have decreased by one-half since the Mississippi Valley was first settled by Euro-pean colonists, mostly from the construction since 1950 oflarge reservoirs on the sediment-laden Missouri and Arkansasrivers (Meade 1995). Other associated cumulative effects ofdams that have either been demonstrated or postulated in-clude alteration of sea level (Chao 1991), generation of green-house gases (St. Louis et al. 2000), and disruption of the hy-drologic flux to the oceans (Sahagian 2000).

Criteria used to describe dams and their scientific limitationsSeveral criteria are used to characterize dams from an engi-neering perspective. Some of these criteria bear more stronglyon the issue of dam removal and river restoration than oth-ers. Chief among these are the size of a dam, its operationalpurpose, and its age. Dam size not only influences such en-gineering considerations as construction and repair costs, italso affects the potential range and magnitude of ecologicaldisturbances to the aquatic ecosystem (ASCE 1997). A damsoperational plan influences the type, magnitude, frequency,and timing of environmental impacts on the riverine ecosys-tem. The age of a dam can affect structural repair costs, as wellas the cumulative magnitude of downstream channel alter-ation because of sediment accumulation within the im-poundment. Traditionally, dam size and operational typehave been discussed among engineers in simple categoricalterms, such as small versus large dams, or storage versus run-of-river dams. In reality, these characteristics are more con-tinuous and multidimensional, and it will be important to an-alyze and synthesize this complexity in developing anecological classification to support the emerging science ofdam removal.

Dam size. Structures have generally been small for most ofthe history of dam building, reflecting preindustrial techni-

cal skills and agrarian social needs. During the 19th and 20thcenturies, however, new technologies allowed the construc-tion of much larger and more complicated structures to gen-erate hydroelectricity, control floods, provide drinking water,support large-scale irrigation, and improve navigation (Smith1971, Schnitter 1994). In the United States, the pace of dambuilding accelerated dramatically after World War II, thoughrelatively few dams have been constructed in the last 10 to 20years (Graf 1999). It is during this period of building largedams that the burgeoning scientific understanding of theenvironmental impacts of river regulation has developed,with its focus on the large structures that dramatically alterriverine ecosystems. Yet most of the dams on the planet arerelatively small structures, and evaluation of their environ-mental impacts is critical to the issue of dam removal.

Dams vary tremendously in size(height and width) and hence intheir reservoir storage volume, fac-tors that have very important directand indirect environmental impacts(see below). Thus it is very temptingto use size as a primary descriptor ofa dams potential ecological impact.Unfortunately, the criteria used bygovernmental agencies and organi-zations to classify dam size do notadequately reflect this variation, andthese criteria are not always used ina consistent manner. For example,the US Army Corps of Engineers National Inventory ofDams (USACE 2000) emphasizes dam safety and definesdams as large if they meet one of three criteria: (1) a high haz-ard potential (i.e., likely loss of human life if the dam fails),regardless of the dams absolute size; (2) a low hazard potentialbut height exceeding 7.6 meters (m) and storage capacitygreater than 18,500 cubic meters (m3); or (3) a low hazard po-tential but height exceeding about 1.8 m and storage ex-ceeding 61,700 m3. Other organizations have adopted quitedifferent criteria for defining dam size. For example, the In-ternational Commission on Large Dams classifies dams aslarge if either their height exceeds 15 m or their height is be-tween 5 and 15 m and a reservoir greater than 3 x 106 m3 isimpounded (WCD 2000). Yet another classification defineshydropower dams as either low-head or high-head, depend-ing on whether their height is less than 30 m or greater than30 m, respectively (EnergyIdeas 2001). The criteria for clas-sifying dams even differ among states.

There are at least two reasons why these criteria are prob-lematic for defining dam characteristics from the perspectiveof environmental effects. First, as illustrated above, the samedam can be classified as large according to one definition andsmall according to another. Second, even if only one defini-tion is adopted, dams that are grouped together can varytremendously in size. For example, the USACE (2000) data-base of large dams includes structures with heights rangingfrom less than 2 m to more than 200 m, and storage volumes

from less than 100 m3 to 3.7 x 1010 m3. Such marked differ-ences in dam size will necessarily translate into very differentuses and environmental effects.

Dam operations. Although designed to meet many dif-ferent human needs, the two basic functions of dams are tostore water and raise water levels (McCully 1996). The stor-age ability of dams allows runoff to be retained for subsequentcontrolled release, whereas the ability to raise upstream wa-ter levels permits water diversion, increases hydraulic head forhydropower generation, creates impoundments for recre-ation, and so on. The most common classification of opera-tional characteristics divides dams into two groups, storageand run-of-river, based in large part on these functional dif-ferences (USBR 2001). For example, a storage dam typically

has a large hydraulic head and stor-age volume, long hydraulic resi-dence time, and control over therate at which water is released fromthe impoundment. By contrast, arun-of-river dam usually has a smallhydraulic head and storage volume,short residence time, and little orno control over the water-releaserate (EPA 2001).

As with dam size, however, thisdichotomous classification has sev-eral limitations. First, different cri-teria are sometimes used to place

dams in an operational class. For instance, the state of Penn-sylvania defines run-of-river dams as relatively small struc-tures whose impoundments are confined completely withinthe banks at normal flow levels (Pennsylvania Fish and BoatCommission 2001), a much more restricted definition thanthat used by most federal agencies. Second, membership ina single class can conceal large and important variation. Forinstance, storage dams can include flood-control dams thatdramatically alter seasonal flow patterns, as well as hydropowerdams that impact flow regimes primarily on a time scale ofhours to days, in response to fluctuating electrical demand.Likewise, run-of-river dams can have whole-reservoir turnovertimes ranging from a few hours to many weeks, and im-poundment depths ranging from 1 m to more than 30 m. Fi-nally, many multipurpose dams are used for flood control,irrigation, navigation, power generation, and recreation anddo not fit neatly in either operational class.

Despite the challenges involved in creating a simple clas-sification system that effectively describes variation in the sizeand operational characteristics of dams, such variation canhave markedly different ecological effects (Hart et al. 2002).For example, the flow regime below a flood-control dam 50m high will be moderated to reduce peak flows, increase baseflows, and alter natural seasonal timing of flow variations (Petts1984). By contrast, a run-of-river hydropower dam that is 10m high may only occasionally modify peak flows and is un-likely to substantially alter thermal regimes downstream;


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Dams vary tremendously in size (height

and width) and hence in their reservoir

storage volume, factors that have very

important direct and indirect

environmental impacts....

however, it will capture the coarser fraction of transported sed-iment.Very small dams, such as a 2-m-high diversion dam andrun-of-river mill dam, are likely to have relatively limited ef-fects on peak flows or downstream sediment regime by virtueof their small storage volume, although they may still reducelow flows downstream and prevent upstream movement ofsmall fishes. Thus the development of a more complete un-derstanding of dam effects, as well as responses to dam re-moval, will require improvements in our ability to characterizevariation in ecologically important dam characteristics suchas size and operational mode.

Damage. Dams have a finite life span, so dam age can bean important factor affecting removal decisions. Two of themajor factors influencing the aging process are the deterio-ration of construction materials and the accumulation ofsediment within the dams impoundment.

