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-13- VII. Alluvial Valley Floors -- Rules 2.06.8 and 4.24 AVF Determination The applicant has provided the results of an alluvial valley floor reconnaissance investigation on pages 54 through 58 of the application. The study area, which included all lands lying within a i 1/2 mile radius of the permit area boundary, is shown on Figure 1, page 56 of the application. Three perennial streams occur within the study area -- Harvey Gap Drainage, West Elk Creek and the Colorado River. The mine is located on Harvey Gap Drainage, which is used as a conduit for irrigation water from Grass Valley Reservoir approximately 1/2 mile north of the mine portals to irrigated terraces of the Colorado River approximately 1 mile south of the mine portals. The applicant has mapped unconsolidated streamlaid deposits on terraces of the Colorado River approximately 1/2 mile south of the 5 year permit area and along West Elk Creek approximately 2 miles to the northeast of the permit area. A narrow strip of alluvium has also been identified along the Harvey Gap Drainage in the permit area. Unnamed ephemeral channels draining the Grand Hogback in the permit and adjacent area contain very limited alluvial deiwsits and are too steep, rocky, and narrow for agricultural use or alluvial valley floor consideration. The alluvial valley floor potential for the three major drainages in the permit and adjacent area is assessed below. Harvey Gap Drainage The applicant has indicated that a narrow band of alluvium exists along Harvey Gap Drainage in the permit and adjacent area. As such, portions of the Harvey Gap Drainage from Grass Ualley Reservoir downstream approximately one mile may meet the geomorphic criteria of an alluvial valley floor. However, information in the application documents that the narrow valley width and steep side slopes would effectively preclude flood irrigation. The width of alluvial material is such that, on the 1:24,000 scale map, the unconsolidated streamlaid deposits could not be delineated. No continuous body of alluvium of at least 50 feet in width and 10 acres in extent occurs along Harvey Gap. Vegetation information in the application is sufficient to document an absence of subirrigation, with the exception of scattered cottonwood trees immediately adjacent to the stream channel. The dominant vegetation type along the valley bottom and sideslopes is big sagebrush/cheatgrass, which is not indicative of subirrigation. The stream channel through the permit and adjacent area has down cut through the alluvium to a depth of over b0 feet, which would effectively preclude the occurrence of a near surface ground water table. 3ased on negative determinations regarding flood irrigation potential and subirrigation, Harvey Gap Drainage from Grass Valley Reservoir downstream approximately one mile (to where! the valley broadens out and meets ttie terror?s of the Colorado River) Ls_ determined not' to be an alluvial Valley floor. (See Figure 1, ~armit Application.)