Description

32—Northern Intermountain Desertic Basins MLRA 32 (fig. 32-1) is in Wyoming (95 percent) and Montana (5 percent). It makes up about 10,763 square miles (27,875 square kilometers). It consists of a syncline between anticlinal mountain ranges that encompasses both the Bighorn and Wind River Basins. The Bighorn Basin is bordered by MLRA 43B (Central Rocky Mountains) to the east, west, and south and MLRA 46 (Northern and Central Rocky Mountain Foothills) to the north. The Wind River Basin is bordered by MLRA 43B to the north, MLRA 46 to the west, and MLRA 34A (Cool Central Desertic Basins and Plateaus) to the south and east. Physiography The northern two-thirds of this MLRA, in the Bighorn Basin, is in the Middle Rocky Mountains province of the .Rocky Mountain System. This part of the MLRA is an elevated, dissected basin surrounded by mountain ranges to the east, west, and south. The Owl Creek and Bridger Mountains separate the northern two-thirds of the MLRA from the southern third. The southern third is in the Wind River Basin, an elevated, dissected plain with mountains to the north, west, and south. This part of the MLRA is in the Wyoming Basin province of the Rocky Mountain System. Some isolated low mountains are in each part of the MLRA. Elevation ranges from 3,600 to 7,300 feet (1,100 to 2,240 meters). Piedmont plains and pediments slope from the mountains to the stream terraces of the Wind River and Bighorn Basins. The plains are eroded to the clay shale bedrock in some areas, and there are areas of badlands. The Beaver and Wind Rivers join to form the Bighorn River in the southern third of this area. The Bighorn River cuts through the Owl Creek Mountains and continues into the northern part of the MLRA. The Shoshone and Greybull Rivers join the Bighorn River in the northern part of the area. Clark’s Fork of the Yellowstone River exits the area in the northwest corner. Geology The surface of this area is covered with old deposits of sand and gravel washed into the basins by the streams and rivers draining the surrounding mountains. The present-day rivers and streams have excavated old pediment surfaces, forming terraces. Alluvial fan deposits grade into the valley fill pediments. The igneous and sedimentary rocks exposed in the adjacent mountains occur beneath the surface of the Bighorn Basin. Tertiary sandstones and shales are exposed where the overlying alluvium has eroded away. Older sandstones, shales, and carbonate rocks are exposed as steeply dipping beds on the mountainsides. The core of most of the mountain ranges is granite. The granite may be exposed at the higher elevations along the margin of the basins. Climate The average annual precipitation in most parts of the basins is 7 to 12 inches (176 to 310 millimeters). It is as high as 22 inches (560 millimeters) in the higher areas within the basins. The maximum precipitation from frontal storms occurs in spring and fall. The surrounding mountain ranges block many of the regional precipitation events. The average annual temperature is 40 to 47 degrees F (4.6 to 8.5 degrees C). The temperature can vary widely within short periods because of the drainage of cooler mountain air into the basins. The freeze-free period averages 133 days and ranges from 110 to 155 days. Water The low and erratic precipitation provides only a small amount of the surface water used in this area. The Wind and Bighorn Rivers and their tributaries bring good-quality irrigation water into the area from the bordering mountains. A few reservoirs store water, but most of the surface water used is diverted directly from the streams. Supplies become scarce late in the growing season, from July through September. Deep artesian wells provide some water for irrigation on the eastern side of the Bighorn Basin. These wells are finished either in sandstone units in the Dakota Formation or in the carbonate rocks of the Madison Group. The well water is very hard. Ground water occurs in the alluvial basin fill deposits near the surface. This water can be soft to very hard. It is not used in the area. Soils The dominant soil orders are Aridisols and Entisols. The soils in the area dominantly have a mesic temperature regime, an aridic moisture regime, and mixed mineralogy. They generally are shallow to very deep, well drained, and loamy. The main soils and their series: Haplargids that formed in alluvium and slope alluvium on alluvial fans, fan remnants, and stream terraces (Griffy, Hiland, and Wallson series) Haplocalcids that formed in alluvium and colluvium on alluvial fans, fan remnants, and stream terraces (Emblem and Trook series) Haplocambids that formed in alluvium and colluvium on alluvial fans, fan remnants, stream terraces, and basin floor remnants (Kinnear, Pavillion, and Zigweid series) Natrargids that formed in alluvium or colluvium over residuum on alluvial fans, stream terraces, hills, fan remnants, and strath terraces (Effington, Muff, and Uffens series) Torrifluvents that formed in alluvium on flood plains, alluvial fans, and stream terraces (Lostwells and Youngston series) Torriorthents that formed in alluvium and colluvium on alluvial fans, fan remnants, stream terraces, and hills (Apron and Kishona series); that formed in residuum and colluvium on hills, escarpments, eroded fan remnants, side slopes, dip slopes, scarp slopes, and pediments (Greybull, Oceanet, Persayo, Shingle, and Worland series) Biological Resources This area supports shrub-grass vegetation. Big sagebrush, Gardner’s saltbush, rhizomatous wheatgrasses, Indian ricegrass, and needle and thread are the dominant species. Black sage, Gardner’s saltbush, and bluebunch wheatgrass are common on shallow soils in the uplands. Major wildlife species include antelope, coyote, jackrabbit, and sage grouse. Land Use Nearly one-half of this area is federally owned. The rest is in farms and ranches. Most of the land is used for grazing (fig. 32-2). The rangeland consists of desert shrubs and short grasses. About 5 percent of the area is irrigated. Most of the irrigated areas are used for alfalfa and other feed crops, but dry beans, malt barley, sugar beets, and corn are important cash crops. The major soil resource concerns are water erosion, water quality, rangeland health, and soil quality. Conservation practices on cropland include irrigation water management and installation of water-conserving irrigation systems. United States Department of Agriculture, Natural Resources Conservation Service. 2022. Land resource regions and major land resource areas of the United States, the Caribbean, and the Pacific Basin. U.S. Department of Agriculture, Agriculture Handbook 296.