Infrastructure safety and repair. As dams age, they becomemore prone to failure. For example, the failure of three damsduring the 1970s (Buffalo Creek, Teton, and Toccoa Creek)resulted in 175 fatalities and more than $1 billion in losses(ASCE 2001a). More recently, heavy rains from a single trop-ical storm in 1994 caused more than 230 dams to fail inGeorgia (FEMA 2001). Because of the boom in US dam con-struction that occurred from 1950 to 1980, we now faceproblems stemming from aging dams. This challenge is ex-acerbated by the fact that one-third of high-hazard damshave not even undergone safety inspections in the last 8 years(ASCE 2001b). Although the failure of a small dam maythreaten fewer lives and cause less property damage than a largedam, many small dams are much older and in poorer con-dition than large dams. Of course, the life span of some damscan be substantially increased by continuous maintenance, butthe associated costs can be high. For example, the cost of re-pairing a small dam can be as much as three times greater thanthe cost of removing it (Born et al. 1998). We emphasize, how-ever, that the relative costs of repair and removal are likely tovary markedly, depending on the regulatory policies of dif-ferent states, especially as they address potential concernsabout the quantity and quality of accumulated sediments. Nev-ertheless, these safety and repair issues underscore the chal-lenges of maintaining an aging dam infrastructure.

Sedimentation.Sediment capture by dams reduces reser-voir storage capacity and impairs dam functionality. Formodern dams, this process generally happens at a muchfaster rate than the loss of structural integrity of constructionmaterials. Thus sedimentation is often a factor limiting adams useful life (Morris and Fan 1998). For example, high sed-imentation rates have reduced the storage capacity of Matil-ija Dam in southern California by about 50% since it was builtin 1948 (Matilija Coalition 2000). By contrast, some dams withlow sedimentation rates have remained functional for ex-tremely long periods, in some cases up to many hundreds ofyears (Schnitter 1994).

The importance of sedimentation is now widely recognized,but sedimentation rates were not consistently factored into

dam design criteria until the 1960s (Morris and Fan 1998),and many dams are expected to fill in with sediment at ratesexceeding design expectations (Dendy 1968). Sedimentationrates vary greatly from watershed to watershed, however, be-cause of spatial variation in sediment supply and delivery thatis controlled by basin geology, slope, drainage density, and landuse or cover. Erosion occurs largely in response to large pre-cipitation events, so climate is also an important controllingfactor in dam aging. Engineers now typically design reservoirsto incorporate a 100-year sediment storage pool, but humandisturbance of land surfaces can greatly increase sediment yieldand thus reduce a reservoirs effective life span. For example,sediment yield can increase by two orders of magnitude in re-gions with extensive road construction (Morris and Fan1998).

Patterns of variation in dam characteristicsVarious agencies and organizations are responsible for main-taining inventories of dams and their characteristics, partic-ularly for purposes such as dam safety and water supply. Forexample, the International Commission on Large Dams hasa global inventory of about 45,000 large dams (WCD 2000).In the United States, the Army Corps of Engineers maintainsthe National Inventory of Dams (USACE 2000), which in-cludes more than 76,500 largestructures. In addition to thesestructures are an estimated 2,000,000 or more small damsin the United States that are not included in this national data-base (Graf 1993). Information for these smaller structures iscompiled and maintained largely by state regulatory agenciesand is therefore much more dispersed and uneven in geo-graphic coverage. Indeed, only a few states have compiled com-prehensive state-wide electronic databases for these smallerstructures.

We examined variations in characteristics of dams in thefederal database and then compared them with dam charac-teristics for two states, Wisconsin and Utah. The size (height)distribution of federally cataloged dams is illustrated in fig-ure 1. Almost half the dams in the federal database are in the4 to 16 m height range. The smallest dams (< 2 m) are rela-tively rare in the federal database, especially when comparedwith their estimated abundance on the landscape (Graf 1993).Dams in different parts of the United States are often oper-ated in a different fashion because of regional variation in cli-mate and economic activity. Such operational differences areclearly seen by dividing the United States into eight geo-graphic regions that reflect broad differences in physical set-ting (climate, topography) and settlement history (figure 2).

The picture of operational purposes of dams shown in fig-ure 2 is unlikely to represent operations for the 2,000,000 orso smaller dams that are not in a national database. In an ef-fort to evaluate this expectation, we analyzed data for Wis-consin and Utah, two states that have relatively complete in-ventories and that differ markedly in climate and topography.These two states might offer some measure of the range ofvariation in operational purposes of small dams (although we


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do not argue they are statistically representative of the UnitedStates as a whole). By comparing the overlap of dams inthese statewide databases with the more comprehensive national database, one can get a sense of the adequacy of us-ing the national database to evaluate the distribution and func-tion of the much more numerous small dams, which aremore likely to be prime candidates for removal in the future.

Figure 3a compares the size distribution of the 3843 Wis-consin dams for which height is recorded in the state data-base with the 655 Wisconsin dams listed in the national data-base (USACE 2000). As expected, the national databaseunder-represents the proportion of smaller structures (< 2 m)and overrepresents the proportion of larger structures (> 8m). Moreover, the correspondence between the state and na-tional databases in terms of operational purpose is poor.Most (39.4%) dams are classified by the state as protection,stock or small farm pond, a use category represented byonly 2% in the national database. By contrast, the national in-ventory overestimates recreation, fish and wildlife ponds,flood control, and hydropower categories, but is reasonablyrepresentative for dams classified as irrigation, which is nota major use in Wisconsin (data not shown).

In the Utah database, 1641 dams are listed, of which only104 are included in the national inventory. As shown in fig-ure 3b, the size distribution of dams in the state database isvery poorly represented by the national database, with the proportion of dams less than 4 m in height being under-represented and dams greater than 8 m in height being over-represented in the national database. In both the state and na-tional databases, dams designated as primarily irrigation arethe most prevalent use category (data not shown), althoughthe national database overestimates their proportional rep-resentation by a factor of two relative to the state database.Stock ponds constitute 22% of state-identified dams, but are

completely absent from the national inventory. Similarly,the national database underestimates the occurrence offlood control structures in Utah by a factor of six relativeto the state database.

Thus, in summary, the national database for large damsdoes a relatively poor job of characterizing small dams interms of size distribution and operational purpose forboth Utah and Wisconsin.

The need for an ecological classification of dams A formal characterization of how dams modify riverecosystems represents a major scientific challenge, espe-cially because the type and magnitude of environmentalalteration stems from interactions among naturalprocesses, dam characteristics, and management practices.At present, little empirical data are available to allowmeaningful generalization. This reflects, in part, the factthat readily available, simple descriptors for dams (e.g.,


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Figure 1. Distribution of US dams by structure height. Data arefrom the National Inventory of Dams (solid bars; USACE 2000) and estimated by USACE for dams less than 2 meters in height(diagonally hatched bar; Graf 1993).

Figure 2. Percentage distribution by geographic region ofdams falling into five categories of primary operationalpurpose, as defined in the national inventory of dams(USACE 2000). Dam uses are defined as flood control(stippled), hydropower (diagonally hatched), irrigation(solid black), recreation (solid white), and public supply(vertically hatched). These five uses represent 71% of thedams (54,903 dams).

Dam height (meters)

1,000,000

100,000

10,000

Perc

enta

ge

size) are not adequate for building a robust classification.More fundamentally, the framework for identifying the crit-ical variables needed to form a classification does not exist;therefore, meaningful classification variables have not beensystematically identified and collected.

