The SRP is a large, arcuate-shaped, topographic depression, mostly filled with volcanic rocks, that crosses the entire state (Fig. 1). The westward-flowing Snake River lies near the southern boundary of the SRP and historically has provided water for much of the region's agriculture (Fig. 3). The river has formed either a steep-walled canyon where it incises thick piles of basaltic lava flows, or a more open valley where it cuts Tertiary and Quaternary lacustrine and fluvial sediments. The eastern part of the SRP is higher in elevation and too cold for V. Vinifera, so most Idaho wine originates from the WSRP, most notably the Sunnyslope area in Canyon County (Fig. 2), where the vineyards are located on the tops and flanks of a series of ridges between Homedale and Lake Lowell, just east of the town of Marsing (43°33'N, 116°48'W) on the Snake River (Fig. 4). Though the vineyards are located within a few kilometers of the Snake River, most slopes would support only native vegetation, such as sagebrush, rabbitbrush, and bunch grasses, were it not for widespread irrigation from a large number of irrigation canals. A second cluster of vineyards is located near Glenns Ferry (42°57'N, 115°18'W) in Elmore County (Fig. 2), and there also are vineyards near Lewiston (46°25'N, 117°01'W; Fig. 1) in northern Idaho.

The geologic history of southwestern Idaho resembles that of eastern Washington with its flood basalts, north-west-trending structures, extensive loess blankets, and glacial outburst floods (Meinert and Busacca, 2000, 2002). However, Idaho's geologic history includes Lake Idaho, a paleo-system of lakes and floodplains which, at its maximum, stretched 240 km northwest-to-southeast from what is now the Oregon-Idaho state line (117°W) to just west of Twin Falls (42°33.5'N, 114°28'W; Fig. 2).

North of the SRP are Cretaceous granites of the Idaho Batholith, along with assorted Eocene volcanic rocks, older sedimentary rocks, and the Miocene (14-17 Ma) Columbia River Basalts of the Weiser embayment (Figs. 1, 2). South of the WSRP are 12-15 Ma volcanic rocks of the Owyhee Mountains that overlie the southern extension of the granitic basement. The WSRP is a northwest-trending, 300-km long and 70-km wide intracontinental rift basin, whose margins are well-defined boundary faults that parallel other structural zones such as the Olympic-Wallowa lineament and Brothers fault zone (Fig. 1) in Oregon and Washington (Wood and Clemens, 2002). In contrast, the eastern SRP is a structural downwarp, associated with extension and magmatism along the track of the Yellowstone hot spot. This mantle plume helped generate the voluminous basalt-rhyolite volcanism of the SRP, and the Pleistocene-Recent Yellowstone caldera of Yellowstone National Park (Pierce et al., 2002).

The major faulting which down-dropped the centre of the WSRP began about 12 Ma and ended by approximately 9 Ma, although minor warping and structural adjustment may have continued locally (Wood, 2004). Rhyolitic flow domes mark the margins; basaltic volcanic rocks interbedded with the earliest sediments show that volcanism and early basin formation were contemporaneous (Wood and Clemens, 2002). A generalized stratigraphic sequence is given in Table 1. Vineyards are planted on soils derived from many units, but most notably the sands and silts of the Pliocene Glenns Ferry Formation, local basalts, the Tenmile and Terrace gravels, and finer grained Holocene deposits of the Bonneville Flood (Fig. 4).

As the rift formed, water accumulated in a series of lakes, floodplains and wetlands, with marginal beaches and streams. Fish and terrestrial vertebrate fossils are abundant locally in the complex sequence of lacustrine and related floodplain-to-shoreline facies sedimentary rocks which make up the Tertiary Idaho Group deposits of paleo-Lake Idaho. Sediments deposited in Lake Idaho include sand, silt, and clay as well as local volcanic ash. Lake level rose and fell as the basin subsided; the maximum extent of Lake Idaho was about 4 million years ago, near what is currently the 3600-ft (1100-m) elevation contour (Wood and Clemens, 2002).

