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Deccmher L x003 Fage 3-7 Raven Creek has a small alluvial system associated with the stream course. This alluvial material transports near surface ground water that in turn supports perennial flow in the stream. 3.4.2 Environmental Consequences Alternative 1 -Proposed Action In the West Flatiron LBA tract, coal would be mined by longwall techniques, but the proposed mine plan would not subside beneath perennial drainages. Any subsidence in the unnamed drainage in Section 13 could cause minor impact to surface water flows due to sedimentation, but stream flow in this ephemeral drainage would be limited to snowmelt runoff or runoff caused by severe thunderstorms. There would be no negative impact to surface water flow in this unnamed drainage from longwall mining. Subsidence could cause some changes in channel morphology to the unnamed drainage, resulting in alteration to the general surface gradient. In turn, such subsidence could cause minor cutting, pooling, soil erosion and sedimentation. Surface tension cracks have the potential to develop within and surrounding this drainage. These changes could cause increased soil erosion and sedimentation during the initial periods of runoff after subsidence. Surtace tension cracks may also intercept precipitation. In these specific conditions, it is expected that the cracks would temporarily divert precipitation away from the drainage, but that given the limited depth of tension cracks, the flow would remain in the drainage. Given the ephemeral nature of the unnamed drainage in Section 13, as well as its steep gradient, the thickness of overburden, and the natural geologic instability of the area, subsidence would have minimal impact to this drainage. Under the presumed mining plans for the LBA tract, no mining would occur beneath Raven Creek in the northeast portion of the lease tract (see Section 2.9, Reasonably Foreseeable Development Scenario). However, because a lease would include all coal seams, it is reasonable to assume that mining could occur in a seam other than the B seam, and a mine plan could include subsidence in the Raven Creek area. If longwall mining was to be conducted under Raven Creek where overburden depths over the B seam are projected to be less than 600 feet (see Figure 2-2, Overburden Isopach Map), there would be a potential for mine subsidence to cause changes in the channel morphology of Raven Creek; such subsidence could trigger head cutting, pooling and channel adjustment. Surtace tension cracks could also develop within and surrounding the drainage. These changes could cause increased soil erosion and sedimentation. With the lower overburden, there is a higher risk that strata fracturing that forms above the mine level could connect to the land surface. If subsidence cracking were to form in Raven Creek, surface flows and near-surface ground water in the alluvial sediments could be intercepted. This could translate effects to other resources, including potential loss of associated riparian vegetation, reduced habitat for riparian obligate species, and loss of aquatic biota habitat. The time it would take for fractures of this nature to "heal" is unknown. Therefore, the duration of the potential loss cannot be calculated. Water may be encountered when the underground mining intersects geologic faults on the LBA tract. It is projected that four faults are found on the tract, and each fault, when encountered, would initially release approximately 1,000 gallons per minute (Koontz, 2003). This flow would be diminished after three days. Envircmmertal „ssessment t'Vasf Flati[nn lc~A Trait Gunr;isari Gin~nty, Cplorada