Figure 4 provides a conceptual framework for how criti-cal biophysical processes are modified by dams and reservoirs.The natural river system can be considered as a set of base-line conditions, characterized by temporal patterns of waterflow, sediment (and organic matter) transport, temperatureconditions, biogeochemical cycling, and biotic movements.These conditions are functions of climate, geology, landcover/use, and biogeography, and they show substantial ge-ographic variation (see Poff 1996 for an example of flowregimes). In theory, the effect of a particular dam could be de-fined by the ways it modifies these natural regimes. In many

instances, however, the prevailing biophysi-cal regime already reflects the impact of up-stream dams, which greatly complicates thetask of characterizing how a particular down-stream dam is modifying the natural riverecosystem (figure 4). Although not shown,downstream dams can also modify a givendams impacts because of their effects on theupstream movements of river biota (Pringle etal. 2000). A further consideration in assessinga dams effect on baseline conditions is theposition of a reservoir in the drainage basin.For example, the degree of thermal deviationfrom natural conditions below a deep releasereservoir is much greater in warmer, down-stream reaches of a river than in cooler head-waters (Ward and Stanford 1983). Thus riversize can be an important consideration inclassifying the effects of dams on riverineecosystems.

Many of the effects that dams have on thebiophysical regime are related to the damssize and operational mode. Dam size (height,width) strongly influences many environ-mental effects, such as the likelihood of tem-perature stratification and thermal regimemodification, the dams effectiveness as a bar-rier to biotic migration and sediment trans-port, and its ability to store peak flows. Damsize also interacts with dam operations to in-fluence a key variable, the hydraulic residencetime (HRT), which in turn affects many dif-ferent facets of the biophysical regime. TheHRT is defined as the ratio of the storage vol-ume (m3) of the reservoir to its flow-throughrate (m3 per year), the latter being a functionof natural inflow to, and human controlledoutflow from, the reservoir. The HRT can po-tentially influence the settlement of sedimentwithin the reservoir, the development of plank-

tonic assemblages and processes, the transport of biotathrough the reservoir to downstream reaches, the type and rateof biogeochemical cycling, and the occurrence of thermal strat-ification (Morris and Fan 1998, Kalff 2002). Thus dams of sim-ilar sizes can potentially have different ecological effects be-cause of differences in their HRTs. Further, seasonal variationin reservoir operations can result in HRT being seasonally vari-able (e.g., if a reservoir is drawn down before annual springflooding).

Although information on HRT is critical to the develop-ment of ecological classification of dams, HRT data are notdirectly available for most impoundments. The situationarises in part because information on seasonal inflows intothe reservoir or operational rules for reservoir discharge areoften not reported, especially for smaller dams. Moreover, onlyabout one-third of the dams in the national database (USACE


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Figure 3. Percentage distribution by dam height for dams in state databases(solid black) and the National Inventory of Dams (diagonally hatched; USACE 2000) for (a) Wisconsin and (b) Utah.

Dam height (meters)

Dam height (meters)

State databaseNational databaseP

erce

ntag

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tot

alPe

rcen

tage

of t

otal

b

a

2000) have reliable reported values for reser-voir storage volume.

Indirect measures of HRT might pro-vide an avenue for dam characterization;however, such measures are themselveslimited. For example, in natural lakes,about 33% of the variation in HRT is sta-tistically explained by variation in lakevolume (Kalff 2002), so an indirect mea-sure of reservoir volume might provide arough estimate of HRT. Unfortunately, themost reasonable predictor variable, damheight, is only weakly correlated (r2 = 0.21for loglog data) for that portion of the na-tional database containing values for bothvariables. Thus HRT is unlikely to be pre-dicted meaningfully from dam height. Innatural lakes, the unexplained 67% of thevariation between HRT and lake volumeprobably reflects differences in regionalrunoff patterns and in lake morphometry(surface area to volume ratio) (Kalff 2002).Similarly, with reservoirs, regional differ-ences in inflows will affect HRT. For ex-ample, Graf (1999) estimated maximumreservoir capacity (m3) to store mean an-nual runoff (m3 per year) to range from0.25 to 0.37 years of storage in the upperMidwest and Northeast to 3.8 years in thearid Southwest. These values provide asense of how HRT is regionally variable; however, predict-ing HRT for individual reservoirs will require that opera-tional mode also be taken into account, since human con-trol over dam outflows are a determinant of active reservoirstorage and HRT.

Ultimately, efforts to categorize dam operations (and thuskey variables like HRT) from a scientific perspective must ac-count for differences in management practices that reflect vari-able social settings, economic conditions, and human pref-erences. Beyond the regional differences in climate and runoff,individual reservoirs are often managed for multiple purposesthat can vary over time. Clearly, different types of opera-tions can have very different environmental effects. For ex-ample, flood storage dams are often drawn down before a pre-dictable flooding season and they are thus able to store peakflows, thereby modifying downstream flow and sedimentregimes. Run-of-river dams of similar size, by contrast, tendto pass peak flows and are therefore less likely to detain finesediment or modify downstream high flows. Alternatively,dams of very different sizes can have similar downstream hy-drologic effects depending on how they store and releasewater over time. However, characterizing dam operations ina meaningful way may be easier for smaller structures (e.g.,many of those not included in the national database), becauseof their smaller storage capacity and limited range of man-agement options.

The influence of dam characteristics on removal decisionsAccording to a recent compilation, 467 dams have been com-pletely or partially removed in the United States in the 20thcentury (AR/FE/TU 1999).At least another 30 dams have beencompletely removed through 2001 (Molly Pohl, Departmentof Geography, San Diego State University, personal commu-nication, 5 March 2002). What kinds of dams are being re-moved, and how might future dam-removal decisions be re-lated to variation in dam characteristics?

There are two striking dam-removal patterns: Dams are be-ing removed at an accelerating rate (figure 5a), and the ma-jority of dams being removed are less than 5 m in height (fig-ure 5b). Several factors suggest that small dams will continueto be removed more often than large dams: As indicated bythe Wisconsin and Utah databases, dams less than 5 m inheight are far more numerous than large dams. Most of thesesmall dams do not generate hydroelectricity or control floods,so the economic benefits of maintaining them are not asgreat when compared with large dams. Small dams are oftenolder than large dams, which makes it more likely that theywill be in poor condition. In fact, concerns about publicsafety, as well as high repair costs, were major factors affect-ing decisions to remove a number of old dams (average age> 100 years) in Wisconsin (Born et al. 1998). Small dams aremore likely to be abandoned, so that financial burdens asso-


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Figure 4. Flow chart illustrating how attributes of damreservoir systems,especially dam size and operations, modify fundamental riverine biophysicalprocesses to cause alterations with local and landscape environmental effects.

ciated with their safety, repair, and maintenance often fall tolocal governments and, ultimately, to taxpayers. Indeed, manydams that have been removed were previously abandoned(Shuman 1995). These patterns clearly demonstrate that thecurrent focus on small dam removal is influenced by socialand economic factors, as well as by concerns about the envi-ronmental effects of small dams (AR/FE/TU 1999, Doyle etal. 2000).

Sediment accumulation in reservoirs is another factor thatcan influence many dam-removal decisions. This issue can becomplicated, depending on the quality and quantity of ac-cumulated sediments, as well as on public and agency attitudesabout potential downstream effects of sediment. For exam-ple, if toxic contaminants are present in the sediment, thereare certain to be concerns about the risks associated withthe downstream release of sediments following dam removal,and the potential effects of these sediments on human andecosystem health (Shuman 1995).

