2019-05-29_REVISION - C1981018 (11)

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39 its associated alluvium are as follows: Red Wash Alluvial Valley Floor Determination The Division has determined that Red Wash and its associated alluvium meets the geomorphic criteria, but not the irrigation requirements of an AVF, as outlined in OSM's "Alluvial Valley Floor Guidelines" established pursuant to the Surface Mining Control and Reclamation Act of 1977. Specifically, pursuant to Rule 2.06.8(3)(c), the Division finds that there is not sufficient water to support agricultural activities. The Division therefore concludes that Red Wash is not an Alluvial Valley Floor. Scullion Gulch Water Availability Criteria - Artificial Flood Irrigation As with Red Wash, flow events along Scullion Gulch were monitored using a crest stage gage. During the two year inventory period conducted by the permittee, only three runoff events were recorded, the highest event measuring 185 cfs. Again, however, crest-gage data has only limited application for AVF purposes. The mean annual yield estimations provided by the permittee are more applicable for identifying the amount of water that may be expected for Scullion Gulch. The permittee’s estimations, derived using Grunsky's Formula, indicate that the potential average annual water yield from the Scullion Gulch watershed is 202 acre feet. This corresponds to approximately 0.34 inches of runoff. The amount of water available annually, as estimated through this method, indicates that Scullion Gulch warrants further consideration as a potential alluvial valley floor. Subirrigation As with Red Wash, the presence of subirrigation along Scullion Gulch was investigated by the permittee based on the type of vegetation present along the drainage. The permittee has identified a big sage brush - greasewood community along the entire length of unconsolidated alluvial deposits identified on Map 102, and concludes that water is available to the vegetation "on a limited and sporadic basis". The big sage brush-greasewood community, however, is more sparse compared to that community found along Red Wash, 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.