37March 2006Volume 33 Number 1GEOSCIENCE CANADA

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Virginia S. Gillerman1, DavidWilkins2, Krista Shellie3 and RonBitner41Idaho Geological Survey, Boise StateUniversity, Boise, ID 83725-1535,vgillerm@boisestate.edu2Department of Geosciences, Boise StateUniversity, Boise, ID 83725-1535,dwilkins@boisestate.edu3USDA-ARS Horticultural Crops ResearchLaboratory, Parma, ID 83660,kshellie@uidaho.edu4Bitner Vineyards, Caldwell, ID 83607,rmbitner@bitnervineyards.com

SSUUMMMMAARRYYThis article explores the unique combi-nation of factors that shape the terroirof Idaho’s principal wine grape-growingdistrict. Most Idaho wine grape vine-yards are located in the Western SnakeRiver Plain (WSRP) rift basin (~43°N,~117°W) on soils derived from lake,river, volcanic and wind-blown sedi-ments. The underlying Tertiary andQuaternary rocks record the geologichistory of ancient Lake Idaho, its inter-action with basaltic volcanism, and sub-

sequent Pleistocene fluvial processes andcatastrophic floods. The arid to semi-arid, mid-latitude steppe climate of theWSRP provides fewer growing degreedays than American Viticultural Areas(AVAs) in Walla Walla, Washington andNapa Valley, California, but still allowscultivation of Vitis vinifera grapes. Otherdifferences include lower precipitation,higher solar radiation during the growingseason, and greater threat of cold injury.Wine grapes grown in the WSRP requireirrigation, and irrigation is used to man-age canopy size and manipulate vinephysiology. Wine grape acreage inIdaho has increased dramatically since1993 and is estimated, in 2003, at about500 ha with the white wine cultivarsRiesling, Chardonnay, andGewürztraminer comprising about 60%of production, and Cabernet Sauvignon,Merlot and Syrah as principal red winecultivars.

RRÉÉSSUUMMÉÉLe présent article porte sur la combinai-son particulière de facteurs qui définit leterroir du principal district viticole del’État d’Idaho. La plupart des vignoblesde l’Idaho sont situés dans le bassin defossé tectonique (~43°N, ~117°O) de laWestern Snake River Plain (WSRP), surdes sols formés de sédiments lacustres,fluviatiles, volcaniques et éoliens. Lescouches tertiaires et quaternaires sous-jacentes témoignent des événementsconstitutifs de l’histoire géologique del’ancien lac Idaho, de phénomènes inter-actifs dont il a été le théâtre, soit un vol-canisme basaltique, ainsi que des proces-sus fluviatiles et des inondations cata-strophiques pléistocènes. Bien que leclimat aride à semi-aride de steppe enaltitude moyenne de la WSRP comportemoins de degrés-jours de croissance queles zone les zones viticoles étasuniennes(AVA) de Walla Walla de l’État de

Washhington et de la vallée de Napa del’État de Californie, il permet tout demême la culture des raisins de Vitisvinifera. De plus, cette région reçoitmoins de précipitations, plus d’en-soleillement durant la saison de crois-sance, et elle est davantage exposée auxmeurtrissures du froid. Les vignes deraisins de cuve cultivés dans la WSRPdoivent être irriguées, l’irrigation perme-ttant d’agir sur l’ampleur du feuillage etsur la physiologie du vin. La superficiede culture du raisin de cuve en Idahos’est considérablement accrue depuis1993 pour atteindre 500 ha environ en2003, les cultivars à vin blanc deRiesling, Chardonnay, etGewürztraminer constituant 60 % de laproduction, et ceux du CabernetSauvignon, du Merlot et du Syrah con-stituant les principaux cultivars à vinrouge.

IINNTTRROODDUUCCTTIIOONNBest known for scenic beauty, white-water rivers, and quality potatoes, thestate of Idaho is receiving medals forpremium red, white, and ice wines pro-duced from Idaho-grown Vitis vinifera L.grapevines. Idaho normally is associatedwith high mountains and cold tempera-tures, but southwestern Idaho’s low ele-vation and relatively moderate climate issuitable for growing European winegrapes. The Snake River Plain (SRP) is acrescent-shaped belt of sagebrush-cov-ered volcanic rocks ranging in widthfrom 65 to 100 km, and stretchingroughly 600 km across southern Idaho;the principal wine grape-growing districtis located in the Western part of theSnake River Plain (WSRP, Figs 1, 2).Geologically, the WSRP is distinguishedfrom the Eastern Snake River Plain(ESRP, Fig. 1) by the much greater pro-portion of sedimentary rocks relative tobasalts and a more fault-bounded, rift-

SERIES

basin geometry. Yellowstone and GrandTeton National Parks lie at the easternterminus of the SRP and contain theheadwaters of the Snake River, whichdrains about 283,000 km2 during its~1120 km trek across the SRP anddown Hells Canyon, before joining theColumbia River en route to the PacificOcean (Fig. 3). Irrigation from theSnake River has been instrumental formuch of Idaho agriculture, includingviticulture.

The WSRP includes parts of 10Idaho counties and a small part ofOregon, but 75% of commercial winegrape acreage, including the largest vine-yard at 150 ha, is located in Idaho’sCanyon County [United StatesDepartment of Agriculture (USDA),Idaho Agricultural Statistics Service,1999; Fig. 2]. Elevation in the WSRPranges from about 660 to 1100 m a.s.l.(USDA, 1972) and vineyards are locatedat elevations ranging from 695 to 890 m.The WSRP is at a similar latitude (43°Nto 44°N) as wine regions in France, Italy

and Spain, and chapters of its geologichistory are similar to the history of theneighbouring states of Washington andOregon (Meinert and Busacca, 2000).In Idaho, European wine grape produc-tion north (~47°N) or east (~114°W) ofthe WSRP is limited by low winter mini-mum temperatures and limited length ofgrowing season.