Even when contaminants are absent, accumulated sedimentcan still influence the likelihood of dam removal. For exam-ple, as reservoirs fill with sediment, they often become less ef-fective in controlling floods, storing water, and generating hy-dropower, which could accelerate calls for dam removal. A

useful index of the operational problems caused by accu-mulated sediments is the time it takes for 50% of the storagecapacity of the reservoir to be lost to sediment deposition(Morris and Fan 1998). The proportional rate at which areservoirs storage volume fills with sediment depends onbasinwide erosion rates (which vary regionally), but also ex-hibits an inverse relationship to dam size. Empirical datacollected for reservoirs across the country by Dendy and col-leagues (1973) showed that those having a storage capacity be-tween 1.2 x 106 and 12 x 106 m3 had a median time to half-filling of 91 years (based on data reported in Morris and Fan1998). Taking the median value of this size range, 92% of theapproximately 76,500 dams in the national database are ex-pected to become half-filled with sediment in an average of91 years. The regional distribution of these short-lived damsvaries somewhat (figure 6), with between 74% (California,Nevada) and 94% (Southeast) of dams falling into this cat-egory. The age of existing dams also shows regional variation,with as many as 50% of dams having construction dates be-fore 1920 in the Northeast, and as few as 5% in the Plainsstates.

The extent of this sediment problem is even greater if weconsider the estimated 2,000,000 small dams not in the na-tional inventory. These structures are defined as having lessthan 6.2 x 104 m3 of storage (Graf 1993) and thus would beexpected to become half full of sediment within roughly 25to 40 years. Dendy (1968) estimated that if present siltationrates continue, about 20% of the Nations small reservoirs willbe half filled with sediment...in about 30 years. The lack ofa national database for these structures precludes an estima-tion of their retirement times. Many in the Northeast are al-ready full of sediment, however (Laura Wildman, AmericanRivers, Northeast Field Office, personal communication, 22May 2002), and literally thousands more nationwide will fillin the coming decades. For example, in Wisconsin alone,over 800 dams are less than 2 m high, and about one-third ofthese were built before 1960. On the basis of the previous es-timates, these dams should already have lost more than 50%of their storage capacities.

Sediment accumulation can be a factor that either in-creases or decreases the likelihood of dam removal, depend-ing in part on local circumstances. For example, in situa-tions where sediment accumulation has reduced the functionalability of dams (e.g., for flood control) and disrupted down-stream geomorphic processes, there have been increased callsfor dam removal (Matilija Coalition 2000). By contrast, con-cerns have sometimes been raised about the possibility thatdownstream habitats, species, and ecosystem processes couldbe adversely affected (at least in the short term) by the releaseof large volumes of sediment during dam removal. Mecha-nized removal before dam breaching is one alternative tosediment release (ASCE 1997), although this can be very ex-pensive. For many of the smaller dams currently being re-moved, however, the volume of accumulated sediment maybe similar to the average annual sediment flux. In these situ-ations, no special management practices are employed, and


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Figure 5. Dam removal in the United States by (a) decade and(b) structure height. (Data taken from AR/FE/TU 1999 andDoyle et al. 2000.)

Dam height (meters)

Year of removal

Num

ber

rem

oved

sediments are allowed to move downstream following damremoval.

The future of dam removals The rapid aging of dams (especially small ones) and the costsof maintaining old dams practically ensures that dam re-moval will continue at a brisk pace for the foreseeable future.An open question is whether these removals will be guidedby scientific principles aimed at river restoration and con-servation or whether they will simply follow utilitarian eco-nomic principles (Pejchar and Warner 2001).

In the last decade, an understanding about how dams se-verely impair free-flowing rivers has become firmly establishedboth in the United States and abroad (Ligon et al. 1995, Col-lier et al. 1996, NRC 1996, Pringle et al. 2000,WCD 2000). Thisknowledge has entered into the public debate on river con-servation, both in terms of greater willingness of reservoirmanagers to minimize downstream ecological effects (Muthet al. 2000) and of increased calls for outright dam removal(Pyle 1995, Joseph 1998, AR/FE/TU 1999). These scientific andsocial currents have led some to call for a new water ethicof increasing water-use efficiency through nonstructuralmeans (Gleick 1998, Postel 2000). Such an ethic is needed ifhuman demands for freshwater continue to grow in the com-ing decades (Postel 2000) and if society wishes to maintainthe long-term sustainability of river ecosystems (Naimanand Turner 2000, Baron et al. 2002).The growing pressure fordam removal represents a real opportunity for scientists.Certainly, dam removals provide excellent opportunities for

scientists to perform large-scale experiments in river restora-tion (Grant 2001, Hart et al. 2002) and thus expand our em-pirical knowledge base. Moreover, scientists are increasinglylikely to be asked to predict the success of dam removal in spe-cific situations where controversy exists over potential ben-efits and costs. Because dam removal can sometimes be ex-pensive and its ecological effects hard to predict, scientists needto develop a better framework for characterizing dams ac-cording to their current environmental effects, as well as tothe potential environmental benefits that could accrue fol-lowing removal. For example, Hart and colleagues (2002)present a graphical model for examining how potential re-sponses to dam removal vary with dam and watershed char-acteristics. This scientific challenge is made more difficult be-cause the effects of dams result both from their alteration ofnatural biophysical processes and from human managementpractices. In this article, we have attempted to highlight someof the more salient attributes of this complex, multidimen-sional challenge. Developing a more predictive environmen-tal science of dam removal is needed to help society decidewhere to spend limited resources to maximize restoration po-tential for impaired river systems in the United States and else-where.

AcknowledgmentsWe thank Dave Pepin for assistance with database analysis andgraphics and Chester Watson for providing literature on damaging. The ideas in this article have benefited greatly from dis-cussions with many colleagues in the Patrick Centers Insti-tute for River Restoration. We thank Matt Greenstone, LauraWildman, Emily Stanley, and two anonymous reviewers forproviding valuable comments on an earlier draft of this man-uscript. N. L. P.s contribution to this project was supported,in part, by grants from the National Science Foundation (# DEB-0075352) and the US Environmental ProtectionAgency (SPO # BS0056363). D. D. H.s work was partially sup-ported by the Patrick Center Endowment Fund. Completionof our manuscript was also facilitated by support from ThePew Charitable Trusts.

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Figure 6. Number of dams by geographic region in the NationalInventory of Dams database (USACE 2000) with estimated average time to half-filling with sediment being 91 years or less,on the basis of reservoir storage volumes. Dams are divided intothose constructed before (solid black) and after (diagonallyhatched) 1920. Refer to figure 2 for regional key.