Geographic subunits

Land Resource Unit 1 32X01 (WY): This LRU is the core of the Big Horn Basin, comprised of the eroded basin floor. As the LRU shifts toward the outer edges, aspect and relation to the major bodies of water and taller landforms create minor shifts in soil chemistry influencing the variety of ecological sites and to plant interactions. The extent of soils currently correlated to this ecological site does not fit within the digitized boundary. Many of the noted soils are provisional and will be reviewed and corrected in mapping update projects. Other map units are correlated as small inclusions within other MLRAs and LRUs based on elevation, landform, and biological references.

Land Resource Unit 2 32X02 (WY): This LRU is the Wind River Basin within MLRA 32X. This LRU is tends to be just a fraction higher in elevation, slightly cooler (by 1-degree Celsius), and spring snowpack tends to persist longer into the spring than the Big Horn Basin (LRU 01). This LRU was originally divided into two LRU's - LRU C which was the core and LRU D which was the rim. With the most current standards, this LRU is divided into two Subsets. The extent of soils currently correlated to this ecological site does not fit within the current subset or LRU boundary. Many of the map units are correlated to ecological sites outside of this MLRA, but will be reviewed and corrected during mapping update projects.

Land Resource Unit Subunit A 32X01A (WY): This Subset is the core of the Big Horn Basin and is comprised of eroded basin floor. As this LRU shifts towards the outer edges, aspect and relation to mountains create minor shifts in soil chemistry influencing the variety of ecological sites and plant interactions. Moisture Regime: Typic Aridic. Prior to 2012, there were map units that cross over to ustic aridic or ustic aridic was correlated into this core area. As progressive mapping continues and when the opportunity arises to do update projects, these overlapping map units will be corrected. Temperature Regime: Mesic Dominant Cover: Rangeland, with saltbush flats as the dominant vegetative cover for this LRU/ESD Representative Value (RV) Effective Precipitation: 5-9 inches (127 –229 mm) RV Frost-Free Days: 110-150 days

Land Resource Unit Subunit A 32X02A (WY): This Subset is the core of the Wind River Basin and is comprised of eroded basin floor. As this LRU shifts towards the outer edges, aspect and relation to mountains create minor shifts in soil chemistry influencing the variety of ecological sites and plant interactions. Moisture Regime: Typic Aridic. Prior to 2012, there were map units that cross over to ustic aridic or ustic aridic was correlated into this core area. As progressive mapping continues and when the opportunity arises to do update projects, these overlapping map units will be corrected. Temperature Regime: Mesic Dominant Cover: Rangeland, with saltbush flats as the dominant vegetative cover for this LRU/ESD Representative Value (RV) Effective Precipitation: 5-9 inches (127 –229 mm) RV Frost-Free Days: 105-130 days

Land Resource Unit Subunit B 32X01B (WY): This Subset is the rim of the Big Horn Basin and is comprised of eroded fan remnants and stream terraces. This subset is driven by the relation to the mountains creating minor shifts in soil chemistry influencing the variety of ecological sites and plant interactions. Moisture Regime: Ustic Aridic. Prior to 2012, there were map units that cross over to aridic ustic or ustic aridic was correlated into the core area. As progressive mapping continues and when the opportunity arises to do update projects, these overlapping map units will be corrected. Temperature Regime: Mesic Dominant Cover: Rangeland, with saltbush flats as the dominant vegetative cover for this LRU/ESD Representative Value (RV) Effective Precipitation: 10-14 inches (254 –355 mm) RV Frost-Free Days: 105-125 days

Land Resource Unit Subunit B 32X02B (WY): This Subset is the rim of the Wind River Basin and is comprised of eroded fan remnants and stream terraces. This subset is driven by the relation to the mountains creating minor shifts in soil chemistry influencing the variety of ecological sites and plant interactions. Moisture Regime: Ustic Aridic Temperature Regime: Mesic Dominant Cover: Rangeland, with sagebrush steppe intermixed with saltbush flats, is the dominant vegetative cover. Representative Value (RV) Effective Precipitation: 10-14 inches (254 – 355 mm) RV Frost-Free Days: 85-115 days