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2.3.2. Area Gradient Index <br />• The area gradient index (AGI) is the product of drainage area and <br />gradient at a point on a stream channel. This index provides a relative <br />means of comparing the erosive power of stream flow on existing and <br />approved topography. Any significant increase in AGI caused by the <br />mining and reclamation process will indicate a potential for increased <br />erosion and channel scour. <br />Five major stream channels were analyzed within the mine area <br />consisting of areas 9/10, 7, 31, and 51. The major channel in area 41 <br />was also analyzed. Area and stream gradient were measured at 500 to <br />1000 foot intervals from the mouth up the main channel such that <br />approximately 6 to 8 values were measured for each channel. Actual <br />sampling locations were selected such that critical reaches of the <br />• stream profiles were included. Area Gradient Index (AGI) is the area in <br />acres multiplied by the channel slope in percent (see Appendix). <br />The mean and standard deviation of AGI for approved topography are <br />10.6 and 6.0, respectively, and the mean and standard deviation for <br />existing topography are 10.8 and 10.4, respectively. The standard <br />deviations bracket the means of both existing and approved AGI and this <br />shows that the means are not significantly different. <br />Another method for comparison of approved and existing AGI is to <br />plot channel slope versus drainage area. Figure 2.8 shows data for <br />approved stream channels with a line defining an upper envelope of <br />slope-drainage area combinations for approved natural channels. The two <br />points lying above the line represent the approved condition for the <br />• <br />21 <br />