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http://www.energyideas.org/glossary/terms.cfm?vcategoryID=7http://www.energyideas.org/glossary/terms.cfm?vcategoryID=7http://www.energyideas.org/glossary/terms.cfm?vcategoryID=7http://www.epa.gov/nps/MMGI/Chapter6/ch6-3a.htmlhttp://www.epa.gov/nps/MMGI/Chapter6/ch6-3a.htmlhttp://www.fema.gov/mit/damsafe/about.htmhttp://www.rain.org/%7Epjenkin/matilijahttp://www.rain.org/%7Epjenkin/matilijahttp://sites.state.pa.us/PA_Exec/Fish_Boat/damhaz.htmhttp://crunch.tec.army.mil/nid/webpages/nid.cfmhttp://www.usbr.gov/cdams/glossary.htmlhttp://giorgio.catchword.com/nw=1/rpsv/cgi-bin/linker?ext=a&reqidx=/0364-152X^28^2922L.359[aid=2853439]http://giorgio.catchword.com/nw=1/rpsv/cgi-bin/linker?ext=a&reqidx=/0006-3568^28^2952L.483[aid=2853532]http://giorgio.catchword.com/nw=1/rpsv/cgi-bin/linker?ext=a&reqidx=/0043-1397^28^2935L.1305[aid=2853533]http://giorgio.catchword.com/nw=1/rpsv/cgi-bin/linker?ext=a&reqidx=/0885-6087^28^2915L.1531[aid=2068411]http://giorgio.catchword.com/nw=1/rpsv/cgi-bin/linker?ext=a&reqidx=/0006-3568^28^2952L.669[aid=2853526]http://giorgio.catchword.com/nw=1/rpsv/cgi-bin/linker?ext=a&reqidx=/0006-3568^28^2945L.183[aid=655085]http://giorgio.catchword.com/nw=1/rpsv/cgi-bin/linker?ext=a&reqidx=/0006-3568^28^2950L.783[aid=9422]http://giorgio.catchword.com/nw=1/rpsv/cgi-bin/linker?ext=a&reqidx=/0364-152X^28^2928L.561[aid=2853535]http://giorgio.catchword.com/nw=1/rpsv/cgi-bin/linker?ext=a&reqidx=/0046-5070^28^2936L.71[aid=2714863]http://giorgio.catchword.com/nw=1/rpsv/cgi-bin/linker?ext=a&reqidx=/0006-3568^28^2947L.769[aid=8628]http://giorgio.catchword.com/nw=1/rpsv/cgi-bin/linker?ext=a&reqidx=/0006-3568^28^2950L.807[aid=9426]http://giorgio.catchword.com/nw=1/rpsv/cgi-bin/linker?ext=a&reqidx=/0006-3568^28^2950L.746[aid=9450]http://giorgio.catchword.com/nw=1/rpsv/cgi-bin/linker?ext=a&reqidx=/0921-8181^28^2925L.39[aid=2853538]http://giorgio.catchword.com/nw=1/rpsv/cgi-bin/linker?ext=a&reqidx=/0169-555X^28^2914L.327[aid=2714873]http://giorgio.catchword.com/nw=1/rpsv/cgi-bin/linker?ext=a&reqidx=/0006-3568^28^2950L.766[aid=9429]http://giorgio.catchword.com/nw=1/rpsv/cgi-bin/linker?ext=a&reqidx=/0036-8075^28^29273L.1558[aid=2714878]http://giorgio.catchword.com/nw=1/rpsv/cgi-bin/linker?ext=a&reqidx=/0006-3568^28^2952L.483[aid=2853532]http://giorgio.catchword.com/nw=1/rpsv/cgi-bin/linker?ext=a&reqidx=/0006-3568^28^2952L.669[aid=2853526]http://giorgio.catchword.com/nw=1/rpsv/cgi-bin/linker?ext=a&reqidx=/0364-152X^28^2928L.561[aid=2853535]http://giorgio.catchword.com/nw=1/rpsv/cgi-bin/linker?ext=a&reqidx=/0921-8181^28^2925L.39[aid=2853538]


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Although dams provide a variety of economic goodsand services, including electric power, flood control, wa-ter supply, reservoir recreation, and navigational services,they also have detrimental effects on riverine ecosystems(Petts 1984). As a result, many people want to know the so-cioeconomic and ecological benefits and costs of rehabilitatingor restoring rivers through dam modification or removal(AR/FE/TU 1999).

Costbenefit analysis is one economic tool that helps de-cisionmakers choose among policy alternatives (Boardmanet al. 1996). Ideally, costbenefit analysis includes all of the costsand benefits associated with each policy alternative. In fact,however, costs and benefits can be difficult to measureestimating the value of an endangered species, for exampleor may not be fully recognized at the time a study is conducted.Thus using costbenefit analysis in evaluating the removal ofa dam can challenge even seasoned analysts.

In spite of these limitations, decisionmakers and stake-holders frequently rely on costbenefit analysis for insights intothe potential consequences of modifying or removing dams.The Edwards Dam on the Kennebec River in Maine, removedin 1999, illustrates the point. A costbenefit study concludedthat necessary structural repairs would have cost 1.7 times thecost of removing the dam and restoring anadromous fish pas-sages (AR/FE/TU 1999).

Edwards Dam is small compared with other dams recentlyunder consideration for removal. The US Army Corps ofEngineers completed a draft costbenefit analysis of a proposalto remove a series of four large dams on the lower Snake Riverin the Pacific Northwest. Wild salmon stocks have dipped per-ilously low on the river, and many people believe the costs ofkeeping the dams outweigh the benefits.

In this article, we describe principles we believe are effec-tive in assessing the economic consequences of environ-mental management decisions. We then describe how thoseprinciples might be used for a costbenefit analysis regard-

ing dam removal using the dams on the lower Snake River asa case study. We examine parts of the US Army Corps of En-gineers draft costbenefit analysis for these dams and suggestmodifications to the Corps analysis that would more fully ac-count for relevant costs and benefits.

Analytical principlesOn 9 September 1998, 78 economists sent a letter to the gov-ernors of the four Pacific states and the premier of British Co-lumbia, urging them to consider the full range of economicconsequences when they and members of their administra-tions make salmon-management decisions (Whitelaw et al.1998). Box 1 presents the six principles that the economistsemphasized should guide an assessment or costbenefitanalysis of the economic consequences of practically any en-vironmental management decision, including whether tokeep or remove a dam.

Ed Whitelaw (e-mail: whitelaw@eugene .econw.com) is a professor of economicsat the University of Oregon, Eugene, OR 97403, and president of ECONorth-west, an economic consulting firm with offices in Eugene and Portland, OR,and Seattle, WA. Ed MacMullan is a project manager and economist atECONorthwest. 2002 American Institute of Biological Sciences.

A Framework for Estimatingthe Costs and Benefits ofDam Removal

ED WHITELAW AND ED MACMULLAN

SOUND COSTBENEFIT ANALYSES OF

REMOVING DAMS ACCOUNT FOR SUBSIDIES

AND EXTERNALITIES, FOR BOTH THE

SHORT AND LONG RUN, AND PLACE THE

ESTIMATED COSTS AND BENEFITS IN THE

APPROPRIATE ECONOMIC CONTEXT


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Two of the six principles play primary roles by addressingthe two key effects of a decision on a dam: (1) the effect of thedecision on the value of the goods and services derived fromthe environmental resources; and (2) the effect on jobs andassociated variables, such as incomes and the well-being ofcommunities. The other four play secondary roles, offeringguidance on the issues that should be addressed when applyingthe first two principles. The four secondary principles are justas important as the primary ones, but they play a different role,defining the range of issues that should be taken into accountas one looks at the benefits, costs, and effects on jobs.

The first principlefirst in order and first in priorityad-monishes decisionmakers to consider both the benefits andthe costs. Though this may seem eminently reasonable, theeconomists observed that many economic studies of envi-ronmental management decisions predominantly empha-size the costs (Whitelaw et al. 1998). Doing so reduces the per-ceived economic importance of the environmental resources.When weighing the benefits and costs, decisionmakers shouldtake into account how their decision would affect all goodsand services with economic value, not just those traded inmarkets with monetary prices. In addition, a full accountingmust be provided of the true value of each affected good orservice, taking into account the market price, as well as all fac-tors, such as subsidies, taxes, and environmental externalities,that distort the level of supply or demand. (Environmental ex-ternalities occur, for example, when those who generate pol-lution benefit financially while others downstream or down-

wind pay the costs of the pollution.) Finally, the estimates ofeconomic impactscosts, benefits, employment conse-quences, and so onshould be placed in the context of thesize and makeup of local and regional economies. Consider-ing impacts without the proper context limits the usefulnessof information to decisionmakers or stakeholders.

