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" 'il t) Subbasin Description PHYSICAL CHARACTERISTICS The Missouri River is formed at the confluence of the Jefferson, Madison, and Gallatin Rivers in southwestern Mon- tana. From its source, the Missouri flows northward, then north- eastward, before turning eastward to its junction with the Yel- lowstone River. The topography is essentially of two types-a series of intermontane valleys descending from the Rocky Moun- tains in the western portion of the subbasin and a large tableland with prairie characteristics, broken by deeply entrenched, narrow, meandering streams in the northern and eastern portions. Eleva- tions range from 1,860 feet at the confluence of the Missouri and Yellowstone Rivers up to 11,293 feet at Koch Peak in soufhwest- ern Montana. The Upper Missouri Subbasin is in the northwest part of the Missouri River Basin and includes the land drained by the Missouri River and its tributaries upstream from its confluence with the Yellowstone River at extreme western edge of North Dakota. The subbasin extends 500 miles in an east-west direc- tion and has a maximum north-south extent of 350 miles for a total area of 57.7 million acres. Some 90 percent of the subbasin lies in northern and eastern Montana, 8 percent in southern Aiberta and Saskatchewan provinces in Canada, and about 1 percent each in western North Dakota and northwestern Wyoming. The Cana- dian area will not be considered in the remainder of this report. The principai tributaries of the Missouri River in this sub- basin include the Jefferson, Madison, Sun and Gallatin Rivers, which join to form the Missouri River; the Marias River in the northwestern part of the subbasin; the Musselshell River, which drains central Montana northward to the Missouri River; and the Milk River, an international stream flowing eastward through northern Montana and southern Canada into the Missouri River just downstream of the Fort Peck Reservoir in northeastern Mon- tana. These rivers generally originate in mountainous areas and flow through level to gently rolling plains. Figure 8 shows the subbasin and its major tributary streams. Normal annual precipitation generally ranges from 12 inches on the plains in the northeastern part of the subbasin to over 20 inches in the western part. Some places in the mountains receive over 50 inches. Winter snow accumulations of 15 feet are common in the mountains. The average annual lake evaporation ranges from 26 inches in the mountains of the western portion to 38 inches on the plains in the eastern portion. CHAPTER 6 UPPER MISSOURI SUBBASIN - , ~ h _ _~ . . .~._-_.~ ik--'~ '- ."..,.. Sri., R.nge nMf Bozem.n, Mont'nI January is normally the coldest month, and July the war- mest. Mean daily minimum temperatures (in degrees Fahrenheit) vary from -4 degrees in the northeast to 16 degrees in the south- west in January, and 40 degrees in the west to 60 degrees in the east in July. Mean daily maximum temperatures vary from 18 degrees in the northeast to 32 degrees in the southwest in January, and 72 degrees in the northwest to 90 degrees in the southeast in July. The frost-free period ranges from about 120 days on the plains in the northeastern part to less than 30 days in the higher elevations. The eastern plains receive about 2,900 hours of sun- shine annually while in the western mountains about 2,400 hours occur annually. ENVIRONMENTAL RESOURCES Terrestrial Biological Resources This subbasin is endowed with a high degree of ecosys- tem diversity which supports a rich floristic and faunal resource base. In addition, many remote parts of the subbasin are in near pristine condition due to the few stresses placed on them by the sparse human population. Selected environmental resource areas are shown on Figure 8. Grassland ecosystems are vast on the plains and are only occasionally interrupted by isolated low mountain ranges or but- tes. Extensive prairie areas of the mixed, short, or sagebrush 53