The history of wine productionin Idaho is similar to that of neighbour-ing states and the province of BritishColumbia, Canada (Meinert andBusacca, 2000; Taylor et al., 2002), dat-ing to the mid-1800s when French andGerman immigrants cultivated Europeangrapes and produced wines near theconfluence of the Snake and Columbiarivers. Native North American grapespecies that host pests detrimental toEuropean vines, like the insect phyllox-era [Daktulosphaira vitifoliae (Fitch)], werenot present in this region, and own-root-ed European vines were successfully cul-tivated. After the United States’ prohibi-tion of alcohol (1920-1933), wine grape

production did not recover until around1970. Idaho created a state commissionin 1984 to promote growth and develop-ment of the grape and wine industry.Acreage and number of wineriesincreased steadily such that by 1998,wine grapes were Idaho’s fourth largestfruit crop (USDA, Idaho AgriculturalStatistics Service, 1999). Between 1993and 1999, the latest year for which offi-cial statistics are available, acreage andnumber of vineyards doubled to 262 hain 27 vineyards. An informal 2003 sur-vey by the Idaho Grape Growers andWine Producers Commission of 35 ofits ~50 grower members, suggested fur-ther doubling of acreage to about 489ha with > 50% of vineyards at < 6 ha.Cultivar acreage, based on 65% of esti-mated total acreage, suggests a predomi-nance of white wine cultivars (60%)including Riesling (32%), Chardonnay(18%), and Gewürztraminer (7%).Principal red wine cultivars by acreageare Cabernet Sauvignon (19%), Merlot(12%), and Syrah (5%). An extensive

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Figure 1. Map showing the location of the Western Snake River Plain (WSRP) relative to other major grape-growing regionsand tectonic elements in the Pacific Northwest. Hells Canyon of the Snake River lies along the Idaho-Oregon border from theWSRP north to Lewiston. Physiographic and structural features (Ball and bar symbols indicate the downthrown side of thefault): BFZ, Brothers Fault Zone; ESRP, Eastern Snake River Plain; IdB, Idaho Batholith; O-WL, Olympic-Wallowa Lineament;Ow M, Owyhee Mountains; Stn M, Steens Mountains; VFZ, Vale Fault Zone; WSRP, Western Snake River Plain; YC,Yellowstone Caldera. Cities: B, Boise; L, Lewiston; R, Roseburg; P, Portland; TF, Twin Falls; WW, Walla Walla. AVA refers toAmerican Viticultural Area, as designated by the Alcohol and Tobacco Tax and Trade Bureau (TTB) formerly called the Bureauof Alcohol, Tobacco and Firearms (BATF). Geology modified from Wood and Clemens (2002).

amount of arable land is available forfuture plantings, limited more by accessto water for irrigation than by land suit-ability. The Idaho Grape Growers andWine Producers Commission currentlyis petitioning for the first designatedAmerican Viticultural Area (AVA) inIdaho, to be delimited by the 1050 ma.s.l. elevation boundary of ancient LakeIdaho (Fig. 2).

TTEERRRROOIIRR OOFF IIDDAAHHOO’’SS WWEESSTTEERRNNSSNNAAKKEE RRIIVVEERR PPLLAAIINNPPhhyyssiiooggrraapphhyyThe SRP is a large, arcuate-shaped,topographic depression, mostly filledwith volcanic rocks, that crosses theentire state (Fig. 1). The westward-flow-

ing Snake River lies near the southernboundary of the SRP and historicallyhas provided water for much of theregion’s agriculture (Fig. 3). The riverhas formed either a steep-walled canyonwhere it incises thick piles of basalticlava flows, or a more open valley whereit cuts Tertiary and Quaternary lacus-trine and fluvial sediments. The easternpart of the SRP is higher in elevationand too cold for V. Vinifera, so mostIdaho wine originates from the WSRP,most notably the Sunnyslope area inCanyon County (Fig. 2), where the vine-yards are located on the tops and flanksof a series of ridges between Homedaleand Lake Lowell, just east of the townof Marsing (43°33’N, 116°48’W) on the

Snake River (Fig. 4). Though the vine-yards are located within a few kilometersof the Snake River, most slopes wouldsupport only native vegetation, such assagebrush, rabbitbrush, and bunchgrasses, were it not for widespread irri-gation from a large number of irrigationcanals. A second cluster of vineyards islocated near Glenns Ferry (42°57’N,115°18’W) in Elmore County (Fig. 2),and there also are vineyards nearLewiston (46°25’N, 117°01’W; Fig. 1) innorthern Idaho.

GGeeoollooggiicc SSeettttiinnggThe geologic history of southwesternIdaho resembles that of easternWashington with its flood basalts, north-

39March 2006Volume 33 Number 1GEOSCIENCE CANADA

Figure 2. Map of the Western Snake River Plain grape-growing region, noting the locations of vineyards. Shaded area marksthe extent (at an elevation of 1050 m a.s.l.) of Pliocene Lake Idaho, and is the boundary of the proposed Snake River ValleyAVA. The Sunnyslope area referred to in the text is the cluster of vineyards west of Deer Flat (DF), which is located inCanyon County. Towns shown are Weiser (W), Boise (B), Parma (P), Marsing (M), Glenns Ferry (GF), Hagerman (H), and TwinFalls (TF).

west-trending structures, extensive loessblankets, and glacial outburst floods(Meinert and Busacca, 2000, 2002).However, Idaho’s geologic historyincludes Lake Idaho, a paleo-system oflakes and floodplains which, at its maxi-mum, stretched 240 km northwest-to-southeast from what is now the Oregon-Idaho state line (117°W) to just west ofTwin Falls (42°33.5’N, 114°28’W; Fig. 2).