To the east, in the Hagerman Fossil National Monument (42°49'N, 114°57'W), basalts interbedded with several hundred feet of Glenns Ferry sediments have been dated at 3.4 Ma (Hart et al., 1999). By approximately 2 Ma, floodplain and marsh sediments of the later part of the Glenns Ferry Formation were deposited east of Marsing on the Chalk Hills topographic ridge (Wood, 2004; Reppening et al., 1994) that underlies the Bitner vineyards. Lizard Butte (Fig. 4) is a phreatomagmatic basaltic volcano that erupted through wet lake and floodplain sediments. A short distance southeast of Marsing, near Pickles Butte and Idaho's largest vineyard, is a subaerial basalt flow that buries stream gravels of the ancestral Snake River. The age of the basalt flow, dated at 1.58 Ma by the Ar⁴⁰/Ar³⁹method, demonstrates that by early Pleistocene time, the WSRP had completely transformed from a filling rift basin to an incised lowland (Othberg, 1994). The draining of Lake Idaho was a consequence of headward erosion of ancestral Hells Canyon by the Snake-Salmon river system coupled with Lake Idaho overtopping a divide and draining northward through the ancestral Hells Canyon. Although timing of this event is poorly constrained, the initial spillover most likely occurred between 2 and 4 Ma with subsequent slow downcutting of the divide (Wood and Clemens, 2002).

Draining of the lake and lowering base level allowed incision of the old lacustrine sediments by the ancestral Boise River, forming a stepped series of Quaternary stream terrace gravels that mark previous river base level stands (Othberg, 1994; Othberg and Stanford, 1992). The oldest, the Tenmile Gravel (Table 1), caps the northwest-trending ridge of Glenns Ferry sediments at Sunnyslope, where many vineyards are located (near 43°35'N, 116°47'W), and towards Chalk Hills where additional vineyards are located (Fig. 4). A younger, lower elevation terrace gravel, known as the Gravel of Deer Flat Terrace, overlies the Tertiary sediments and flanks the ridges. Several vineyards are planted on the sandy pebble gravel of the Deer Flat Terrace, which is locally overlain by loess. Terrace elevations were controlled by paleo-base levels, and in turn, those terrace elevations dictate the vineyard elevations and influence land use.

The Bonneville Flood, which occurred 14,500 years ago, is the most recent geologic event important to the vineyards of Idaho (Scott et al., 1982). This catastrophic flood resulted from erosion of a low divide that was over-topped by a northern arm of Lake Bonneville, the ancestral Great Salt Lake in Utah (Fig. 3). The resulting deluge of water down the Snake River lasted 6 months (D. Currey, personal communication, 2005) and the discharge peaked at about 1 million m³•s^-1at the Lake Bonneville outlet (O'Connor, 1993). This single flood event created the western Snake River Canyon of today. Because the discharge was variable over the duration of the flood, multi-metre-sized boulders to sand to silt-sized sediments were deposited (Fig. 5). During the highest flood discharges, water was hydraulically backed up by constricted flow through Hells Canyon, causing relatively quiet water to pond in the lower reaches of the Snake, Payette, Boise, and Owyhee river valleys (Othberg, 1994; Fig. 3). As a result, from Hells Canyon south to Marsing, fine-grained slackwater silt blankets the late Pleistocene terraces below an elevation of 747 m a.s.l. (2,450 feet), which is lower than the early Pleistocene Deer Flat Terrace (Othberg and Stanford, 1992). Vineyards in Arena Valley northwest of Marsing are located adjacent to a circular erosion feature where the late Pleistocene Whitney terrace was scoured by late stages of the Bonneville Flood (Fig. 4).

Soil type in the WSRP varies somewhat according to the lithology of the parent material, and the timeframe and climate under which the soil developed. Surficial loess, sand, and Bonneville Flood slackwater silts are the predominant parent materials at the vineyards, and the soils normally contain abundant quartz and feldspar grains derived from the Tertiary units, though fields near the basaltic vents may contain more clay and mafic minerals.

