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with a sedimentation pond 15 feet deep, and proceed to the north in cells. Graveling will proceed with the west <br />excavated embankment a minimum of 50 feet from the main Animas River Channel. The entire site is a grave] <br />bar and flood plain of the Animas River, unmediately adjacent to and south of the alluvial fan of Maggie <br />Gulch, emanating from the high slopes to the east. <br />Through evaluation of aerial photos, detailed topographic maps of the site, and other mapping data, and <br />through personal observation in the past, we would categorize this stretch of the Animas River as a "D4" or <br />"DA4" type stream (reference Rosgren, Dave, Applied River Morphology, 1996; excerpted as Appendix A). <br />This reach of the Animas from the north end of pit site, approximately one mile to the south, goes from a <br />channelized reach where Maggie Gulch alluvial fan pinches the river to the west flank of the valley, to an <br />anastimosing, braided stream adjacent and south of the site. The river divides into numerous sub channels <br />which appear to be erosionally stable, overgrown with willow and other phreatophyte vegetation. <br />The stream "flood plain" is not mapped on FEMA Documents. Figure I is a duplication of a FEMA Map <br />showing upstream and downstream reaches, which have been categorized. Based on our observations and this <br />mapping, we believe the "100 Year Flood Plain" adjacent to the site may encroach into the side channel, <br />traversing the tip from north to south. However, the flood plain widens from approximately 250 feet near the <br />north end of the site, to over 400 feet neaz the south end. This means high peak flows of the Animas would <br />spread laterally along the site, and decrease in erosional potential. The slope of the Animas above the site is <br />approximately .Ol feet per foot, or 1.0 %, whereas from the pit site southward, it decreases to approximately <br />.005 feet, or 0.5%. <br />USGS River Gauging Stations at Howazdsville and Silverton (above Cement and Mineral Creeks) are in Table <br />1 attached. This summarizes the historic peak flows at these two stations. The Howardsville Gauge was in <br />operation from 1936 to 1982 (46 years) when it was abandoned. The gauge at Silverton (as wall as Cement <br />Creek) were installed in 1992 and have a period of record from 1992-2004 (12 years). Maximum peak flow <br />recorded at the Howardsville Gauge was 1980 cfs, June 18, 1949. Average maximum peak flow at the time of <br />record was 1026 cfs. The gauge at Silverton has recorded historic high o£ 1470 cfs, June 4, 1997. Average peak <br />flow is 1035 cfs. <br />Based on the historic peak of 1980 cfs, 27 years have recorded a peak value over the average (46% of the time). <br />Eighty percent of peak flow (1584 cfs) has only been exceeded four times in 59 years of record, or 7%. No <br />definition of volume for 100-yeaz flood peak flows is provided in the FEMA Documents. Therefore we would <br />expect peak flows of 1000 cfs to 1600 cfs during spring runoff on an average to above average snowpack yeaz. <br />Erosional Potential: The proposed mining plan includes maintaining the excavation a minimum of 50 feet from <br />the "main stem" of the Animas River. Detailed site plan shows a natural (and we beNeve man-made) small side <br />channel, starting approximately 350 feet from the north end of the pit, transversing the proposed pit in a <br />south southeasterly direction. Concems raised by DMG are that, should this side channel receive high flows in <br />the springtime, headward erosion could occur, thus capturing main flow of the Animas River. Concerns were <br />also expressed regarding erosion at the terminus end of pit, which could potentially cause downstream flooding <br />due to breach in an excavated area between the pit (sedimentation) and the main river. The pit reclarnation plan <br />calls for revegatation of willow and other phreatoyphyte vegetation, (as ruining progresses) from south to north <br />on the 50-foot berm between the pit and the main Animas River. <br />Mitigation: 1n order to assure DMG that adequate measures will be taken to prevent river capture and erosion <br />on the flanks of the pit, we would propose armoring of the existing channel at the upper end, and creation of a <br />"spill way" out of the sedimentation pond at the south end. Detailed survey of the ephemeral channel inlet will <br />provide elevations which can be maintained through armoring and creation of a "floodway inlet". These <br />