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Sediment yields from Yellow Creek are quite high. The annual sediment yield <br />for water year 1980 was 12,495 tons. Daily extremes for Yellow Creek for <br />the period of record from 1976 to 1980 vary from 0 to 290,000 tons. The <br />latter was an extreme event which occurred on 7 September 1978, and <br />corresponds with the maximum discharge for Yellow Creek. The event was a <br />result of a high intensity, localized thunderstorm which occurred in the <br />lower portion of the basin (BLM 1983). <br />Some specific values for surface water quality of Yellow Creek are shown on <br />Table G-1. Site-specific samples on the lease area have not been taken, due <br />to the lack of perennial stream flow and absence of important water bodies <br />in the area. <br />G.2.3 Ground Water <br />G.2.3.1 Regional Sel:ting <br />C• The principal water bearing strata within the Piceance Basin are the Uinta <br />and Green River Formation (see Figure G-18); the Green River Formation being <br />the most important. The ground water system within the Piceance Basin is <br />highly fractured and extremely complex. For description purposes, the <br />aquifer system can be considered to be divided into Upper and Lower <br />Aquifers. These two systems are separated by a less permeable zone rich in <br />oil shale, known as l:he Mahogany Zone. Estimates of groundwater storage of <br />the basin range up to 25 million acre feet (IEC 1981). In addition to the <br />Upper and Lower Aquifers described above, an alluvial aquifer system also <br />exists. Each are described below. <br />G.2.3.2 Aquifer Systems <br />G.2.3.2.1 Alluvial P,quifer <br /> Although not a continuous regional aquifer, the alluvial deposits in the <br /> Piceance Creek Basin provide a significant source of groundwater, <br />• particularly in the region's major stream valleys. The alluvial deposits, <br />G-24 <br />