North of the SRP are

Cretaceous granites of the IdahoBatholith, along with assorted Eocenevolcanic rocks, older sedimentary rocks,and the Miocene (14-17 Ma) ColumbiaRiver Basalts of the Weiser embayment(Figs. 1, 2). South of the WSRP are 12-15 Ma volcanic rocks of the OwyheeMountains that overlie the southernextension of the granitic basement. TheWSRP is a northwest-trending, 300-kmlong and 70-km wide intracontinental

rift basin, whose margins are well-defined boundary faults that parallelother structural zones such as theOlympic-Wallowa lineament andBrothers fault zone (Fig. 1) in Oregonand Washington (Wood and Clemens,2002). In contrast, the eastern SRP is astructural downwarp, associated withextension and magmatism along thetrack of the Yellowstone hot spot. Thismantle plume helped generate the volu-minous basalt-rhyolite volcanism of theSRP, and the Pleistocene-RecentYellowstone caldera of YellowstoneNational Park (Pierce et al., 2002).

The major faulting which down-dropped the centre of the WSRP beganabout 12 Ma and ended by approximate-ly 9 Ma, although minor warping andstructural adjustment may have contin-ued locally (Wood, 2004). Rhyoliticflow domes mark the margins; basalticvolcanic rocks interbedded with the ear-liest sediments show that volcanism andearly basin formation were contempora-neous (Wood and Clemens, 2002). Ageneralized stratigraphic sequence isgiven in Table 1. Vineyards are plantedon soils derived from many units, butmost notably the sands and silts of thePliocene Glenns Ferry Formation, localbasalts, the Tenmile and Terrace gravels,and finer grained Holocene deposits ofthe Bonneville Flood (Fig. 4).

As the rift formed, water accu-mulated in a series of lakes, floodplainsand wetlands, with marginal beaches andstreams. Fish and terrestrial vertebratefossils are abundant locally in the com-plex sequence of lacustrine and relatedfloodplain-to-shoreline facies sedimenta-ry rocks which make up the TertiaryIdaho Group deposits of paleo-LakeIdaho. Sediments deposited in LakeIdaho include sand, silt, and clay as wellas local volcanic ash. Lake level roseand fell as the basin subsided; the maxi-mum extent of Lake Idaho was about 4million years ago, near what is currentlythe 3600-ft (1100-m) elevation contour(Wood and Clemens, 2002).

To the east, in the HagermanFossil National Monument (42°49’N,114°57’W), basalts interbedded with sev-eral hundred feet of Glenns Ferry sedi-ments have been dated at 3.4 Ma (Hartet al., 1999). By approximately 2 Ma,floodplain and marsh sediments of thelater part of the Glenns FerryFormation were deposited east of

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Figure 3. Regional map of the WSRP showing modern river drainages and outlineof late Pleistocene Lake Bonneville with its associated outburst flood (flood pathand extent), after O’Connor (1993). Abbreviations: O (Owyhee River), B (Boise), S(Salt Lake City), GSL (modern Great Salt Lake).

Marsing on the Chalk Hills topographicridge (Wood, 2004; Reppening et al.,1994) that underlies the Bitner vineyards.Lizard Butte (Fig. 4) is a phreatomag-matic basaltic volcano that eruptedthrough wet lake and floodplain sedi-ments. A short distance southeast ofMarsing, near Pickles Butte and Idaho’slargest vineyard, is a subaerial basalt flowthat buries stream gravels of the ances-tral Snake River. The age of the basaltflow, dated at 1.58 Ma by the Ar40/Ar39

method, demonstrates that by earlyPleistocene time, the WSRP had com-pletely transformed from a filling riftbasin to an incised lowland (Othberg,1994). The draining of Lake Idaho wasa consequence of headward erosion ofancestral Hells Canyon by the Snake-Salmon river system coupled with LakeIdaho overtopping a divide and drainingnorthward through the ancestral HellsCanyon. Although timing of this eventis poorly constrained, the initial spillovermost likely occurred between 2 and 4

41March 2006Volume 33 Number 1GEOSCIENCE CANADA

Epoch/Period Age (Ma) Group Rock UnitHolocene Q

Pleistocene

14,500 yrs.

0-2 Ma

< 2 Ma

Bonneville Flood

Terrace Gravels

Snake River Group

Sediments

River Gravels w/ local basalt

Basalt flows, hydrovolcanoes

Pliocene T

(2-5 Ma) 2-5 Ma?

