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/unt 1991 Cypnas TCC "Analysir o/Subsidence E/jau on Hydrology ~ 2 ` Stream flow monitoring for Fish Creek has been performed within the general study azea at two stations: Station 16, located upstream of Panels 5 and 6, and Station 307, located downstream of the srudy area. Historic minimum flow for Station 16, which replaced United States Geological Survey (USGS) Station 1001 after monitoring was discontinued in 1973, is 2.0 cfs, and historic maximum flow is 108 cfs during a period of record from March, 1987 through September, 1991. High flow occurs during and following snowmelt, typically in April through June. Boettcher, (1977) indicated that 25% of the streamflow in Fish Creek originates from ground water discharge, much of which occurs downstream from the study area. Therefore, Fish Creek is gaining stream through portions of its watershed azea. Surface water usage from Fish Creek is predominately for irrigation and stock watering. Agricultural aspetts of water usage are discussed in Exhibit 46, Fish Creek AVF/Stream Study, Vegetation Analysis. Potential changes in the Fish Creek gradient resulting from longwall mining in Panels 5 and 6 and related surface subsidence were analyzed. This analysis reflects consideration of site-specific chazatteristiu of Fish Creek within the study area and the results of the surface subsidence predictions previously referenced. Several stream channel profiles were plotted for Fish Creek as it meanders through the study area. Available mapping was initially used for the channel profile as shown on Figure 1, Potential Changes in Channel Profile Due to Subsidence, as "Original Profile." Afield survey of the study azea was completed in May 1992. Results of the field Srrvey were incorporated into Map 1, Subsidence Impact on Hydrology, showing current stream alignment and flood plain configuration. Current stream profile, based on the 1992 survey, was also plotted on • Figure 1. During the period from approximately 1984 to 1992 the Fish Creek Stream channel changed considerably. Several meanders were cut off, reducing the stream length through the srudy azea. It also appears that elevation of the stream section through the study azea was reduced by subsequent down-cutting, however, this conclusion can not be verified because original mapping was based on five foot contours and the profile was extrapolated to one foot intervals. Fish Creek meanders through the study area intersecting the areas overlying proposed Panel 6 and a rib of Panel 5. The locations of these panel alignments depicted on Figure 1 are based on the stream m~guration as recently surveyed (1992). Predicted gradient changes resulting from undermining Panels 5 and 6 were estimated using subsidence contours shown on Map 1. The surveyed profile shows two slope segmentg Segment 1 with a slope of 0.65%, and Segment 2 with a slope of 0.27%. Changes to the channel gradient after undermining Panel 5 using the same segments as the surveyed profile, show a slope of 0.70% (an increase of 0.05%) for Segment 1 and a slope of 0.23% (a deaeau of 0.05%) for Segment 2. Gradient changes of this magnitude are not expected to have a significant impact on Fish Creek. Any headcutting or deposition resulting from gradient changes would occur within a short period of time due to the fine grained nature of the alluvium, Changes to the Fish Creek stream channel would not be expected to be of greater magnirude than natural stream channel variations. Given these considerations, no significant impacts on water avai]ability or essential hydrologic functions of the Fish Creek Alluvial Valley Floor (A VF) are expected. ACZ Inc. ' P.O. Box 779018 • Suamboat Springs. Colorado 8047) '(303)87%260