A decision to remove a dam also would have both positiveand negative effects on jobs and incomes. When examiningthese effects, decisionmakers should take into account theeconomys ability to adjust over time to exploit the positiveand attenuate the negative. The decision to remove a dam mayexpand or contract the demand for labor and, hence, in-crease or decrease job opportunities. The actions taken mayaffect the quality of life in a local area or region and, hence,influence where people prefer to live, work, play, and shop.Analysis of the employment effects associated with a man-agement decision on a dam must also separate these conse-quences from the employment consequences associated withlarger economic forces and trends unrelated to decisions onthe dam, but which may affect local and regional economiesin proximity to the dam.

Because the decision on a dam generates both benefitsand costs, and produces both positive and negative effects onjobs and incomes, it creates both winners and losers. The econ-omists recommended that such distributional effects not beoverlooked. They also emphasized the importance of havinga clear understanding of how the decision affects the rightsand responsibilities of landowners and resource users. The

Primary analytical principles

1. Benefits as well as costsRemoving or keeping a dam would generate economic benefits as well as economic costs. Consider them both to understand the full effect on the value of the goods and services derived from streams, forests, and other resources.

2. Positive as well as negative impacts on jobsDealing with a dam would have both positive and negative effects on job opportunities. Consider them both to understand the full effect on workers, their families, and their communities.

Secondary analytical principles

3. Distribution of consequences and fairnessThose who enjoy the benefits or jobs of a decision on a dam would not necessarily be the same as those who would bear the costs or job losses. Consider the full distribution of economic consequences to understand who wins, who loses, and the fairness of the distribution.

4. Rights and responsibilitiesWith any decision on a dam, property owners and resource users behave differently than they otherwise would.Consider whether these changes represent infringement of their rights or enforcement of their responsibilities.

5. Uncertainty and sustainabilityAny decision on a dam would rely unavoidably on information insufficient to guarantee the outcome. Consider fully the potentially high costs from decisions yielding undesirable outcomes that are irreversible or extremely difficult to reverse.

6. More than just salmon conservationRemoving or keeping a dam would have a variety of ecological and economic effects, such as changes in the quality of stream water used for other purposes, that may seem peripheral. But consider all the effects.

Box 1. Six principles that should guide the analysis of the economic consequences of removing or keeping adam. From a 1998 letter by concerned economists (Whitelaw et al.) to Governors Kitzhaber, Knowles, Locke,

and Wilson and Premier Clarke regarding the economic issues of salmon recovery.


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value society places on the decision that restricts property own-ers rights can differ markedly from the value of otherwisecomparable measures that induce the property owners tocomply with their responsibilities. In addition, the economistsobserved that, given the uncertainty regarding a decision toremove a dam, there always is the possibility it would yield un-desired outcomes, and care should be taken to avoid outcomesthat are costlyor even impossibleto reverse. Finally, theeconomists stressed that although the primary economicconsequences of an environmental management decisionhave to do with the specific environmental resource itself, oth-ers do not. A full analysis should include the costs, benefits,and effects peripheral to the specific resource at issue.

The six principles identified in the economists letterprovide an analytical foundation for assessing the costs andbenefits of removing a dam. The analytical approach out-lined in this section describes the economic implications ofremoving a dam, from large dams that facilitate barge traf-fic to small dams such as those that provided water to long-abandoned mills. Removing larger dams is likely to gener-ate more significant impacts than removing smaller dams;therefore, depending on the specifics of the dam, a full-blown economic analysis may not be appropriate. (SeeTrout Unlimited 2001 for information on the economicbenefits of removing small dams.) However, a costbenefitanalysis of removing a dam using the principles describedin box 1 ensures that the analysis captures the full range ofeconomic consequences.

Application to dams on the lower Snake RiverIn this section we describe the application of the analytical ap-proach described above to the question of removing the fourdams on the lower Snake River in Washington.

Background regarding the impacts of dams onendangered salmon. Four dams are situated in the lowerSnake River, between the Snakes confluence with the Co-lumbia River at Pasco, Washington, and Lewiston, Idaho.The US Army Corps of Engineers (USACE) constructed thedams between 1962 and 1975, primarily to create a series ofponds so barges could reach Lewiston, and secondarily to pro-vide easy access to water for irrigation and to generate hy-droelectricity.

Wild salmon stocks returning to the Snake River haveplummeted since the dams construction, and a chorus of fish-eries biologists and others has called for breaching or bypassingthe dams, that is, removing the earthen mounds adjacent tothe concrete portions of the dams and letting the rivers runfree. Proponents argue that breaching the dams would, amongother things, restore endangered wild salmon, return tradi-tional sites and fisheries to Indian tribes, improve water qual-ity, reduce taxpayer subsidies to corporate irrigators andbarging companies, and comply with numerous laws andtreaties. Opponents claim such actions would prove prohib-itively costly, even wreck the Northwests economy.

In December 1999, the US Army Corps of Engineers re-leased a draft Feasibility Report/Environmental Impact State-ment (FR/EIS), which, among other things, provides an es-timate of the economic effects of breaching the four dams onthe lower Snake River (USACE 1999a). The FR/EIS describes,to varying degrees, the costs and benefits of dam removal ondifferent sectors of the regional and national economies, in-cluding tribal interests; recreational use; anadromous fisheries;irrigated agriculture; transportation; electrical utilities; andmunicipal, industrial, and private water use.

In the remainder of this article, we evaluate the Corps analy-sis of the economic effects of breaching the dams in light ofthe analytical principles described in box 1. We have limitedour critique of the Corps analysis to the primary analyticalprinciples of the overall costs and benefits of removing thedams and the associated impacts on jobs. Readers interestedin a more in-depth discussion of the primary analytical prin-ciples and the related secondary principles as they apply to theCorps costbenefit analysis should consult ECONorthwest(1999, 2000).

Evaluating the Corps analysis of economic effects.First we describe the overall structure of the Corps costben-efit analysis, and then we review the Corps analysis of costsand benefits. This section concludes with our critique of theCorps analysis of the employment impacts of removing thedams.

The Corps overall analytical approach. When federalagencies such as the Corps conduct costbenefit analyses ofproposed water projects, they typically follow the Principlesand Guidelines developed by the US Water Resources Coun-cil (USWRC) in the early 1970s to provide guidance on de-cisionmaking and analytical procedures as they apply towater resources. The Principles and Guidelines, which re-placed the Principles and Standards, were last updated in1983 (USWRC 1983). Other federal agencies that use thePrinciples and Guidelines are the Bureau of Reclamation, theNatural Resources Conservation Service, and the TennesseeValley Authority (NRC 1999). According to the Corps (USACE 1999b), the Principles and Guidelines recommendthat a costbenefit analysis include the following socioeco-nomic factors:

National economic development (NED) effects, whichdescribe the changes in the economic value of thenational output of goods and services

Environmental quality effects, which describe nonmon-etary consequences for significant natural and culturalresources

Regional economic development (RED) effects, whichaddress changes in the distribution of regional econom-ic activity such as jobs and income

Other social effects, which describe potential effectsfrom relevant perspectives that are not reflected in theother three types of effects

In spite of the comprehensive approach outlined in thePrinciples and Guidelines, the Corps considered only a por-tion of this information in its decisionmaking process for thedams on the lower Snake. The Corps FR/EIS states,The NEDaccount is the only account required under the WRC [Wa-ter Resources Council] guidelines (appendix I, pp. I1-1I1-2). As calculated by the Corps, the impact of dam removal onthe value of the nations goods and services apparently de-termined the outcome of its costbenefit analysis.

