Description

MLRA 25 lies within the Intermontane Plateaus physiographic province. The southern half is in the Great Basin Section of the Basin and Range Province. This part of the MLRA is characterized by isolated, uplifted fault-block mountain ranges separated by narrow, aggraded desert plains. This geologically older terrain has been dissected by numerous streams draining to the Humboldt River. The northern half of the area lies within the Columbia Plateaus geologic province. This part of the MLRA forms the southern boundary of the extensive Columbia Plateau basalt flows. Deep, narrow canyons drain to the Snake River which incise the broad volcanic plain. The Humboldt River, route of a major western pioneer trail, crosses the southern half of this area. Reaches of the Owyhee River in this area have been designated as National Wild and Scenic Rivers. Nevada’s climate is predominantly arid, with large daily ranges of temperature, infrequent severe storms, heavy snowfall in the higher mountains, and great location variations with elevation. Three basic geographical factors largely influence Nevada’s climate: continentality, latitude, and elevation. Continentality is the most important factor. The strong continental effect is expressed in the form of both dryness and large temperature variations. Nevada lies on the eastern, lee side of the Sierra Nevada Range, a massive mountain barrier that markedly influences the climate of the State.

Geographic subunits

Land Resource Unit 2. Topography: This LRU represents the lower elevation, dryer portions of the northern half of MLRA 25. The dissected lava plateau forms the southern boundary of the extensive Columbia Plateau basalt flows. This part of the MLRA is characterized by deep, narrow canyons incised into broad basalt plains draining to the Snake River. Topography is characterized by extensive flat summits with limited steep sloping side-slopes/ vertical walls, elevations are less than 5400' (<1650m). Median slope value is 3% (50% of samples slope values are 3% or less), 95% of slope values are <30% (n=3919). Soil characteristics: Soils formed in volcanic ash and/or loess over residuum derived from incised basalt flows forming broad plateaus. Soils in this LRU commonly have vitric soil properties which contain >5% volcanic glass with appreciable amounts of pedogenic glass alteration. This area is characterized by a mesic soil temperature regime and aridic bordering on xeric soil moisture regime. Typically soil profiles have an ochric epipedon or a thin (~18cm) mollic epipedon. Thicker mollic epipedon development is found in soils that experience increased moisture from run on or run through. Soils forming on the tops of plateaus average moderately deep (50 to 100 cm) to a duripan or lithic contact and range from very shallow (<25 cm) to very deep (> 150 cm). Soils forming on the side slopes of plateaus are moderately deep to deep (100 to 150 cm) to a lithic contact. Duripan development in this LRU is dominated by calcium carbonate accumulations above the lithic contact with appreciable amounts of silica accumulation in the forms of laminar caps, bridges between sand grains, and accumulations on rock fragments. Calcium carbonate in these profiles originate from aeolian deposits. Representative soil types of this LRU include: Calciargids (Willhill series), soils that form in loess over alluvium and residuum derived from rhyolitic tuff on hills and plateaus and Haplargids (Plush and Dougal series), soils that form in colluvium and slope alluvium from welded rhyolitic tuff on plateaus, terraces and foothills. Climate: Mean annual precipitation ranges from 10-14" (270-350 mm). Precipitation comes mostly as snow in the winter. Frost free days 110-140 days, evapotranspiration equals or exceeds precipitation across the entire area and mean annual air temperature ranges from 7.6-9.1 C (46-48 F). Biological resources: This area supports shrub-grass vegetation characterized by Wyoming big sagebrush associated with bluebunch wheatgrass, western wheatgrass and Thurber’s needlegrass. Other important plants include bluegrass, squirreltail, penstemon, phlox, milkvetch, lupine, Indian paintbrush, aster, and rabbitbrush.

