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Evaluation of Potential Subsidence Impacts of Longwall Mining in the Spruce Stomp Lease Area <br />to Aquatic Life and Water Supply <br />potential subsidence located approximately 7,800 feet above sea level and the downstream limits <br />of potential subsidence located at approximately 7,200 feet above sea level. <br />4.2.1 Physical Characteristics <br />I. Stream Flows <br />Available hydrologic informational data from the USGS, Bowie and others was investigated to <br />identify typical and dry -year flows in West Fork Terror Creek in the Spruce Stomp Lease Area. <br />Stream flows in West Fork Terror Creek are highly variable depending on season and year (refer <br />to Figures 11 and 12 and Table 6). In addition to being influenced by climatic influences, West <br />Fork Terror Creek flows are manipulated by transbasin diversions, and tributary wells that <br />collect water and transport it out of the watershed (see Figure 8). Based on direct measurements <br />made by Bowie, average daily mean flows are highest in March (19.9 cubic feet per second [cfs]) <br />and are lowest during late summer and early fall (approximately 1 cfs in July, August and <br />September). Based on an analysis of diversion records and USGS flow measurements, dry -year <br />conditions can result in flow reductions in West Fork Terror Creek of 70 percent or more <br />compared to average conditions during late summer months when flows are at or near their <br />lowest monthly levels. <br />Based on communication with individuals familiar with the location, West Fork Terror Creek <br />can have short, extreme high flow events. These events are reportedly capable of overtopping <br />the culvert at the Terror Creek road and would not be measurable with the current staff gage at <br />this location. It is valuable to note that in order to overtop the culvert at the Terror Creek Road, <br />flows would be in excess of approximately 230 cfs (based on hydrologic modeling at this <br />location). This type of flow in the West Fork Terror Creek channel would cause scouring, <br />sediment transport and movement of large rocks and boulders, all of which would result in <br />significant aquatic habitat disturbance. <br />Channel Profile, Slope, Sinuosity, Bed and Bank Composition, etc. <br />Within the Spruce Stomp Lease Area, the West Fork Terror Creek channel contains primarily <br />boulder dominated riffles with some interspersed cobble dominated riffles and small pools (see <br />121 - 014.000 Wright Water Engineers, Inc. Page 7 <br />January 2013 <br />