CHARACTERISTICS OF THE CIRCLE WEST STUDY AREA 



CLIMATE AND WEATHER 



The study area has a typical continental climate having hot summers, cold 

 winters, and rapid seasonal transitions. At Circle, January temperatures aver- 

 age -10.5°C (13.1°F) with a mean minimum temperature of -16.7°C (2°F). Temper- 

 atures in July average 21.1°C (70°F) with a mean maximum temperature of 31.1 C 

 (88 F). The frost-free season lasts about 99 days (Cordell 1971). 



Precipitation for the area is variable and unevenly distributed and drought 

 strongly influences vegetation. The fifty-year precipitation average at Circle 

 is 31.65 cm (12.46 inches). Between 1952 and 1977, the average was 32.8 cm 

 (12.9 inches) with a standard deviation of 9.1 cm (3.6 inches). At Fort Peck, 

 the average precipitation between 1955 and 1975 was 28.7 cm (11.3 inches) with 

 a standard deviation of 8.6 cm (3.4 inches). The annual precipitation of the 

 study areas averages between 30 cm - 40 cm (12 and 16 inches). The proposed 

 mine area has an average annual precipitation of about 35.6 cm (14 inches) 

 (USDA-DNRC 1977). On the average, 82% of the annual precipitation falls between 

 April and September, and 55% falls between May and July (Cordell 1971). 



Of the two winters included in the baseline period, the first (1976-77) 

 was "moderate" while the second (1977-78) was "severe"; therefore, data for each 

 winter were treated separately. The National Weather Service station at Glas- 

 gow, 48 km (thirty mi les) northwest of the study area, recorded eight days with 

 at least 13 cm (5 inches) of snow on the ground during December and January of 

 1976-77 compared to fifty such days during December and January of 1977-78. 

 20 cm (eight inches) of precipitation were recorded for these two months the 

 first winter compared to 43 cm (17 inches) the second winter. The average temp- 

 erature in January, 1977 was -16°C (3°F), which is 3C0 (6F0) below the norm, 

 while the average in January, 1973 was -19°C (-2°F), 60° (IIFO) below the norm. 



The spring and summer of 1977 were unusually hot and dry. A single storm 

 on June 14 and 15 produced most the precipitation for the two seasons. 



GEOLOGY AiJD SOILS 



The Circle West study area is underlain by nearly horizontal beds of sedi- 

 mentary rock which are poorly exposed except in local badlands in the western 

 half of the area. The oldest exposed formation, the Bearpaw Shale, of Late 

 Cretaceous age, is exposed in bluffs along the Missouri River and near Fort 

 Peck. Overlying the Bearpaw are the Fox Hills Sandstone (Cretaceous), the Hell 

 Creek formation (sandstone and claystone, some lignite; Late Cretaceous age), 

 and the Fort Union formation (shale, sandstone, and lignite; Paleocene age). 

 The Fox Hills and Hell Creek formations have outcrops along the Big Dry Arm of 

 Fort Peck Reservior and along the breaks of the Missouri River. The Fort Union 

 formation occupies the central and southern parts of the Circle West study area. 



