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SECTION 1 - <br />• INTRODUCTION <br />Tlris report describes the results of a study to evaluate stream valleys in llre areas of Pittsburg <br />and Midway Coal Mining Company's (P&M's) proposed Fish Creek Mine as alluvial valley <br />floors. The evaluation was conducted in accordance with the AVF regulations of the Colorado <br />Mined land Reclamation Board (section 2.06.8) and with the Office of Surface Mining's AVF <br />Identification and Study Guidelines and focuses on the Fish Creek drainage. The report also <br />describes the results of studies concerning the significance of designated AVF's to agriculture. <br />Study Area <br />The study area for this evaluation was determined in consultation with P&M and the Colorado <br />Mined Land Reclamation Division (CMLRD). More specifically, the study area covers an area <br />extending approximately one mile around the boundary of the proposed Fish Creek Mine (see <br />Figure 1). <br />AVF Identification Criteria and Study Approach <br />As defined in applicable regulations and guidelines, alluvial valley floors are those areas in the <br />western United States which are (1) located in topographic valley's, (2) are underlain by <br />unconsolidated deposits where the land surface consists of floodplains and terrace landforms, (3) <br />leave an associated sVeam channel, and (4) have sufficient water to support agricultural activities <br />as evidenced by existing flood irrigation, the capability of [he area to be flood irrigated, or the <br />presence of subirrigation. <br />i The two principal criteria to determine the existence of an AVF include certain geomorphic and <br />water availability characteristics. The basic geomorphic characteristics of AVF's, as specified in <br />the definition of the term "alluvial valley floor" in the Colorado Regulations, include the <br />following: "unconsolidated streamlaid deposits holding streams ... but does not include upland <br />area which are generally overlain by a thin veneer of colluvial deposits cornposed chiefly of <br />debris from a sheet erosion, deposits formed by unconcentrated runoff of slope wash, together <br />with talus, or other mass-movement accumulations, and windblown deposits." (section 1.04) <br />Itr addition, terraces and floodplains are important geomorphic characteristics of AVF's since <br />these landforms are constructed by fluvial processes usually underlain by streamlaid (or <br />alluvial) deposits. Therefore, the geomorphic characteristics of alluvial valley floors were <br />evaluated in this study by identifying the surface extent of all floodplains and terraces, and also <br />shallow sloping colluvial areas which were considered likely to be underlain by unconsolidated <br />deposits. It should be noted that these geomorphic evaluations were only carried out in valleys <br />wlriclr contain identifiable stream channels since the definition of AVF's includes unconsolidated <br />streamlaid deposits holding streams. <br />In addition to meeting certain geomorphic criteria, AVF's must also meet certain water <br />availability criteria. More specifically, the latter criteria include the requirement that there be <br />sufficient water present in the valley to support flood irrigation or subirrigation agricultural <br />activities. Therefore, in the course of this study, evaluations of existing flood irrigation practices <br />were undertaken in the study area and an assessment of regional flood irrigation practices and <br />. other site-specific characteristics was made to determine the potential for flood irrigation in the <br />study area. In order to evaluate the extent of subirrigation agricultural activities, test pits were <br />I:env iron\tracy\documen VExhibTA <br />0 I! I G/98 <br />APPR®iJED uur~ 2 s 2000 <br />