Tenmile Gravel

Idaho Group

~~~~~~~~~~~

Gravels

Glenns Ferry Formation, or

Terteling Springs Fm. (N. side)

Miocene T

(5-24 Ma)

7-10 Ma?

11-12 Ma?

12-15 Ma

14-17 Ma

Idaho Group

Older Basalts/Seds

Margin Rhyolites

Idavada Volcanics

Columbia River Group

Chalk Hills Formation

inc. Banbury/Poison Creek Fm.

“Dry Rhyolites”

Rhyolites, Volcanics of Owyhee Co.

Basalts

Table 1: General stratigraphy of the WSRP, modified from Wood and Clemens(2002). Q = Quaternary; T = Tertiary.

Figure 4. Simplified geologic map for major WSRP grape-growing region, modified from Othberg and Stanford (1992).Towns are Homedale (H), Marsing (M), Caldwell (C), Nampa (N), with other features of Sunnyslope (SS), Lake Lowell (LL),Arena Valley (AV), and Lizard Butte (LB). Wineries and general areas of vineyards shown schematically by grape clusters; oth-ers are east of this map. Geologic units: Qs, Quaternary alluvium/surficial deposits; Qbf, Bonneville Flood deposits undiffer-entiated; Qg, Quaternary terrace gravels, younger; Qdg, Deer Flat Terrace Gravel; Qb, Quaternary basalts; QTtg, TenmileGravel; and Ts, Tertiary lacustrine and fluvial sediments, Idaho Group. Heavy black lines are faults. Table 1 gives generalizedstratigraphy.

Ma with subsequent slow downcuttingof the divide (Wood and Clemens,2002).

Draining of the lake and lower-ing base level allowed incision of the oldlacustrine sediments by the ancestralBoise River, forming a stepped series ofQuaternary stream terrace gravels thatmark previous river base level stands(Othberg, 1994; Othberg and Stanford,1992). The oldest, the Tenmile Gravel(Table 1), caps the northwest-trendingridge of Glenns Ferry sediments atSunnyslope, where many vineyards arelocated (near 43°35’N, 116°47’W), andtowards Chalk Hills where additionalvineyards are located (Fig. 4). Ayounger, lower elevation terrace gravel,known as the Gravel of Deer FlatTerrace, overlies the Tertiary sedimentsand flanks the ridges. Several vineyardsare planted on the sandy pebble gravelof the Deer Flat Terrace, which is locallyoverlain by loess. Terrace elevationswere controlled by paleo-base levels, andin turn, those terrace elevations dictatethe vineyard elevations and influenceland use.

The Bonneville Flood, whichoccurred 14,500 years ago, is the mostrecent geologic event important to thevineyards of Idaho (Scott et al., 1982).This catastrophic flood resulted fromerosion of a low divide that was over-topped by a northern arm of LakeBonneville, the ancestral Great Salt Lakein Utah (Fig. 3). The resulting deluge ofwater down the Snake River lasted 6

months (D. Currey, personal communi-cation, 2005) and the discharge peakedat about 1 million m3•s-1 at the LakeBonneville outlet (O’Connor, 1993).This single flood event created the west-ern Snake River Canyon of today.Because the discharge was variable overthe duration of the flood, multi-metre-sized boulders to sand to silt-sized sedi-ments were deposited (Fig. 5). Duringthe highest flood discharges, water washydraulically backed up by constrictedflow through Hells Canyon, causing rela-tively quiet water to pond in the lowerreaches of the Snake, Payette, Boise, andOwyhee river valleys (Othberg, 1994;Fig. 3). As a result, from Hells Canyonsouth to Marsing, fine-grained slackwa-ter silt blankets the late Pleistocene ter-races below an elevation of 747 m a.s.l.(2,450 feet), which is lower than theearly Pleistocene Deer Flat Terrace(Othberg and Stanford, 1992).Vineyards in Arena Valley northwest ofMarsing are located adjacent to a circularerosion feature where the latePleistocene Whitney terrace was scouredby late stages of the Bonneville Flood(Fig. 4).

PPeeddoollooggiiccaall DDeessccrriippttiioonnSoil type in the WSRP varies somewhataccording to the lithology of the parentmaterial, and the timeframe and climateunder which the soil developed.Surficial loess, sand, and BonnevilleFlood slackwater silts are the predomi-nant parent materials at the vineyards,

and the soils normally contain abundantquartz and feldspar grains derived fromthe Tertiary units, though fields near thebasaltic vents may contain more clay andmafic minerals.

Older soils generally tend to bemore complex and show more extensiveduripan (caliche) development and clay-rich B-horizons (Othberg, 1994). Insome areas, soil has developed on ablanket of loess up to 4 m thick(Othberg and Stanford, 1992). TheBonneville Flood sediments are youngerthan the loess and typically show littlesoil development. Soils on the Deer FlatTerrace Gravel, which underlies somevineyards, contain more than 25% pedo-genic clay and a buried duripan greaterthan one metre thick. Soils on the olderTenmile Gravel, which underlies ridge-top vineyards, may have 50% clay and a2-m thick duripan (Othberg, 1994).The thicker, platy duripans promotealkaline soils and may impede subsurfacedrainage and root penetration by thevines.

Vineyard locations in theSunnyslope area were spatially comparedwith their underlying soil characteristicslisted in the NRCS Soil SurveyGeographic (SSURGO) Database. Thesoil database for Canyon County listsover 50 soil series and eight associations.The GIS analysis revealed no single soilseries common among the vineyards; 19vineyards are located on 11 soil seriesthat share many characteristics. Mostsoils underlying the vineyards in theSunnyslope area are silty to sandy loamswhere silt percentages range from 58%to 67% in the upper horizons (USDA,1972). The series are characterized asmoderately to extremely well-drained,moderately calcareous and alkaline sub-groups of aridisols and entisols; theyhave moderate to high cation exchangecapacities. Most of the soils are fairlyshallow (<1 m) with soil depth aninverse function of slope (i.e., steeperslopes have shallower soil depths). Allthe soils used for agriculture (that is, notrange or urban) require surface irrigation(USDA, 1972). The combination ofmoderate to steep slopes, moderately toexcessively drained soils, and easily erod-ed sediments places further limitationson land use in the region. The soilsassociated with the vineyard sites typical-ly are characterized as not being primefarm land (i.e., having moderate to

