Riparian Resources for Aspinall PDEIS

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Operation governed by action- alternatives D can be expected to local -scale improvement on the regional -scale impact of the current operation. Operation under action- alternatives will have a Tong -term impact on park resources; the duration of successive intervals without a shearing flow is too long. The intensity is moderate. Moderate intensity improvement coupled with the long -term duration is not sufficient to preclude the encroachment of woody vegetation and the type of response is adverse. Table 2. • Alternative Context Duration Intensity Type No- Action Regional Long -term Negligible Adverse Alternative A Local Long -term Moderate Adverse Alternative B Local Long -term Moderate Adverse Alternative C Local Long -term Moderate Advere Alternative D Local Long -term Moderate Adverse NEPA DEFINITIONS: RIPARIAN RESOURCES ANALYSIS, COMMUNITY COMPOSITION Context is herein defined as a function of the percentage change in bottomland community composition of herbaceous vegetation. Extent of and community composition of herbaceous vegetation is controlled by the supply of water. The riparian corridor receives little precipitation and the water balance for native plants is a function of peak and duration flows. Four plant community types are identified within the riverine corridor of Black Canyon. 1. Xeric community is a community of very dry site plants. 2. Mesic (moderately wet site) is a. community of grass and sedges. 3. Hydric community is a community if plants that grow in saturated to nearly saturated soil conditions. 4. Aquatic community is a community of plants that grow in water. The aquatic community is spatially small in area and unchanged with dam operations. It makes up about 1 percent of the bottomland. The hydric community is much greater in extent, but is also unchanged by dam operation within the variation embedded in a 31 year record that includes no darn operations. The principal changes effect by darn operations is in the percentage of bottomland in xeric and mesic plant communities. A local context change is herein defined in a percentage change of ± 50 or Tess in the occurrence of mesic plants. A regional context is defined as a ± 50 percent or greater change in the occurrence of mesic plants. Duration: The change in percentage of mesic community composition is directly a measure • of the time that the bottomland is at or above inundation. Occurrence of a mesic community implies that the bottomland is inundated for at least 3 days and for no more than approximately 75 days per year.