2012-09-06_REVISION - M2008070 (22)

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of the White River. Alluvial fans consist mainly of clay, silt, and sand deposited on the sides of the White River valley at the mouths of smaller tributaries, draws, and gullies. In some areas, fans from adjacent tribu- taries have coalesced, creating a continuous deposit that mantles the White River alluvium for miles. Terrace alluvium exists at low levels adjacent to the White River and at higher levels some distance from the river. The ter- race deposits are remnants of former flood plains and are similar in composi- tion to White River alluvium. Material found at the base of cliffs or steep valley walls that has slumped or fallen to the flood plain, known as collu- vium, is found in significant quantities in the study area only near the Meeker dome. METHODS OF STUDY Several methods were used to acquire information for this report. Black- and -white aerial photographs at scales of 1:32,000 and color aerial photographs at scales of 1:12,000 were studied and annotated to delineate the extent of the valley -fill deposits. Geologic mapping was conducted in the field in some areas to check contacts plotted on the basis of aerial - photographic interpretation and to ascertain the hydraulic relationship between terrace alluvium and White River alluvium. Bulk samples of White River alluvium were collected at various sites and analyzed for grain -size distribution. An inventory of all known water wells, oil and gas wells, and test holes in the study area was compiled to provide information on ground- water quality, alluvial well yields, depth to bedrock, depth to water, and vertical lithologic changes within the alluvium. The inventory data are summarized in table 7 in the Supplemental Information section at the back of this report. Two methods of surface - geophysical investigation were conducted to provide information on depth to bedrock in areas where inventory data were incomplete or not available. Vertical electrical - resistivity soundings con- ducted at 50 sites throughout the study area yielded useful data; however, seismic - refraction techniques produced no usable results. A summary of the theory, field procedure, and methods of interpretation involved in the ver- tical electrical soundings is given in the Supplemental Information section, followed by the interpreted depth to bedrock for each of the soundings (tables 8 and 9 in the Supplemental Information section at the back of this report). Pumping tests were conducted at several sites to determine the hydraulic characteristics of certain areas of the alluvial aquifer and to obtain ground- water samples for chemical analysis of dissolved constituents. Procedures involved setting a submersible pump in the well to be tested, pumping the well at a nearly- constant rate for a specified period of time, and measuring water - level changes in the pumped well and in any nearby observation wells. The pumping rate was measured with an in -line flow meter and controlled with a gate valve in the discharge line. During some tests, pumping rate was checked by measuring the time to fill a bucket of known volume. At specified times after pumping began, water levels were measured with a steel tape or with an electric sounder. Temperature and specific conductance of the discharge water were measured throughout the test, and samples were sent to the laboratory for more detailed analysis. After the pump was shut off, the recovery of water levels in wells also was measured. 8