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Figure 5. Boulders in Bonneville Flood Melon Gravels, at Celebration Park alongSnake River, south of Melba, Idaho.

severe limitations; USDA, 1972), withprescribed uses ranging from irrigatedpasture to fruit orchards (where hardfreezes are not a danger). Prime farmland in the area (i.e., having higher soilwater availability, deeper soil depths,lower gradient slopes) is used primarilyfor large scale row crop operations suchas sugar beets, alfalfa, corn, and onions.More recently, prime farmland on theurban fringes has been converted tohousing subdivisions.

CClliimmaattee The climatic factors of precipitation(amount and seasonal pattern), growingseason length, and growing degree days(e.g., Winkler et al., 1974) all affect grapeand wine quality (van Leeuwen et al.,2004; Jones and Davis, 2000) and thuscontribute strongly to terroir. TheWSRP is located in the continental inte-rior of the western U.S. approximately500 km east of the Cascade Range (Fig.1). Even with its continental interiorlocation, the region is on a climatichinge line and exhibits influences ofboth continental and marine climates.Winter months (November 1 throughMarch 30) provide two-thirds of theprecipitation for the region (Fig. 6).Winter precipitation is caused both bystorms from the Gulf of Alaska track-ing under the influence of the dominantwesterlies at this latitude (Godfrey,1999), and more tropical moisture origi-nating near Hawaii tracking under theinfluence of the subtropical jet streamand producing what is colloquiallyreferred to as the “Pineapple Express.”Whereas winters may be cold and over-cast, the summer growing season (April1 through September 30) is character-ized by warm, dry days with a possibleaverage of 70% of sunshine [WesternRegional Climate Center (WRCC), 2005;http://www.wrcc.dri.edu/].

CClliimmaattiicc CCoommppaarriissoonnss Thirty-year monthly climate normals,covering the period from 1971-2000,were obtained from the NationalClimate Data Center’s (NCDC) onlinearchives (http://www.ncdc.noaa.gov/oa/ncdc.html) for four locations in theWSRP (Glenns Ferry, Weiser, Parma,and Deer Flat Dam, Idaho). TheseIdaho climate normals were comparedto climate data from other grape-grow-ing regions (Walla Walla, Washington;

Roseburg, Oregon; and Napa,California) in the western United States(Table 2; Fig. 6). The NCDC climatenormals summarize mean values formonthly maximum and minimum tem-peratures and precipitation.

Despite latitudinal and situation-al variation among the regions, seasonali-ty of precipitation is strikingly similar(Fig. 6). All four regions experience apronounced summer precipitation mini-mum, with the Napa climate stationrecording only a trace of July precipita-tion. Despite a similar seasonal precipi-tation pattern, the WSRP receives abouthalf the annual precipitation of theother regions (Fig. 6). The lower annualprecipitation in the WSRP may be partlyattributed to the rain shadows created bythe Cascade, Sierra Nevada, and morelocally the Owyhee ranges.

Temperature is what most distin-guishes the different regions, resulting atleast in part from differences in eleva-

tion. The WSRP ranges from 640 to 765m, compared to a low of 12 m at Napaand to 365 m at Walla Walla. The meanannual temperature in the WSRP of10.8°C is the lowest in the regional com-parison group, and is close to the 10°Cisotherm described as the poleward tem-perature limit for cultivation ofEuropean grapes (Jones et al., 2004; DeBlij, 1983). The WSRP also has signifi-cantly lower mid-winter (January) meanminimum temperatures than the otherwestern US districts, and two months,December and January, have mean tem-peratures below 0°C (Fig. 6). Whereasthese mean temperatures are not solelylimiting, they provide evidence for thepotential of damage from severe coldtemperatures (i.e., <-18°C, Table 2;Winkler et al., 1974). This potential fordeep freezing temperatures has implica-tions for viticultural practices.

The temperature contrastbetween the WSRP and the other

43March 2006Volume 33 Number 1GEOSCIENCE CANADA

Figure 6. Comparison of climate variables, with mean precipitation as bars andmean temperature as lines, for the averaged WSRP stations; Walla Walla,Washington; Napa, California; and Roseburg, Oregon, in the western United States.Data were compiled from online archives available through the National ClimateData Center and the Western Regional Climate Center, Desert Research Institute,Reno, Nevada (http://www.wrcc.dri.edu/summary/).

regions also translates into differences inlength of growing season (i.e., numberof days during the growing season withless than 50% probability of reaching0°C; Winkler et al., 1974). With its high-er elevation and more interior locationresulting in colder winters, the WSRPhas the shortest growing season of theregions in Table 2, and this may be alimiting factor for some grape varieties.

Another climate factor, conti-nentality, is defined by the annual rangein temperature and reflects remotenessfrom moderating ocean influences; high-er temperature ranges indicate a greaterdegree of continentality. Napa andRoseburg have the most moderate tem-perature ranges, reflecting their proximi-ty to the ocean, whereas the WSRP hasmean monthly temperatures that vary byalmost 25°C (Fig. 6). This range isslightly greater than at Walla Walla andmuch greater than at Roseburg, which isat the same 43°N latitude but has amonthly mean temperature range ofonly 16°C.

