Climate, precipitation, and vegetation

The climate within the WBRB is primarily a function of altitude – to a lesser degree latitude and topography – and ranges from semi-arid continental within the basin interiors to humid-alpine in the bordering mountain ranges. The mountain ranges surrounding the basins tend to catch much of the atmospheric moisture flow through precipitation caused by orographic uplift, substantially decreasing precipitation in the basin interiors. Temperature varies by season from well below zero degrees Fahrenheit in the winter to more than 100 degrees in the summer. Most precipitation within the WBRB occurs during winter as snowfall and during spring and summer as thunderstorms (Libra et al., 1981). Average annual precipitation ranges from 6 to 10 inches in the basin interiors, 11 to 20 inches along the elevated foothills of the basins, 21 to 45 inches in the mountain ranges, and up to 70 inches along the mountain peaks above approximately 10,000 feet elevation (Figure 3-3). Average annual precipitation within Yellowstone National Park ranges from 13 to 70 inches (Figure 3-3; Cox, 1976).

Average annual precipitation (1961 – 1990), Wind/Bighorn River Basin.
Average annual precipitation (1961 – 1990),
Wind/Bighorn River Basin.

Vegetation in the WBRB is influenced by elevation, soil, exposure, and precipitation. In the lowland basin areas, mixedgrassland and sagebrush steppe vegetation dominates, with grasses, sagebrush, saltbush, greasewood, and desert shrub; and with cottonwood and Russian olive trees along drainages. Some of the lowland areas, especially along perennial streams, have been converted to cropland. Higher-altitude desert vegetation along the uplifts and foothills adjacent to the surrounding mountains includes a greater abundance of grasses, sagebrush, saltbush, greasewood, and desert shrub, along with woodland species that include cottonwood, willow, boxelder, juniper, and limber pine. Alpine forest and alpine tundra are characteristic of the higher mountain regions; vegetation includes a variety of grasses, sagebrush, lodgepole pine, ponderosa pine, spruce, fir, and aspen. Above timberline (approximately 10,000 feet), tundra supports alpine grasses (Peterson, Mora, et al., 1987).


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