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I I I I I I I I I I I I I I I I I I I 11-3 Groundwater is also a source of supply for municipal, industrial, domestic, stock and agricultural use in the Basin. The primary aquifer in the Basin is in the val ley alluvium of St. Vrain Creek and its plains area tribu- taries. The alluvium is recharged by precipitation, applied irrigation water, and leakage from canals and reservoirs. It is estimated that approximately 9,000 ac-ft per year may be used for irrigated agriculture in the Study Area and that lesser amounts of groundwater are used for rural residential and other uses. C. Environmental Resources 1. Terrestrial Biology The St. Vrain Basin encompasses a diverse array of vegetation types due to the wide range in elevat ion and corresponding changes in life zones. The Siting Area is divided into three regional zones, each zone characterized by generally similar sensitivities of the environmental resources. The three life zones identified are the mountain zone (elevations above 7000 feet), the foothi lis zone (elevations between 5500 and 7000 feet), and the plains zone (elevations below 5500 feet). The mountain zone contains vegetation and wi Idlife habitat that is interrelated wi th the high scenic and recreational land use values. Vegeta- tion in the mountain zone consists primari Iy of Douglas fir, lodgepole pine and aspen mountain forests. Riparian vegetation is usually characterized by wi II ow and alder stands. Elk and mule deer, important wi Idl i fe species, use higher elevations of the mountain habitat during the summer. The foothi lis zone is a transitional area between the mountain and plain zones. Vegetation in this zone becomes dominated by ponderosa pine in association with aspen and Douglas fir. Riparian vegetation consists primari- ly of willow and alder. Essentially the same wildlife species utilize the foothi lis zone as the mountain zone. Seasonal use by game species is impor- tant in this zone. Much of the plains zone was originally represented by plains grass- land with occasional sagebrush and mixed prairie species and now is mostly converted to agricultural production. Blue gramma, weed grasses and other mid- to tall-grasses dominate the native rangelands. Dense riparian vegeta- tion dominated by cottonwoods grows along water courses and around some of the numerous lakes and reservoi rs. Wi Idl i fe associated wi th this zone includes coyote, fox, mule deer, whitetai I deer, and numerous species of bi rds and smal I mammals. Plant communities and open water provide suitable habitat for migratory waterfowl, shorebi rds and raptors, including the peregrine falcon and bald eagle. No plant species I isted as threatened or endangered by the Federal government or the State of Colorado are known to occur within the St. Vrain Basin (U.S. Fish and Wildlife Service, 1983). However, butterfly weed is a