GGrroowwiinngg DDeeggrreeee DDaayyssGrowing degree days (GDD) is a sum-

mation of accumulated heat units asmeasured by days during the growingseason (April 1 to October 31 in thenorthern hemisphere) with a mean tem-perature over an established base (10°Cfor grapes; Winkler et al., 1974); underthe 10°C temperature base threshold, lit-tle growth or development of winegrapes occurs. Growing degree days alsocan be used as an indicator of the tim-ing, independent of the calendar, ofphenological events including dates ofbudbreak, bloom, veraison (onset ofberry ripening) and harvest.

The climate data indicate thatthe Walla Walla Valley and Napa districtseach fall within the Winkler Region IIIrange (1666-1944 Base 10°C GDD),with higher growing degree days thanthe WSRP and Roseburg districts, whichare within the Winkler Region II range(1389-1665 Base 10°C GDD; Table 2;Fig. 7). The higher growing degree daysrating for Napa Valley reflects the rela-tively warm early growing season,whereas values for Walla Walla reflecthigh temperatures during June throughAugust. In contrast, Roseburg has thelowest growing degree day rating,

reflecting that district’s marine influenceand generally low summer temperatures.Seasonal temperatures rise quickly in theWSRP, with an average last and first dayof 0°C frost on May 10 and September29, respectively. Thus, the WSRP has ashorter growing season than the otherthree districts.

VViittiiccuullttuurraall PPrraaccttiicceessSuccessful production of wine grapes inthe WSRP requires careful considerationof site and cultivar selection, number offrost-free days, and availability of sup-plemental water. Risk of frost damageis minimized by locating vineyards onslopes with good air drainage towardsriver or valley bottoms (Fig. 8a).Vineyards tend to be located on hillsideswith a southern or southwestern aspectto maximize heat unit accumulation andto avoid direct exposure to prevailingnorthwesterly winds. Vines are generallyplanted in north-south rows to facilitatecold air drainage and to provide equalsunlight exposure on both sides of thevine canopy (Fig. 8b). The predominantvine training system is cordon-trained,spur-pruned, with vertical shoot posi-tioning on a six wire trellis. Commonin-row spacing is 2.1 m with 2.7 mbetween rows. Target shoot length isabout 1.5 m. Dormant vines typicallyare pruned manually to two (red culti-vars) or three (white cultivars) bud spurs,yielding 24 to 28 (red) or 36-42 (white)buds per vine. Some growers mechani-cally pre-prune dormant vines, and manymechanically harvest their grapes.

An estimated 60% of total pro-ducing wine grape acreage in the WSRPis composed of fairly cold hardy, whitewine cultivars (Riesling, Chardonnay, andGewürztraminer) that have a low heatunit requirement to reach maturity(Wolfe, 1998). The red wine cultivarsCabernet Sauvignon, a late maturing cul-tivar with a relatively high heat unitrequirement (Wolfe, 1998), and Merlot, aless cold hardy cultivar (Wolfe, 1998),comprise about a third of producingacreage, and many new plantings includered wine cultivars with similar tempera-ture requirements.

The large range (about 280) inGDD among WSRP weather stationsites as depicted in Table 2, as well asthe successful commercial production ofCabernet Sauvignon and Merlot, high-lights the importance of the vineyard

44

StationName

Elev(m)

Location(lat/long)

MAT(°C)

MAP(mm)

GDD10°C

GSL0°C

XMT(°C)

CNT(°C)

Parma Exp.Station,Idaho

698 43.8°N116.95°W

9.9 283 1342 140 -32(1990)

25

Weiser, Idaho 646 43.67°N116.68°W

11.0 307 1637 136 -34(1990)

27

Deer FlatDam, Idaho

765 43.59°N116.75°W

11.6 258 1626 165 -30(1989)

24

Glenns Ferry,Idaho

752 43.61°N116.57°W

10.5 248 1413 125 -32(1989)

24

WSRP 715 10.8 274 1504 142 25Roseburg,Oregon

130 43.2°N123.36°W

13.0 855 1484 218 -16(1989)

16

Walla WallaAirport,Washington

355 46.05°N118.28°W

12.3 530 1715 206 -11(1985)

23

Napa StateHospital,California

11 38.28°N122.27°W

15.0 672 1753 259 -10(1990)

12

Table 2: Climate Data for the WSRP and other grape growing districts in the west-ern United States. Column heading abbreviations: MAT, Mean Annual Temperature;MAP, Mean Annual Precipitation; GDD, Growing Degree Days; GSL, GrowingSeason Length; XMT, 30-year extreme minimum winter temperature (event year inparenthesis); CNT, Continentality (mean annual temperature range). Data compiledfrom the National Climate Data Center (http://www.ncdc.noaa.gov).

mesoclimate. For example, a 140-daygrowing season with 1442 growingdegree days is reported for the IdahoParma Experimental weather station inTable 2 (Fig. 2). However, temperaturedata collected in a hillside vineyardplanted in 1997 with a northern aspectand north-south row orientation record-ed 1581, 1851, and 1644 growing degreedays at the Parma Experiment Station(simple average, base 10°C, daily temper-ature), respectively over three vintages(2002-2004). The average number offrost-free days needed in this vineyardfor fruit to reach maturity was 147-150for Merlot, 150-154 for Chardonnay, and160 or more for Cabernet Sauvignonand Syrah (Table 3). Fruit compositionof each cultivar at harvest reached opti-mum brix (~23%; this is a measure ofsugar concentration in the grapes), pH(~3.6), and titratable acidity (0.6 g/dl),suggesting adequate season duration andtemperature accumulation. The growingseason and heat unit accumulation datain Table 2 suggest that the climate in

45March 2006Volume 33 Number 1GEOSCIENCE CANADA

Figure 7. Mean cumulative growing degree days (GDD) from 1971-2000 for the four averaged WSRP stations; Walla Walla,Washington; Napa, California; and Roseburg, Oregon, calculated from National Climate Data Center from average monthlymaximum and minimum temperatures between April 1 and October 31 using a base temperature of 10°C.

