2009-05-12_REVISION - C1981025 (8)

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There are two criteria used by the Division to identify alluvial valley floors. They are: 1. Sufficient unconsolidated streamlaid deposits holding streams, and 2. Sufficient water to support agricultural activities. Hydrologic data available for North Thompson Creek indicates an annual runoff of 16.2 cfs or 0.6 cfs per square mile. Eighty percent of this runoff occurs during the snowmelt season of April through June. Low flows during late summer average less than one cubic foot per second. Although water is limited within the Thompson Creek drainage, the Division determined that sufficient water is available to support limited agricultural activity. Therefore, the Thompson Creek drainage satisfies one of the criteria for alluvial valley floors. The second and final alluvial valley floor criterion that needs to be considered is that of unconsolidated streamlaid deposits. Using current Federal Office of Surface Mining guidelines, the Division identified three areas that are sufficient in size to support agricultural activities. Because there is sufficient water available to support at least a portion of these areas, they all qualify as alluvial valley floors. The first area identified as an alluvial valley floor is the alluvial fan, 14 acres in size, at the confluence of Yank, Sand, and North Thompson Creeks (see Map D-3-7 of the permit application). Its size barely qualifies it as an alluvial valley floor. In addition, subirrigation is restricted to the immediate streambanks and water may not be available to flood irrigate this area, as it is in the upper reach of the North Thompson Creek drainage. However, without more information on water availability, the Division must assume this area is capable of supporting agriculture. The essential hydrologic function of this area is flood irrigation potential. The North Thompson Creek Mines did not impact this area. The site is 2.3 miles upstream from any mine facility and one mile west of the maximum anticipated life-of-mine for the operation. The operator had not identified any actual or potential impact that might have extend to this area. The mines are now in permanent cessation and final stages of reclamation. The second alluvial valley floor body identified by the Division is a 15-acre tract located in Willow Park along Middle Thompson Creek. This area has the potential to be flood irrigated and is of sufficient size to support agricultural activities. With the exception of a pre-permit existing mine vent fan and two water monitoring flumes, no surface disturbance was made by the operator within the Middle Thompson Creek drainage. Although mine workings extended under a small portion of this watershed, the Permit area does not include Willow Park, and the alluvial valley floor was not undermined. Both the Willow Park and above-mentioned Sand/Yank Creek alluvial valley floors are currently undeveloped rangeland which are not significant for farming. The essential hydrologic functions of these areas were preserved. North Thompson Creek C-1981-025 Permit Renewal 05 12 May 2009 34