Riparian Resources for Aspinall PDEIS

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Asp,„aiy cis RIPARAIN RESOURCES BACKGROUND: The net ecological change in Black Canyon is expressed by the establishment of vegetation where it would not have naturally occurred. NPS management policy is to return the environment to the natural condition. Regulation by Aspinall has resulted in more land vegetated with arid species at the expense of Tess land vegetated with moderately wet species, and regulation has resulted in more land area being colonized with woody plants. The desired condition is one where scour and cobble bar inundation from peak flows result in little or no arid community composition coupled with one where moderately wet conditions prevail. Plants themselves are regularly scoured and perennial vegetation is controlled. SIGNIFICANCE: Significance testing is assessed with a paired Student's t analysis; this is characterization of the significance of the difference in the means for an equal length record. Total bottomland cleared - No- Action and alternative A are not significantly different. Action - alternatives B,C and D are significantly different. Total bottomland cleared by scoured, action alternatives A through D are significantly different than the No- Action. For total cleared, that No- Action and action A are not different results because of the impact of inundation flows in the No- Action. METHODOLOGY: Two methods are employed in this evaluation. Method one is computation of the percentage of bottomland scoured of herbaceous and woody plant communities under each alternative. The comparison metric is the functional relationship between scouring of woody vegetation and peak flow and inundation flow, as in Friedman and Auble, 1999. NEPA DEFINITIONS: RIPARIAN RESOURCES ANALYSIS, WOODY VEGETATION Context is herein defined as a function of the removal of bottomland vegetation. An operation that over the 31 year comparison record changes the percentage of bottomland scoured by ±50 or less is defined as having a local context. An operation that removes changes the percentage of bottomland scoured by greater than ± 50 is defined as having a regional context. Duration: Friedman and Auble (1999, p. 473) state that "low -flow periods longer than 10 - 15 years may result in irreversible streamward encroachment of woody vegetation ". Encroachment is irreversible owing to ability of woody species to better withstand removal from a shearing force as they mature. The duration metric is a count of the number of consecutive years in an alternative without a flow capable of clearing 50 percent of the bottomland without a scouring flow. An alternative that is typified by 6 years or Tess without a flow that removes woody vegetation from 50 percent of less is a short -term duration impact. An alternative that is typified by