Cultivar (clone) DTM(days)

Brix (%) pH TA (g/dl) 100 berryweight (g)

Merlot (01) 147 23.6 3.56 0.52 104.5

Merlot (08) 149 24.0 3.70 0.51 105.5

Chardonnay (29) 154 22.9 3.38 0.75 97.9

Chardonnay (49) 150 23.1 3.38 0.74 102.5

CabernetSauvignon (02)

162 24.1 3.65 0.45 95.9

CabernetSauvignon (04)

160 23.3 3.62 0.46 95.0

CabernetSauvignon (11)

171 23.7 3.51 0.59 104.0

Syrah (07) 164 24.3 3.73 0.50 113.7

Table 3: For own-rooted, Vitis vinifera L. vines (wine grapes of European origin)planted in 1997 at the Parma Experimental Station, Idaho, table shows: Averagenumber of days from budburst to harvest (DTM); Harvest percent brix (a measureof sugar concentration in the grapes); pH; titratable acidity (TA); and berry weightof fruit harvested in 2002, 2003, and 2004 from the cultivars listed.

Parma is marginal for cultivation ofMerlot and Chardonnay and not suitablefor production of Cabernet Sauvignonor Syrah, yet vineyard heat unit accumu-lation and number of frost-free dayssuggest a suitable vineyard mesoclimatefor production of these red wine culti-vars.

Vine cold-hardiness is not wellunderstood because of its complexinteraction with environmental condi-tions, including temperature and pho-

toperiod (Howell, 2000), tissue maturity(Goffinet, 2000), and vine water status(Wample et al., 2000), but growers in theWSRP minimize yield loss from coldinjury by adopting preventative cultiva-tion practices. For example, own-rootedcuttings are planted at a depth of 30 to36 cm to facilitate root survival in theevent of above ground vine loss fromprolonged low temperature. The vinesare trained to two trunks (Fig. 8b) witheach trunk forming a unilateral cordon

that extends to one side or other of thevine. In the event of cold damage, thesecond trunk can be used to replenishdamaged wood. The absence of phyl-loxera in the region permits cultivationof own-rooted rather than grafted vines,enabling trunk re-establishment withoutreplanting. Despite these cultivationpractices, injury from prolonged mini-mum mid-winter low temperature wasanecdotally reported in the very coldyears of 1989 and 1990. Another com-mon type of cold injury observed in theWSRP occurs during winter dormancywhen several days of high solar radiationand warm ambient temperature precedean abrupt return to freezing or nearfreezing temperature.

The low annual and growing sea-son precipitation and the shallow soils inthe WSRP facilitate manipulation ofvine physiology through irrigation man-agement. Most irrigation water isobtained from annually recharged snow-pack in mountain ranges surroundingthe WSRP and is delivered through anextensive network of reservoirs andcanals. The majority of vineyards areirrigated with above ground drip lines,although some vineyards utilize over-head sprinklers or furrows. Irrigationscheduling is used to prepare for thefirst fall frost by imposing water stresson the vine to encourage periderm for-mation on green shoots. Periderm is avisible indicator of tissue maturity andhas been associated with bud and canecold hardiness (Goffinet, 2000).Irrigation is then normally applied priorto the first fall frost to bring soil mois-ture up to field capacity in an effort toprotect roots during the winter.Growers also manipulate vine waterstress during the growing season to shiftgrowth from vegetative to reproductivestructures (Greenspan, 2005) and tocontrol canopy as well as berry size.Regulated deficit irrigation is used tocontrol plant water status and to opti-mize fruit quality during the growingseason. Ongoing research is being con-ducted by researchers at the USDA-Agricultural Research Service in Parma,Idaho, to understand how vine waterstatus influences grape components thatcontribute to wine quality.

CCOONNCCLLUUSSIIOONNSSThe Western Snake River Plain is theprincipal wine grape-growing district in

46

Figure 8a. Cliffside Vineyard planted on gently tilted Lake Idaho sediments of theGlenns Ferry Formation (Pliocene), facing west. The Snake River lies at the base ofthe steep slope, along the line of green trees and houses.

Figure 8b. North-south rows of grape vines at Skyline Vineyard with a basaltic cin-der pit in distance adjacent to Sawtooth Vineyard, facing north. Vines are plantedwith double trunks and require drip irrigation.

Idaho. Formation of the WSRP beganbetween 9 and 12 Ma with major exten-sional faulting and volcanism resulting ina down-dropped topographic basin.Over approximately the next 7 millionyears, Lake Idaho and associated streamsand floodplains deposited a successionof fine-grained sand, silt, and ash, withthe lake high stand reaching up to nearthe present-day 1100-metre topographiccontour of the WSRP. After Lake Idahodrained about 2 Ma, the ancestral BoiseRiver incised these Tertiary sediments,forming a stepped series of Quaternarystream terrace gravels surrounding rem-nant highlands of the earlier sediments.The final major geologic event to shapethe region occurred about 14,500 yearsago when Lake Bonneville dischargedcatastrophically into the Snake Rivercanyon, enlarging it and leaving behindwidespread flood deposits from con-glomerates to silt over-bank deposits.Vineyards are planted on all of theseTertiary to Quaternary units where thelandscape allows cold air drainage.

Low temperatures and a shortgrowing season limit the range ofEuropean wine grape cultivars that aresuitable for production in the WSRP.However, late maturing, less cold hardycultivars like Cabernet Sauvignon andMerlot currently comprise an estimated31% of producing acreage in the WSRP,suggesting that vineyard mesoclimate iscritical for production success.Furthermore, controlled irrigation is acritical tool for managing vine cold har-diness.

