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3. FIELD DATA COLLECTION AND ANALYSIS 3.1. Mathers Hole Field data were collected at Mathers Hole (RM 17.5) between August 26 and 29, 1993 when the discharge in the river was about 300 cfs. Fourteen cross sections were surveyed, using a local datum, through the project reach and considerable additional topographic data were obtained for the entire bar (Figure 3.1). Cross section profiles are presented in Appendix B. Water-surface elevations at the time of the survey were determined for subsequent calibration of the hydraulic model, as were elevations of the 1993 high-water marks (peak discharge 17,600 cfs) and high-water marks for the 1984 flood of record (peak discharge 32,300 cfs). The entire bar was not submerged in 1993. The elevations of slackwater flood deposits (paleofloods) located above the 1984 high-water marks in very coarse colluvial deposits on the left valley wall between cross sections 4 and 5 were also established. The slackwater deposits were located between 4 feet (Elevation 108 ft) and 9 feet (Elevation 113 ft) above the 1984 high-water marks (Elevation 104ft). Using critical depth assumptions at the downstream boundary, HEC-2 modeling indicated that the magnitude of the flood associated with the lower of the paleoflood deposits was on the order of 60,000 cfs and that associated with the upper paleoflood deposits was on the order of 75,000 cfs. These order of magnitude estimates of the paleoflood discharges are preliminary estimates at best since no other high-water mark data are available to develop a water-surface profile for calibration purposes. Eleven Wolman Counts were conducted on surface sediments and six pits were dug to permit bar stratigraphy to be observed and enable fIVe subsurface sediment samples to be collected. The locations of the Wolman Counts, the pits and the sediment samples sites are shown on Figure 3.1. The locations of two Wolman Counts made in 1991 are also shown on Figure 3.1. Clast orientation and imbrication angle measurements were made on the bar surface at various locations. A discharge measurement was made at about cross section 12. A sample of thixotropic flocculated mud was retrieved from the water's edge at cross section 4 (FM1). A cold chisel and hammer were required to obtain a sample of cemented gravels that capped a flat bench on the left margin of the left hand chute channel in the vicinity of cross section 6 (81). The cemented gravels overlaid unconsolidated and uncemented sands and gravels of recent origin. 3.1 Resource Consultants & Engineers, Inc.