Older soils generally tend to be more complex and show more extensive duripan (caliche) development and clay-rich B-horizons (Othberg, 1994). In some areas, soil has developed on a blanket of loess up to 4 m thick (Othberg and Stanford, 1992). The Bonneville Flood sediments are younger than the loess and typically show little soil development. Soils on the Deer Flat Terrace Gravel, which underlies some vineyards, contain more than 25% pedogenic clay and a buried duripan greater than one metre thick. Soils on the older Tenmile Gravel, which underlies ridge-top vineyards, may have 50% clay and a 2-m thick duripan (Othberg, 1994). The thicker, platy duripans promote alkaline soils and may impede subsurface drainage and root penetration by the vines.

Vineyard locations in the Sunnyslope area were spatially compared with their underlying soil characteristics listed in the NRCS Soil Survey Geographic (SSURGO) Database. The soil database for Canyon County lists over 50 soil series and eight associations. The GIS analysis revealed no single soil series common among the vineyards; 19 vineyards are located on 11 soil series that share many characteristics. Most soils underlying the vineyards in the Sunnyslope area are silty to sandy loams where silt percentages range from 58% to 67% in the upper horizons (USDA, 1972). The series are characterized as moderately to extremely well-drained, moderately calcareous and alkaline subgroups of aridisols and entisols; they have moderate to high cation exchange capacities. Most of the soils are fairly shallow (<1 m) with soil depth an inverse function of slope (i.e., steeper slopes have shallower soil depths). All the soils used for agriculture (that is, not range or urban) require surface irrigation (USDA, 1972). The combination of moderate to steep slopes, moderately to excessively drained soils, and easily eroded sediments places further limitations on land use in the region. The soils associated with the vineyard sites typically are characterized as not being prime farm land (i.e., having moderate to severe limitations; USDA, 1972), with prescribed uses ranging from irrigated pasture to fruit orchards (where hard freezes are not a danger). Prime farm land in the area (i.e., having higher soil water availability, deeper soil depths, lower gradient slopes) is used primarily for large scale row crop operations such as sugar beets, alfalfa, corn, and onions. More recently, prime farmland on the urban fringes has been converted to housing subdivisions.

The climatic factors of precipitation (amount and seasonal pattern), growing season length, and growing degree days (e.g., Winkler et al., 1974) all affect grape and wine quality (van Leeuwen et al., 2004; Jones and Davis, 2000) and thus contribute strongly to terroir. The WSRP is located in the continental interior of the western U.S. approximately 500 km east of the Cascade Range (Fig.1). Even with its continental interior location, the region is on a climatic hinge line and exhibits influences of both continental and marine climates. Winter months (November 1 through March 30) provide two-thirds of the precipitation for the region (Fig. 6). Winter precipitation is caused both by storms from the Gulf of Alaska tracking under the influence of the dominant westerlies at this latitude (Godfrey, 1999), and more tropical moisture originating near Hawaii tracking under the influence of the subtropical jet stream and producing what is colloquially referred to as the "Pineapple Express." Whereas winters may be cold and overcast, the summer growing season (April 1 through September 30) is characterized by warm, dry days with a possible average of 70% of sunshine [Western Regional Climate Center (WRCC), 2005; http://www.wrcc.dri.edu/].

Successful production of wine grapes in the WSRP requires careful consideration of site and cultivar selection, number of frost-free days, and availability of supplemental water. Risk of frost damage is minimized by locating vineyards on slopes with good air drainage towards river or valley bottoms (Fig. 8a). Vineyards tend to be located on hillsides with a southern or southwestern aspect to maximize heat unit accumulation and to avoid direct exposure to prevailing northwesterly winds. Vines are generally planted in north-south rows to facilitate cold air drainage and to provide equal sunlight exposure on both sides of the vine canopy (Fig. 8b). The predominant vine training system is cordon-trained, spur-pruned, with vertical shoot positioning on a six wire trellis. Common in-row spacing is 2.1 m with 2.7 m between rows. Target shoot length is about 1.5 m. Dormant vines typically are pruned manually to two (red cultivars) or three (white cultivars) bud spurs, yielding 24 to 28 (red) or 36-42 (white) buds per vine. Some growers mechanically pre-prune dormant vines, and many mechanically harvest their grapes.