Land Resource Unit 3. Topography: This LRU represents the higher elevation plateaus in northern half of MLRA 25. The southern boundary of this LRU boarders the dissected low lava plateau of the Columbia Plateau basalt flows. This part of the MLRA is characterized by deep, narrow canyons that have been incised into the broad basalt plain draining to the Snake River. Topography is characterized by extensive flat summits with limited steep sloping side-slopes/ vertical walls, elevations range from about 5000 to 6000' (1500-1865m). Median slope value is 6% (50% of samples slope values are 6% or less), 95% of slope values are <40% (n=3419). Soil characteristics: Soils form in volcanic ash and/or loess over residuum derived from incised uplifted basalt flows forming broad plateaus. This area is characterized by a frigid soil temperature regime and xeric bordering aridic and xeric soil moisture regime. Soils in this LRU commonly have vitric soil properties which contain >5% volcanic glass with appreciable amounts of pedogenic glass alteration. Some soil profiles have a mollic epipedon that exceeds 40 cm thick. Pachic mollic epipedons are most common in run on landscape positions and highly productive shrub sites. Duripan development in this LRU is dominated by calcium carbonate, originating from aeolian deposits. Representative soil types in this LRU include: Palexerolls (Amboat series), soils that formed in loess over colluvium and residuum derived from welded tuff on summits of plateaus. Haploxeralfs (Wickahoney series), soils that formed in loess over colluvium and residuum derived from extrusive rocks and volcanic ash on foothills and tablelands. Climate: Mean annual precipitation ranges from 12-20" (300-520 mm). Precipitation comes mostly as snow in the winter. Frost free days 95-140 days, evapotranspiration equals or exceeds precipitation across the entire area and mean annual air temperature ranges from 6.6-8.5C (43-47F). Biological resources: This area supports mountain big sagebrush, low sagebrush and/or early sagebrush associated bluebunch wheatgrass and Idaho fescue. Snowberry, curlleaf mountain mahogany and ceanothus are common throughout this LRU. Steep side slopes of incised drainages may be dominated by juniper.

Land Resource Unit 4. Topography: This LRU represents the alluvium dominated piedmont slopes in southern portion of MLRA 25. It is characterized by isolated, uplifted fault-block mountain ranges separated by narrow, aggraded desert plains. This geologically older terrain has been dissected by numerous streams. Topography is characteristic of the basin and range, formed by grabens and half-grabens, fan piedmonts and small to medium sized fluvial systems. This entire area is contained within the hydrologic Great Basin, none of the surface water resources drain external of the Great Basin. Elevations range from 5000'-6500' (1580-1900m). Median slope value is 6%, and 95% of slope values are <36% (n=4384). Soil characteristics: Soils dominantly form in loess or ash deposits over the Humboldt formation (Miocene) which is generally comprised of interbedded lake bed deposits, ash and tuff deposits, and mixed stream deposits. Soil temperature regimes range from mesic to frigid and soil moisture regimes are typically aridic bordering xeric and xeric bordering aridic. Soils commonly have vitric soil properties which contain >5% volcanic glass with appreciable amounts of pedogenic glass alteration. Typical soil profiles have an ochric epipedon or a thin (~18cm) mollic epipedon. Duripan development in this LRU is dominated by calcium carbonate accumulations above the lithic contact with appreciable amounts of silica accumulation in the forms of laminar caps and accumulations on rock fragments. Durinodes are common in deep and very deep soils of stable inset fans. Calcium carbonate in these profiles originate from aeolian deposits. Soils forming in alluvium over rock pediments are not common, but do exist in areas adjacent to limestone/dolomite hills and mountains in some areas. These soils are generally shallow to a moderately to strongly cemented duripan over a deep to very deep lithic contact. Side slopes are characterized by shallow soils. Calcium carbonate accumulations are common throughout the soil profile. Soils forming along the Humboldt River flood plain are characterized by aquic moisture regimes, mesic soil temperatures and a thick mollic epipedon (≥ 50 cm). Soils forming in the flood plain are generally fine textured (silts, loams, and fine sands) with lenses of gravel and cobbles characteristic of river morphology. Water tables along the flood plain fluctuate from below 150 cm in the profile to near surface (25 cm) with changing seasons. Representative soil: Haplodurids (Bioya and Coonskin series), soils formed in loess over alluvium on fan piedmonts. Argidurids (Dacker, Bruncan, and Hunnton series) and Durixerolls (Stampede and Donna series). Haplargids formed in residuum and colluvium on hills, mountain slopes (Vanwyper series) and in alluvium on ballenas (very deep Wieland). Climate: This area is warmer and dryer than the surrounding mountains. Precipitation ranges from 10-13" (260-334mm), evapotranspiration exceeds precipitation across the entire area, mean annual air temperature ranges from 6.0- 8.7C (42 – 48F), the area is characterized by greater than 90 frost free days (88-130). Biological resources: This area supports shrub-grass vegetation characterized by Wyoming big sagebrush associated with bluebunch wheatgrass, Thurber’s needlegrass, and bluegrass. Black sagebrush and low sagebrush are found throughout the area but are less extensive. Areas with deep soils and run in moisture are commonly characterized by basin big sagebrush. Other important plants include squirreltail, basin wildrye, penstemon, phlox, milkvetch, lupine, aster, and rabbitbrush. Minor amounts of singleleaf pinyon and/or Utah juniper occur in limited areas along the southern boundary with MLRA 28B, the northeast corner of Elko County and Box Elder County.