The geologic and physiographicdiversity within the WSRP suggests thatsubregions, such as the Marsing Valley,the Hagerman Valley, Glenns Ferryregion, and Boise Foothills, may emergeas future viticultural areas. Much workremains to document the key elementsof Idaho terroir that may enhance futurevintages.

AACCKKNNOOWWLLEEDDGGEEMMEENNTTSSWe thank Larry Meinert, RogerMacqueen, and an anonymous reviewerfor comments which greatly improvedthe original manuscript. We also thankmembers of the Idaho Grape Growersand Wine Producers Commission forintroducing us to the subject and provid-ing access to the vineyards.

RREEFFEERREENNCCEESSDe Blij, H., 1983, Wine, A Geographic

Appreciation: Rowmand and AllanheldPublishers, New Jersey, 239p.

Godfrey, B., 1999, Delineation of agrocli-mate zones in Idaho: M.S. thesis,University of Idaho, Moscow, Idaho,202p.

Greenspan, M., 2005, Integrated irrigation ofCalifornia winegrapes: Practical Wineryand Vineyard, v. 17, p. 21-79.

Goffinet, M., 2000, The anatomy of low-temperature injury of grapevines:Proceedings of the ASEV 50th

Anniversary Annual Meeting, SeattleWA, American Society for Enology andViticulture, p.94-100.

Hart, W.K., Brueske, M.E., Renne, P.R., andMcDonald, H.G., 1999,Chronostratigraphy of the PlioceneGlenns Ferry Formation, HagermanFossil Beds National Monument, Idaho[abstract]: Geological Society ofAmerica Abstracts with Programs, v. 31:No 4, p. A15.

Howell, G. S., 2000, Grapevine cold hardi-ness: mechanisms of cold acclimation,mid-winter hardiness maintenance, andspring deacclimation: Proceedings of theASEV 50th Anniversary Annual Meeting,Seattle WA, American Society forEnology and Viticulture, p. 35-47.

Jones, G., and Davis, R., 2000, Climate influ-ences on grapevine phenology, grapecomposition, and wine production andquality for Bordeaux, France: AmericanJournal of Enology and Viticulture, v.51, p. 249-261.

Jones, G., Snead, N., and Nelson, P., 2004,Geology and Wine 8. Modelling viticul-tural landscapes: a GIS analysis of theterroir potential in the Umpqua Valley ofOregon: Geoscience Canada, v. 31, p.167-178.

Meinert, L.D., and Busacca, A.J., 2000,Geology and Wine 3. Terroirs of theWalla Walla Valley appellation, southeast-ern Washington State, USA: GeoscienceCanada, v. 27, p. 149-171.

Meinert, L.D., and Busacca, A.J., 2002,Geology and Wine 6. Terroir of the RedMountain Appellation, centralWashington State, U.S.A.: GeoscienceCanada, v. 29, p. 149-168.

O’Connor, J.E., 1993, Hydrology, hydraulics,and geomorphology of the Bonnevilleflood: Geological Society of AmericaSpecial Paper 274, 83p.

Othberg, K.L., 1994, Geology and geomor-phology of the Boise Valley and adjoin-ing areas, western Snake River Plain,Idaho: Idaho Geological Survey Bulletin29, 54p.

Othberg, K.L., and Stanford, L.R., 1992,Geologic map of the Boise Valley and

adjoining area, Western Snake RiverPlain, Idaho: Idaho Geological SurveyGM-18, 1:100,000 scale, 1 sheet.

Pierce, K.L., Morgan, L.A., and Saltus, R.W.,2002, Yellowstone plume head:Postulated tectonic relations to theVancouver slab, continental boundaries,and climate, in Bonnichsen, B., White,C.M., and McCurry, M., eds., Tectonicand Magmatic Evolution of the SnakeRiver Plain Volcanic Province: IdahoGeological Survey Bulletin 30, p. 5-33.

Reppening, C.A., Weasma, T.R., and Scott,G.R., 1994, The early Pleistocene (latestBlancan-earliest Irvingtonian) FromanFerry fauna and history of the GlennsFerry Formation, southwestern Idaho:U.S. Geological Survey Bulletin 2105,86p.

Scott, W.E., Pierce, K.L., Bradbury, J.P., andForester, R.M., 1982, Revised Quaternarystratigraphy and chronology in theAmerican Falls area, Southeastern Idaho,in Bonnichsen, B., and Breckenridge,R.M., eds., Cenozoic Geology of Idaho:Idaho Bureau of Mines and GeologyBulletin 26, p. 581-595.

Taylor, V., Longerich, H., and Greenough, J.,2002, Geology and Wine 5. Provenanceof Okanagan Valley wines, BritishColumbia, using trace elements: promiseand limitations: Geoscience Canada, v.29, p. 110-120.

U.S. Department of Agriculture, IdahoAgricultural Statistics Service, 1999,Idaho fruit tree census 1999, 21p.(http://www.nass.usda.gov/id/).

U.S. Department of Agriculture, SoilConservation Service, 1972, Soil Surveyof Canyon Area, Idaho: Superintendentof Documents, U.S. GovernmentPrinting Office Washington, D.C., 118p.

van Leeuwen, C., Friant, P., Chone, X.,Tregoat, O., Koundouras, S., andDubourdieu, D., 2004, Influence of cli-mate, soil, and cultivar on terroir:American Journal of Enology andViticulture, v. 55, p. 207-217.

Wample, R.L., Hartley, S., and Mills, L., 2000,Dynamics of grapevine cold hardiness:Proceedings of the ASEV 50th

Anniversary Annual Meeting, SeattleWA, American Society for Enology andViticulture, p. 81-93.

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Wolfe, W., 1998, Site selection in easternWashington: optimizing site and varietychoices, in Watson, J., ed., GrowingGrapes in Eastern Washington: GoodFruit Grower, Yakima, WA, USA, p.27-30.

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47March 2006Volume 33 Number 1GEOSCIENCE CANADA

Owyhee Mountains to the BoiseFoothills, in Haller, K.M., and Wood,S.H., eds., Geological Fieldtrips in south-ern Idaho, eastern Oregon, and northernNevada: Dept. of Geosciences, BoiseState University, pp. 82-105 (USGSOFR2004-1222).

Wood, S.H., and Clemens, D.L., 2002,Geologic and tectonic history of thewestern Snake River Plain, Idaho andOregon, in Bonnichsen, B.,White, C.M.,and McCurry, M., eds., 2004, Tectonicand magmatic evolution of the SnakeRiver Plain Volcanic Province: IdahoGeological Survey Bulletin 30, p. 69-103.

Accepted as revised 17 February, 2006

48

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