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approximately 250 feet in the vicinity of the confluence; the stream channel itself is incised <br />throughout the reach, with no apparent flood plain present. Flood flows aze contained within the <br />incised channel; as such, the valley bottom along the channel is in effect a terrace. <br />There is no current flood imgation along the West Fork and no indications of historic flood <br />irrigation. Vegetation on the relatively flat terrace benefits from precipitation and snow melt <br />runoff from the steep valley side slopes. As a result, the predominately clayey soils of the terrace <br />support a relatively lush growth of native grasses, dominated in most areas by basin wildrye. <br />Areas of colluvial deposition that aze not as flat, and exhibit coazse material at the surface <br />support upland shrub communities. Riparian species are restricted to a very narrow band, <br />immediately adjacent to the channel, and species indicative of subirrigated conditions were not <br />observed on the terrace. Absence of subirrigation is further demonstrated by alluvial monitoring <br />wells near the mouth of the West Fork that show saturated conditions aze not present within 10 <br />feet of the soil surface. <br />Although forage production is higher along the valley bottom than on adjacent upland slopes, the <br />increased production appears to be due to "run on" precipitation, and not due to surface or sub- <br />surface irrigation. The valley is contained within a larger rangeland "pasture unit", and is not <br />hayed, fenced, or otherwise developed or managed differently than surrounding uplands. <br />Due to the restricted width of the alluvial deposit along most of the reach, and the absence of <br />sub-irrigated or flood irrigated agricultural activity (current or historic), the Division has <br />determined that the West Fork of Good Spring Creek is not an alluvial valley floor, based on <br />geomorphic and land use criteria. <br />Wilson Creek (Middle Segment) <br />A segment of Wilson Creek approximately four miles in length within the PR-2 expanded permit <br />azea was determined to be an alluvial valley floor during initial permitting of the Colowyo Mine. <br />However, in 1985, this segment of the Wilson Creek valley was altered by severe erosion and <br />mass wasting. The Division's July 1987 Renewal Findings states: <br />During 1985, natural erosion processes created severe mass wasting along <br />Wilson Creek, and for all intents and purposes destroyed this AVF. This impact <br />was not related to the mining operation. <br />However, the Division appazently never made an official determination that the segment of the <br />Wilson Creek valley impacted by the erosion and mass wasting no longer qualified as an alluvial <br />valley floor. This issue was revisited in association with PR-2. As described in Section 2.06.8 of <br />Volume 12, alluvium within the segment was down-cut 20 to 30 feet below the former flood <br />plain, leaving two narrow terraces high above the new incised channel, generally no wider than <br />100 feet and in some areas much narrower. The down-cutting resulted in de-watering of the <br />alluvial aquifer, such that areas which may historically have benefited from sub-irrigation no <br />Proposed Decision and Findings of Compliance 4 May 2007 <br />Permi[ Revision 02 Page 64 <br />