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Stage and Discharge Measurements in the Lees Ferry Reach The modern Lees Ferry Gage is located on the left (east) bank (figs, 1 B-C), just upstream from the gravel bar at the mouth of the Paria River. It is housed in a concrete structure adjacent to the old dugway road to the ferry crossing, near the base of a cliff within the Shinarump Conglomerate member of the Chinle Fonnation (fig, 2). Since December 1966, all discharge measurements have been made from the Modern Cableway located 50 ft upstream from the Lees Ferry Gage (fig.1C), Prior to this date, discharge measurements were made from two eableways fmther upstream. Most measurements were made from the eableway located I mile upstream and infOlmally called the Upper Cableway (figs. IC, 3A-B). At discharges exceeding about 60,000 ft3/s, discharge measurements were typically made from a second cableway located 0.4 miles upstream from the gage house. This second eableway crossed the river at a much wider cross-section and was informally called the Lower Cableway (figs. IC and 3C-D). The recording Lees Ferry Gage became operational on Janumy 19, 1923. Prior to this date, stage was read twice daily on vm'ious staff gages in the I-mile-Iong reach upstream (fig. IC), These historical staff gages were known as the LaRue Gage, the Number I Gage, the Number 2 Gage, the Number 3 Gage, the Number 4 Gage, and the Dugway Gage. Each of these staff gages had a different period of record and datum. The LaRue Gage was installed on May 8, 1921, and was destroyed during the peak ofthe June 1921 flood; the Number I Gage was installed at the site ofthe LaRue Gage on June 24, 1921, but at a different datum; the Number 4 Gage was installed upstream on the left bank on August 3, 1921; and the Dugway Gage was installed on August 5, 1921, near the future location and at the same datum as the Lees Feny Gage, The Number 2 and 3 Gages were used only during June-August 1921. Another staff gage, known as the Cable Gage, was installed on the left bank under the Upper Cableway in April 1924, Stage was read on this gage during most of the Upper Cableway discharge measurements until 1964, In ~eh 1941, a final staff gage, herein referred to as the "LoWer Staff Gage," was installed on the right bank of the Colorado River 1.5 miles downstream from the Lees Ferry Gage, below the mouth of the Paria River (fig, IB). Stage was read on the Lower Staff Gage periodically until 1959, The important events in the history of the Lees Ferry gaging station are listed in Appendix B, The Hydraulic Control and its Effect on the Shape of the Stage-Discharge Rating Curve The location and geometry of the hydraulic control for the Lees Felry Gage changes as a function of stage. During large tloods, these changes cause the water-surface profile in the reach downstream from the gage to flatten with increasing stage (fig, 4A), During the largest tloods, this flattening of the water-surface protile (that is, the development of back watered flow conditions) extends from some unknown point downstream in an upstream direction past the Lees Ferry Gage. This phenomenon produces a reversal in the curvature ofthe stage-discharge rating curve at a stage of about IS ft (figs, 4B-C); above this stage, stage increases at a progressively faster rate than does discharge. This reversal in the curvature of the stage-discharge rating curve has not always been explicit! y considered in estimations of the peak discharges of Im'ge Hoods nor in estimations of paleoflood discharges (O'Connor and others, 1994). The existence of backwatered flow conditions near Lees Felry at high stage is evident not only in the stage measurements at the staff gages, but also in photographs of the reach at higher discharge (ligs, 5 and 6), At stages lower than about 10 ft (that is, discharges less than about 20,000 ft3/s), the gravel bar atthe mouth of the Paria River confines the flow to a channel adjacent to the left bank, and the consttiction at the entrance to this channel forms a stable hydraulic control for the Lees Feny Gage (fig, 5B). Under these conditions, a riffle exists in the left-bank channel, and a second riffle, called the Pm'ia Riffle by modern river runners (Stevens, 1983), exists at a debris fan downstream from the mouth of the Paria River (figs. lB, 4A, and 5B). As the stage increases above 10 ft, these rimes begin to "wash out" as the overall water- surface slope flattens (figs, 6A-B). At stages between 13 and IS ft (that is, discharges between about 40,000 and 60,000 ft3/s), the flow divides into two or more channels across the gravel bar (Jig, SA), and the entire gravel bar forms the hydraulic control for the gage, Due to this change in the location and geumetry of the hydraulic control, the water surface flattens considerably in the 0.3- mile-long reach immediately downstream from the gage (figs, 4A and 6B); and at a stage of about 15 ft, the curvature of the stage-discharge rating curve reverses (fig, 4C).ln contrast to the behavior of the water-surface slope in this reach, the water-surface slope in the reach upstream fi'om the gage generally increases with increasing stage, and begins to flatten only at stages between 20 and 25 ft (figs. 4A, 6C-E), 8 Computatiol18nd Analysis of the Instantaneous-Discharge Record forthe Colorado River at Lees Ferry, Arizona-May 8. 1921, through September 30. 2000