The National Research Council (NRC) reviewed the Corpsuse of the Principles and Guidelines in a number of applica-tions and concluded that the Corps approach ignores im-portant impacts, is out-of-date, and does not reflect currentthinking on the role that water resources play in local, regional,or national economies (NRC 1999). The NRC concluded(pp. 45):

While they were in effect, the P&S [Principles and Stan-dards] were consistently reviewed and updated by fed-eral and other water planning specialists. By contrast,the P&G [Principles and Guidelines] have not receivedthe same degree of attention and, as a result, do notadequately reflect contemporary water resource plan-ning principles and practices.... Movement away fromconsideration of the National Economic Development(NED) account [is] the most important concern. Today,ecological and social considerations are often of greatimportance in project planning and should not neces-sarily be considered secondary to the maximization ofeconomic benefits. Strict adherence to the NED accountmay discourage consideration of innovative and non-structural approaches to water resources planning.... Insummary, the committee recommends that the federalPrinciples and Guidelines be thoroughly reviewed andmodified to incorporate contemporary analytical tech-niques and changes in public values and federal agencyprograms.

Applying the NRCs criticisms of the Corps overall ana-lytical approach to the analysis of impacts of removing theSnake River dams, we see that the Corps analysis provides lim-ited useful information and misleading results. For example,as we describe below, the Corps NED analysis is incompletebecause, among other deficiencies, it excludes the impact ofsubsidies. Thus the Corps violated the first principle in box1 of considering the full range of economic benefits andcosts. Even though the Corps estimated RED impacts, theyignored these impacts during the decisionmaking processand focused exclusively on the NED impacts. By excludingRED impacts from the decisionmaking process, they vio-lated the second principle of considering the full range of em-ployment impacts. Likewise, the Corps excluded the range ofsecondary analytical principles in box 1 from its analysis.

Changes in benefits and costs. The Corps analysis ofcosts and benefitsthe NED effects described in the Princi-ples and Guidelines, and the effects that drove the Corps de-cisionmaking processhas significant deficiencies. We firstdiscuss two of the major drawbacks of the Corps analysis: ex-

cluding subsidies from the costbenefit analysis and ignor-ing benefits associated with tribal circumstances and non-market values. The former overestimates the cost of taking outthe dams. The latter underestimates the benefits of taking outthe dams. We then discuss the Corps results and place theseresults in the context of the regional economy of Washing-ton, Oregon, and Idaho.

Certain sectors of the regions economy that rely on thedams benefit from subsidies provided by the federal gov-ernment. In effect, taxpayers throughout the United States sub-sidize the economic activities and profits of these businesses.Taking out the dams would generate negative economic con-sequences for these businesses, but there are positive economicconsequences for US taxpayers because they would no longerpay subsidies to these businesses.

Transportation is one example. Snake River waterwayusers pay a fuel tax that generates a few hundred thousand dol-lars annually, which covers but a small portion of the actualcosts of using the waterways. Federal taxpayers make up thedifference, contributing $10 million annually to subsidizetransportations share of operations and maintenance costsfor the Snake River dams (Dickey 1999). The Corps did notinclude this and other subsidies in their analysis. For exam-ple, describing the analysis of transportation impacts, theFR/EIS states,The analysis does not take into considerationthe effects of taxes or subsidies, which represent transfer pay-ments within the national economy (USACE 1999b, p. I3-62). Furthermore, subsidies are more wide-reaching thansimply transferring wealth from one group to another. Whena service, such as transportation along the Snake River, issubsidized, so that users do not face a price reflecting the fullproduction cost, they have an economic incentive to consumemore of the service than they would otherwise. The Corpscostbenefit analysis failed to account for this overcon-sumption, which biases the analysis in favor of those sectorsof the economy that receive subsidies.

In estimating the benefits from breaching the dams, theCorps excluded a number of relevant values, including tribe-related benefits and the benefits that all of us gain from theexistence of both the increased salmon runs and a free-flow-ing lower Snake River. First, the Corps estimate of tribe-related benefits included the number of acres of sacred andtraditional sites that the tribes would regain access to, as wellas the number of pounds of fish from treaty-protected sub-sistence and ceremonial fisheries, but it did not include theeconomic benefits that tribal members and other North-westerners and Americans would gain from these changes(USACE 1999b). In not doing so, it overlooked economic ben-efits to tribal members that constitute real increases in thevalue of national goods and services.As a result, the Corps un-derestimated how breaching the dams would benefit thetribes, and how that, in turn, would benefit all of us.

Second, the Corps excluded from its costbenefit tallywhat it calls passive-use benefits that Northwesterners andother Americans would enjoy from both the increased salmonruns and from converting the lower Snake to 140 miles of free-


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flowing river. These values comenot from using the resourcesthe salmon or the riverbutfrom knowing the salmon andthe free-flowing river exist andthat future generations wouldget to enjoy them. These valuesarent trivial. Economists work-ing for the Corps estimated thatthe passive-use values of thesetwo resources range from $486million to nearly $1.3 billion(USACE 1999b). The Corps es-timates of the overall costs andbenefits of taking out the damsranged from a net cost of $300million to a net benefit of $1.3billion, in 1998 dollars (Whitelaw2000). If passive-use values wereincorporated into the Corpsoverall estimate of net costs andbenefits, the range would changeto a low of $186 million net ben-efits and a high of $2.6 billionnet benefits. This huge rangestems largely from the wide rangein the estimates of the benefits from river recreation thatwould result from breaching the dams, a range of $11 millionto $1.5 billion (USACE 1999b).

Personal income provides one measure of the ability of aregion to pay for some good, service, or action. In this caseit serves as a context for the Corps costbenefit results. Com-paring the regions personal income with the net costs orbenefits of taking out the dams provides insights into the rel-ative expense or benefit of the action. Personal income in Ore-gon, Washington, and Idaho in 2000 exceeded $310 billion(USDC 2001). The Corps worst-case estimate of net costs of$300 million represents 0.1% of the regions personal in-come. The actual impacts would be even smaller because, aswe described above, the Corps overestimated the costs and un-derestimated the benefits of taking out the dams.

Employment impacts. In spite of the Corps emphasis onchanges in costs and benefits at the national level, the impactson jobs, especially jobs in the local and regional economy, arewhat concern many people. We discuss how the Corps analy-sis overstates the employment impacts and then place theCorps analytical results in the proper context, in this case, thelocal and regional economies that would be affected by thedecision. We also illustrate the importance of considering rel-evant economic forces and trends in an analysis of employ-ment impacts.

Figure 1 summarizes the Corps estimates of the effects thatbreaching the dams would have on jobs, that is, the regionaleconomic development effects. (See ECONorthwest 1999and Whitelaw 2000 for a detailed discussion of the employ-ment impacts of bypassing the dams.) According to the

Corps, breaching the dams would create 13,400 to 27,700short-term jobs during the decade of deconstruction andconstruction (USACE 1999b).

In the long term, there would be job losses and gains. Thebiggest gains, between 1475 and 3126 jobs, would result fromimproved recreational tourism and angling opportunities. Thelargest losses of long-term jobs would occur in irrigated agri-culture (between 901 and 2256 jobs), the operations of the ex-isting dams and locks (between 1193 and 1651), and reducedspending caused by increased electricity rates (between 1534and 2382). Taking the midpoints of the ranges shown in fig-ure 1, breaching the dams would cause a net loss of 1081 long-term jobs in the Pacific Northwest (4200 jobs gained, 5281 jobslost). The Corps generated ranges of employment impacts byusing low, medium, and high scenarios for various data andassumptions.