An estimated 60% of total producing wine grape acreage in the WSRP is composed of fairly cold hardy, white wine cultivars (Riesling, Chardonnay, and Gewürztraminer) that have a low heat unit requirement to reach maturity (Wolfe, 1998). The red wine cultivars Cabernet Sauvignon, a late maturing cultivar with a relatively high heat unit requirement (Wolfe, 1998), and Merlot, a less cold hardy cultivar (Wolfe, 1998), comprise about a third of producing acreage, and many new plantings include red wine cultivars with similar temperature requirements.

The large range (about 280) in GDD among WSRP weather station sites as depicted in Table 2, as well as the successful commercial production of Cabernet Sauvignon and Merlot, highlights the importance of the vineyard mesoclimate. For example, a 140-day growing season with 1442 growing degree days is reported for the Idaho Parma Experimental weather station in Table 2 (Fig. 2). However, temperature data collected in a hillside vineyard planted in 1997 with a northern aspect and north-south row orientation recorded 1581, 1851, and 1644 growing degree days at the Parma Experiment Station (simple average, base 10°C, daily temperature), respectively over three vintages (2002-2004). The average number of frost-free days needed in this vineyard for fruit to reach maturity was 147-150 for Merlot, 150-154 for Chardonnay, and 160 or more for Cabernet Sauvignon and Syrah (Table 3). Fruit composition of each cultivar at harvest reached optimum brix (~23%; this is a measure of sugar concentration in the grapes), pH (~3.6), and titratable acidity (0.6 g/dl), suggesting adequate season duration and temperature accumulation. The growing season and heat unit accumulation data in Table 2 suggest that the climate in Parma is marginal for cultivation of Merlot and Chardonnay and not suitable for production of Cabernet Sauvignon or Syrah, yet vineyard heat unit accumulation and number of frost-free days suggest a suitable vineyard mesoclimate for production of these red wine cultivars.

Vine cold-hardiness is not well understood because of its complex interaction with environmental conditions, including temperature and photoperiod (Howell, 2000), tissue maturity (Goffinet, 2000), and vine water status (Wample et al., 2000), but growers in the WSRP minimize yield loss from cold injury by adopting preventative cultivation practices. For example, own-rooted cuttings are planted at a depth of 30 to 36 cm to facilitate root survival in the event of above ground vine loss from prolonged low temperature. The vines are trained to two trunks (Fig. 8b) with each trunk forming a unilateral cordon that extends to one side or other of the vine. In the event of cold damage, the second trunk can be used to replenish damaged wood. The absence of phylloxera in the region permits cultivation of own-rooted rather than grafted vines, enabling trunk re-establishment without replanting. Despite these cultivation practices, injury from prolonged minimum mid-winter low temperature was anecdotally reported in the very cold years of 1989 and 1990. Another common type of cold injury observed in the WSRP occurs during winter dormancy when several days of high solar radiation and warm ambient temperature precede an abrupt return to freezing or near freezing temperature.

The low annual and growing season precipitation and the shallow soils in the WSRP facilitate manipulation of vine physiology through irrigation management. Most irrigation water is obtained from annually recharged snow-pack in mountain ranges surrounding the WSRP and is delivered through an extensive network of reservoirs and canals. The majority of vineyards are irrigated with above ground drip lines, although some vineyards utilize overhead sprinklers or furrows. Irrigation scheduling is used to prepare for the first fall frost by imposing water stress on the vine to encourage periderm formation on green shoots. Periderm is a visible indicator of tissue maturity and has been associated with bud and cane cold hardiness (Goffinet, 2000). Irrigation is then normally applied prior to the first fall frost to bring soil moisture up to field capacity in an effort to protect roots during the winter. Growers also manipulate vine water stress during the growing season to shift growth from vegetative to reproductive structures (Greenspan, 2005) and to control canopy as well as berry size. Regulated deficit irrigation is used to control plant water status and to optimize fruit quality during the growing season. Ongoing research is being conducted by researchers at the USDA Agricultural Research Service in Parma, Idaho, to understand how vine water status influences grape components that contribute to wine quality.