OCR Text |
Show for the seismic risk map of the western United States ( Figure B- 3) were abstracted from the Department of Commerce News, January 14, 1969. Considering past evidence of recorded history of seismicity in that area covered by the proposed Bonneville Unit, it is reasonably safe to assume that the area of high seismic risk ( where destructive damage may be incurred on a Unit feature) lies along the west slope of the Wasatch Mountains. The Wasatch Fault Zone is located along the west slope and is recognized in published literature as the controlling natural or physical division where earthquake activity has occurred more persistently than in areas east of this zone. Publications are available for detail study of the seismicity of the area. Some of these are listed in the Bibliography.-'-^' J-'- SJIO, 17 Vegetation ae General A variety of vegetation zones, ranging from subalpine forests to grass- sagebrush lowlands, occur within the area of the proposed Bonneville Unit. Each vegetal zone is usually characterized by several different plant communities. The composition and arrangement of the plant communities within a particular vegetation zone are largely dependent upon the moisture situation. Since settlement of the area in the mid- 18001s various types and degrees of land use have acted to alter native conditions and creat artificial situations that have sometimes been of less desirable quality and reduced utility. Hence, the relatively recent use of the land has largely controlled the short- term evolution of vegetative patterns in many areas. One index of the degree of land use is the presence of exotic or introduced species sometimes referred to as ruderals. In instances of excessive land use no replacement vegetation develops. Another sign of a " healthy" plant community is the presence of a large number of different species. Because of the steepness and the thin soil mantle of some locations along the proposed route of the Unit collection system the plant communities in these areas are very fragile and subject to severe deterioration if not carefully treated. The major vegetation zones and prevailing plant communities from representative sites within the proposed Unit area are presented in Table B- l. A listing of the plant species 122 |