The Corps overestimated the negative employment con-sequences of bypassing the dams because it failed to accountfor the economic forces and trends acting on the relevanteconomies. The tool the Corps employed to estimate theimpact of breaching on jobs and incomes presents a pictureof the economy at a single point in time,the Corps states, andthat point is 1995 (USACE 1999b, p. I6-5). Furthermore, theCorps assumes the long-run effects are permanent and con-tinue for the 100-year period analyzed in this study(USACE1999b, p. I6-3). In other words, the Corps assumed that thebasic structure of the economy would remain fixed in its1995 form, unchanged for the next 100 years. For example,the Corps estimated a maximum of 2256 jobs would be lostin irrigated agriculture (figure 1). To arrive at 2256, the Corps

Figure 1. Employment impacts of the bypass. Adapted by ECONorthwest with data from USACE (1999b).

assumed that, when breaching the dams eliminates reser-voir water for irrigation, the affected 13 corporate farmswould take out of production all 37,000 acres of their farm-land. This assumption ignores other possible outcomes, in-cluding switching to groundwater, adopting different irriga-tion practices, and altering crops. In effect, the Corps assumedthat the owners of these corporate assets would quit and theassets would remain idle for 100 years. Furthermore, theCorps assumed, in effect, that for the next century, thosewho lost their jobs as a result would never work again; localand regional firms that otherwise would have sold goodsand services to those who lost their jobs instead would losethose sales and wouldnt find replacement sales; owners of thefarming enterprises wouldnt switch to any other economicactivities; and those throughout the chain who lost their jobswould act exactly the same way as the original job losers inthat they would never work again.

The Corps rigid analytical structure produces an extremeworst-case scenario, unsupported by economic theory or bythe historical performance of the local and regional economies.The Corps analysis freezes all economic interactions in 1995.Such a constraint ignores the dynamic adjustments thateconomiesemployees and employers, buyers and sellers,savers and investors, and all other economic decisionmakersundertake all the time. For example, since the four dams be-gan service, the agricultural sector experienced four major con-tractions, each of which affected more than 2256 workers, themaximum that breaching the dams would affect, the Corpspredicts. And yet the local and regional economies have ex-panded steadily during this period (USDC 1998, ECONorth-west 1999).

These data and economic trends indicate that a snapshotof lost jobs at a point in time tells very little about how a realeconomy reacts to the breaching of four dams or any otherchanges. Such rigid and unrealistic assumptions cannot pro-duce a credible forecast of economic consequences underthe breaching scenario. University of Montana economistTom Power equates such an analytical approach to driving bylooking in the rearview mirror (Power 1996).

A comprehensive assessment of likely employment con-sequences of bypassing dams would include these elements:

Feasible alternatives to permanently idling assets thatare negatively affected by the bypass

Information on the average periods of unemploymentin the local and regional economies

Likely mitigation options that would reduce negativeemployment consequences

Projected employment demand in economic sectorsunaffected by the bypass

It is instructive to put these estimates in perspective. By theend of 2000, Washington, Oregon, and Idaho had approxi-mately 6.2 million workers (USDC 2001). For the three states,the net loss of 1081 jobs would amount to less than 0.02% of

all jobs. For the counties in southeastern Washington, north-eastern Oregon, and central Idaho near the lower SnakeRiverthe counties the Corps treated as the relevant localeconomythe Corps estimated a net loss of 711 long-termjobs, less than 0.3% of the employment in these 15 counties.(In 1996, the local economy of the 15 counties that border thelower Snake River employed approximately 266,000 workers.)For another perspective, compare the total number of jobs theCorps predicted would be lost5281 gross, not netwith the25,000 jobs lost in Oregon and Washingtons timber indus-try during the past decade (USDC 2000).

Placing the Corps results regarding employment impactsin the context of the size of the local and regional economies(and ignoring the issues raised by the NRC) indicates that by-passing the dams would generate minimal negative employ-ment consequences relative to the size of the local and regionaleconomies. Even though the negative employment impactswould be minimal overall, they represent hardships for the af-fected workers and their families. The limited nature of thenegative impacts, however, means that mitigating the nega-tive employment consequences would be manageable. (Formore information on mitigation options, see ECONorth-west 1999.)

Discussion and conclusionsIn a 27 July 1999 speech, Senator Slade Gorton (RWA)claimed that removing the four Snake River dams would bean unmitigated disaster and an economic nightmare(Hughes 1999). In February 2000, George W. Bush said,Breaching the [Snake River] dams would be a big mis-take....The economy and jobs of much of the Northwest de-pend on the dams (Seattle Times, 26 February 2000, p. A1).In its 1 May 2000 editorial, the Oregonian likened breachingthe dams to taking a sledgehammer to the Northwest econ-omy. The Clinton administration, perhaps sensitive to theseclaims, decided to leave the dams in place while other salmon-recovery methods were attempted.

Just 10 years ago, many politicians offered similar predic-tions on the disastrous effects of protecting the northernspotted owl. Representative Bob Smith (ROR) predicted theowl listing would wreak havoc on the people and economyof the Pacific Northwest (Ulrich and Ota 1990). During acampaign swing through the Pacific Northwest in 1992, Pres-ident George Bush warned, It is time we worried not onlyabout endangered species, but endangered jobs (Hong andYang 1992). President Bush and many of the other politiciansin those yearsSenators Mark Hatfield and Bob Packwoodand Representative Bob Smithembraced the simplisticlogic of owls versus jobs, just as some today frame the dam-breaching debate as salmon versus jobs: We can protect en-dangered jobs, or we can protect endangered species, butnot both.

In fact, the Pacific Northwest economy has boomed, con-sistently outperforming the national economy, whether mea-sured by jobs, income, or sheer exuberance, throughout the1990s. Between 1988 and 1998, logging in Oregon and Wash-


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ington fell 91% on federal lands and 52% overall, and timber-industry employment dropped 20%. But new jobs inother sectors offset these losses. Total employment actually in-creased 31%, while inflation-adjusted per capita income grew26% (USDC 2000, Warren 19902000). Ten years ago this re-gion had never experienced widespread economic changes toprotect a species. The current deliberations on the fate ofthe Snake River dams would benefit from a consideration ofthese experiences and the implications for developing andconducting costbenefit analyses. Ignoring the constraints in-herent in the Corps analysis and the resulting biases thatoverestimate costs and underestimate benefits, the jobs lossespredicted by the Corps do not describe a disaster, night-mare,or sledgehammer for either the local or the regionaleconomy.

An incomplete or otherwise flawed analysis lends itself tosuch misrepresentation. It also fails to characterize accuratelythe range of potential economic consequences. By applyingthe analytical principles in box 1, however, costbenefit an-alysts would meet the relevant professional standards and,not incidentally, provide more useful information to deci-sionmakers. For emphasis and clarification, we describe be-low the important components of the two primary analyt-ical principles.

Measure all relevant costs, benefits, and employmentgains and losses. A policy decision will rarely, if ever, gener-ate only costs or only benefits. The impacts of removingdams, for example, extend far beyond dams, fish, and farm-ers, just as the Pacific Northwest found that the impacts of re-stricting logging extended far beyond owls and timber work-ers. In some cases a policy decision may generate costs orbenefits some distance away from the area directly affected bythe decision. For example, removing a dam may influence pop-ulations of anadromous fish, which in turn influences incomesand employment far downstream in coastal communitiesengaged in commercial fishing.

Account for

how dams vary and why it matters for the emerging science of

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