OCR Text |
Show from forested areas. They create serious problems in intensively developing areas such as Diamond Fork. On highway cuts in Idaho annual losses up to several hundred tons per exposed acre have been measured; in one case 97 tons per acre of soil and 62 percent runoff from 2.5 inches of intense rainfall on a bare highway cut was observed. " Indirect consequences are equally important. Major damage from roads is not limited to sedimentation from the road prism itself. Water generated from the road prism and drained onto adjacent soil provides an equally potent opportunity for erosion. The road prism acts much like a paved catchment in that it greatly increases surface runoff. If the road is surfaced, all of the precipitation which falls upon it may be expressed directly as overland flow. Even if the road is unsurfaced, as much as 90 percent of the precipitation which falls upon the road prism may be expressed as overland flow. To complicate this problem, roads by design concentrate the water prior to draining it from the road. This water, existing in concentrated form, increases both the opportunity to initiate erosion and the capability for sediment transport from areas adjacent to roads. " For example, a road located 100 yards from a stream channel and producing 50 tons of sediment for each acre of exposed prism per 2- inch storm may deliver 80 percent of the sediment or 40 tons per acre directly into the stream. The delivery potential means that not only is the material removed from the road prism, but it is placed directly into the stream. This presents a serious problem particularly in areas where the natural potential for cleansing the stream is eliminated by containment of spring peaks." In addition to the general impacts described above, roads often intersect stream courses, and crossings must be made. Particular care would be exercised in the design and location of stream crossings so that a permanent erosion source would not be created. b• Vegetation The construction of the Bonneville Unit would cause the loss of about 22,000 acres of existing vegetation. About |