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<br /> <br /> <br /> <br /> 26 <br />area included the potential 20+ year life-of-mine area, in addition to the first <br />permitted five-year mine plan area. Also, areas adjacent to the life-of-mine were <br />considered for distances in excess of three miles. The total area considered for this mine <br />investigation was in excess of 75 square miles. Black and white and color aerial photos <br />were used for the study. In addition, areas were field checked for accuracy. <br /> <br />The study identified areas of unconsolidated streamlaid deposits along the North, <br />Middle, and South Thompson Creeks and their tributaries. Surveys and field checks of <br />these deposits revealed that the alluvium is relatively thin, intermittent, and <br />restricted to the immediate stream channels. No identifiable soils were mapped <br />within alluvial areas; the alluvium is primarily boulders and cobbles with minor <br />stretches of sand, gravel, silt, and clay. <br /> <br />There are two criteria used by the Division to identify alluvial valley floors. They are: <br /> <br />1. Sufficient unconsolidated streamlaid deposits holding streams, and <br /> <br />2. Sufficient water to support agricultural activities. <br /> <br />Hydrologic data available for North Thompson Creek indicates an annual runoff of <br />16.2 cfs or 0.6 cfs per square mile. Eighty percent of this runoff occurs during the <br />snowmelt season of April through June. Low flows during late summer average less <br />than one cubic foot per second. Although water is limited within the Thompson <br />Creek drainage, the Division determined that sufficient water is available to support <br />limited agricultural activity. Therefore, the Thompson Creek drainage satisfies one <br />of the criteria for alluvial valley floors. <br /> <br />The second and final alluvial valley floor criterion that needs to be considered is <br />that of unconsolidated streamlaid deposits. Using current Federal Office of Surface <br />Mining guidelines, the Division identified three areas that are sufficient in size to <br />support agricultural activities. Because there is sufficient water available to support at <br />least a portion of these areas, they all qualify as alluvial valley floors. <br /> <br />The first area identified as an alluvial valley floor is the alluvial fan, 14 acres in size, <br />at the confluence of Yank, Sand, and North Thompson Creeks (see Map D-3-7 of the <br />permit application). Its size barely qualifies it as an alluvial valley floor. In addition, <br />subirrigation is restricted to the immediate streambanks and water may not be available <br />to flood irrigate this area, as it is in the upper reach of the North Thompson Creek <br />drainage. However, without more information on water availability, the Division <br />must assume this area is capable of supporting agriculture. The essential <br />hydrologic function of this area is flood irrigation potential. The North Thompson <br />Creek Mines did not impact this area. The site is 2.3 miles upstream from any mine <br />facility and one mile west of the maximum anticipated life-of-mine for the operation. <br />The operator had not identified any actual or potential impact that might have <br />extended to this area. The mines are now in permanent cessation and final stages <br />of reclamation.