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Cresson Pro'ect H dro • eochemist AdrianBrown <br />2.2 Geology <br />The District is located in the Diatreme; a Tertiary -age volcanic caldera filled with phonolites and welded <br />hydrothermal breccia, surrounded by Precambrian granite, granodiorite, schist, and gneiss (Plate 3). <br />The Diatreme is a breccia - filled volcanic pipe that was formed by volcanic activity at three eruptive <br />centers. The volcanic inclusion extends to great depth, narrowing with a subsurface shape identified by <br />drilling and geophysical evaluation as shown in Plate 4. <br />2.3 Surface Water <br />Surface water flow is monitored at a number of locations in the District'. The monitoring indicates that <br />little water runs off from the Diatreme footprint in any of the streams that flow from the District. <br />The principal surface water outflow that exits from the Diatreme occurs to the east from the footprint of <br />Grassy Valley. The flow in Grassy Valley has been monitored continuously at three monitor locations <br />since mid - 2009 The results of the average daily flow measurements for 2010 are presented in Plate 6, <br />together with the daily precipitation. The following conclusions are drawn: <br />1. There is no runoff during mid - October through mid -April, representing the period of the year <br />when the average daily temperature is below freezing. <br />2. The creek in Grassy Valley flows briefly during spring snowmelt, which occurred in 2010 in <br />mid -April. Flows were modest and short- lived, totaling 2.6 acre feet, the equivalent of 0.028 <br />inches of precipitation over the 1,124 acre catchment area. The snowmelt volume is <br />approximated to be: <br />Spring snowmelt runoff (acre feet) = 0.00232 * Catchment area (acre) <br />3. During the balance of the year, where the average daily temperature is above freezing, runoff is <br />dependent on precipitation, generally occurring when precipitation exceeds 0.24 inches in any <br />' The results of the flow and quality monitoring in the District are presented in the Cresson Project MLE2, Hydrologic Evaluation, Volume <br />II, Appendix 2. <br />2 The flow was monitored by hand on a periodic basis (approximately monthly) in Grassy Valley at five locations from 1997 through the <br />present. The results were reported in the MLE Hydrogeochemical Report (ABC, 2008). However, the monitoring did not provide an <br />accurate measure of the actual streamflow, because the flow was highly intermittent, with the stream not flowing for much of the year <br />except when the average daily temperature is above zero, and there is precipitation of more than approximately 0.24 inches within the <br />prior two days, or the small spring snowmelt is occurring, typically over a few days in mid - April. Further, flow readings are typically <br />concentrated on periods of precipitation or snowmelt, resulting in an over - estimate of the overall flow from the basin. Continuous <br />monitoring was initiated in September 2009, and has been maintained since. The continuous monitoring of the flow in streams at this <br />elevation presents substantial challenges, due to freezing of the stilling well attached to the flume from mid - October through mid -April, <br />and erosion around and siltation within the Parshall flumes during the remainder of the year due to the storm -driven flash -flood nature of <br />much of the flow. As a result, a substantially complete record has only been obtained at one location (GV -03) for one calendar year <br />(2010). These data are used for the evaluation reported herein. <br />1385L.20120125 4 <br />