Land Resource Unit 5. Topography: This LRU is representative of mountains and hills. It includes the Jarbidge, Independence, Mahogee, Ruby, East Humboldt, Santa Rosa, Raft River, Grouse Creek, Owyhee Mountains, and other minor ranges. Lithology of mountain ranges is dominantly volcanic (rhyolite) with areas of igneous intrusions, metamorphic deposits, and calcareous marine deposits. The topography is mountainous with high relief, narrow ridges, broad backslopes and narrow valleys. The highest ranges in this LRU experienced Pleistocene glaciation. Aspect is an important driving factor in this LRU. Vegetation patterns and resulting soil patterns on the landscape are heavily influenced by north-south aspects. The area is dominated by bedrock-controlled landforms. Elevations are greater than 5900' (1800-2350m) with individual peaks as high as 11,000’. Median slope value is 23% and 75% of slope values are >11% (n=5293). Soil Characteristics: Soils form in loess and/or ash deposits over residuum and colluvium derived from mixed parent material. Soil temperature regimes range from frigid to cryic and soil moisture regimes are typically xeric bordering aridic and xeric. Lower elevation soils in this LRU commonly have vitric soil properties which contain >5% volcanic glass with appreciable amounts of pedogenic glass alteration. Some soil profiles have a mollic epipedon that exceeds 40 cm thick. Pachic mollic epipedons are common on less sloping, broad, concave, long mountain side slopes and on concave-concave backslopes. Soils forming on stable geomorphic surfaces tend to have argillic horizons closer to the soil surface. Less stable surfaces, steeply sloping backslopes or historically glaciated areas, lack strong pedogenic development. Soils forming under dense shrub communities are dominated by thick mollic epipedons and generally have lower pH, <6.5. Soils forming under conifer stands are characterized by ochric epipedons and commonly have a soil pH less than 6.5. Soils forming in carbonatic deposits at high elevations lack calcic horizon development in the upper profile due to moisture flushing the matrix of soluble carbonates and depositing them at the wetting front deeper in the profile. Duripans are not common in the mountain and hills LRU. Representative soil types in this LRU include: Argixerolls (McIvey series), soils that formed in alluvium or colluvium on hills, and mountain slopes. Haplocryolls (Hapgood series) formed in colluvium and residuum derived from mixed parent material with a component of loess and volcanic ash on hills and mountains. Climate: Precipitation comes mostly as snow in the winter, with localized late summer thunderstorms making up a small percentage of the total precipitation. Aspect determines thickness and duration of snowpack on the landscape. North aspects and highest elevations regularly hold snow through the month of June. Mean annual precipitation exceeds 12" (320-750mm), frost free days range from 85-125, with a median value of 100. The number of frost free days is not significantly different from the surrounding alluvium of the upper Humboldt plains, but the hills and mountains receive more annual precipitation and are characterized by colder mean annual temperature. This result increased positive effective precipitation (i.e. precipitation exceeds evapotranspiration). Estimates of effective precipitation range from -235 to 258mm (-9 to 10”) across this LRU. Biological Resources: Conifers, aspen, and curl-leaf mountain mahogany are in the Owyhee, Ruby, Santa Rosa and Jarbridge Mountains. The conifers include whitebark pine, Douglas-fir, limber pine, Engelmann spruce, and subalpine fir. Bristlecone pine is present but is limited to Ruby Mountains and East Humboldt Ranges exclusively on limestone parent material. Vast areas support shrub-grass vegetation characterized by Mountain big sagebrush or low sagebrush in association with Idaho fescue, bluebunch wheatgrass, needlegrasses, and/or bluegrass.