2011-01-13_REVISION - C1981038 (8)

Home Browse Search

an Alluvial Valley Floor based on its lack of flood irrigation potential and lack of significant subirrigation. ° An additional area along Steven's Gulch which lies within and to the south of the mine permit area contains areas of alluvium. This area includes alluvial gravel deposits in Sections 24 and 25 (TUS, R92W) and the W 1/2 of Section 30 (T13S, R91W). Four separate deposits have been identified within this area, as shown on Map No. 2-2. The two larger deposits, labeled Qagy and Qago, may meet the geomorphic criteria of Alluvial Valley Floors, but do not meet the water availability criteria for either flood irrigation or subirrigation. Due to the steep slopes, rocky surface, and lack of sufficient water, these areas cannot be flood irrigated. Regarding subirrigation potential, the field investigation to detect subirrigated species along Steven's Gulch did not identify any such species within these areas. Vegetation suggests subirrigation does occur along portions of Steven's Gulch, but these species are confined to the extreme channel bottom areas. Based upon this information, these two areas are found not to meet the criteria for Alluvial Valley Floors. These are older terraced deposits that have been dissected by stream channels, such that the channels have completely cut through the deposit. These areas are found not to be Alluvial Valley Floors. The larger of the two areas totals 11.5 acres, but the residential access road that passes over the area reduces the acreage to below 10 acres. A third area identified as having alluvial deposits is located along the North Fork of the Gunnison River. These consist of floodplain and low terrace alluvial deposits and are identified as (Al) on Plate No. 2. This area meets the geologic and hydrologic criteria of an Alluvial Valley Floor in having unconsolidated stream-laid deposits with water sufficient to sustain agriculture. Portions of the loadout facility and rail spur have been part of the mine plan before August 3, 1977; therefore, their construction is grandfathered (see Alluvial Valley Floor Findings that follow). Adjoining these deposits on both sides of the river are deposits of course-grained outwash (pediment) labeled Qpc. Geomorphically the deposits form lobes and undulating topography that extend down to the low alluvial terrace deposits of the identified Alluvial Valley Floor. These deposits are easily identified on Map 8-3 by the abrupt change in slope as one leaves the flood-plain and terrace deposits along the river. The slope outwash deposits are flood irrigated, which meets the hydrologic criteria of an Alluvial Valley Floor, but geologically they are mixtures Mancos shale and eroded alluvium that were primarily deposited by gravity flow and sheet wash. As such, they do not meet the criteria of being stream laid. During the original review of the permit application, these deposits were identified as part of the AVE But further review and interpretation of additional hydrologic and geologic information during the summer of 1991 indicated that the area did not meet the strict definition of an Alluvial Valley Floor. Deposits at the mouth of Steven's Gulch mapped as Qpc on Plates 1 and 2, of the Geo-Hydro AVF Phase I study (Volume 4) are identified as slope outwash deposits. As such, they also do not meet the strict definition of unconsolidated stream-laid deposits. The U.S. Soil Conservation Service has identified the Aqua Fria soils in this area as "Farmland of Statewide Importance" and not as prime farmland. Because much of the area is occupied by flood-irrigated orchards, the Division feels these areas are worth protecting for their agricultural contribution. Although these areas are not AVF's, the Division will require that these areas be treated as AVF's with respect to assuring that their essential hydrologic functions be restored. On the basis of the information provided, the North Fork of the Gunnison and its associated alluvium, identified as (Al) on Plate No. 2 in those portions of Sections 29, 30 and 31 in T13S, R91W, and Section 36 in T13S, R92W, which lie roughly between the Farmers Ditch and the North Fork of the Gunnison River, are Alluvial 42