1986-06-13_GENERAL DOCUMENTS - C1981018

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-25- indicating that the availability of subsurface water is much more limited in Scullion Gulch. Further evidence of the general lack of subirrigation can be gleaned from the permittee' s surface facilities geotechnical investigation, contained in Illustration 18 of the permit application. Six exploration holes, sited at the proposed D-portal area, were drilled within the alluvial body associated with Scullion Gulch. Although all the holes fully penetrated the alluvium, no evidence 'of alluvial ground water was encountered. This information, coupled with the sparse occurrence of deep-rooted vegetation, generally indicates that subirrigation is of minimal consequence along Scullion Gulch. 3. White River Geomorphic Characteristics The White River, a perennial stream, is located along the southern margin of the permit area. The river actually crosses through the southern most tip of the permit boundary for approximately one-half mile. The river meets the geomorphic criteria for alluvial valley floors, as shown on Map 102, as an extensive area of unconsolidated streamlaid deposits are present. The deposits within and adjacent to the permit area appear to range between 1 ,000 to 1 ,500 feet wide. The alluvium is about 37 feet thick in the general area, consisting of 15 feet of fine silts and clays, with 22 feet of gravel , sand, • silt, and clay located below the upper fine-grained material . The permittee mapped the deposits in the immediate vicinity of the permit area, from a point one-half mile upstream from the eastern margin of the permit area to a point one-half mile downstream from the southwestern portion of the permit area. The deposits are continuous, and extend beyond both the upstream and downstream limits of the area mapped by the permittee. Within the area mapped (approximately 2 miles in length) the alluvial deposits constitute an area in excess of 400 acres. The permittee has indicated that ground water is contained in the alluvium along the White River at depths less than 11 .5 feet below the land surface. Subirrigation of deeper rooted vegetation, including alfalfa, likely occurs within this depth. The permittee has indicated, however, that it is a regional practice to artifcially flood irrigate alfalfa to maintain a productive crop. Irrigation requirements are therefore not satisfied by—suM rrigation alone; it is likely, however that subirrigation does supplement the flood irrigation. Based on the demonstrated suitability of the White River Valley for irrigation, the permittee concludes that the land along the White River constitutes an alluvial valley floor. Alluvial Valley Floor Determination The permittee submitted a request along with the original permit application for a negative alluvial valley floor determination of Red Wash and Scullion Gulch. The permittee proposed that the alluvial deposits located along the White River constitute an alluvial valley floor. • The permittee originally requested a negative alluvial valley floor determination of Red Wash and Scullion Gulch on the basis of a regional reconnaisance survey conducted to address the irrigation potential of the two drainages. The inventory reviewed all areas within a 10 mile radius of the permit area. The results of the inventory led the permittee to conclude that