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. l I; \\ 7.2 ft on July 20, 1966, and the elevations of6.9 ft or above from March 15, 1966, to August 23, 1966, indicate that the channel bed was at roughly the same elevation as the white line for at least 5 months in 1966. Therefore, the line probably represents a fonner channel elevation. Repeat photography.-comparison of photographs taken before and after the flood of September 1970 show significant channel change at this site (figs. 14 A-C). On January 9, 1964 (fig. 14A), there was a stand of riparian trees along the right bank of the stream channel. At that time, the PZF was 4.62 ft. Figure 19B shows the channel in 1970 (exact date unknown) before a large flood that occurred on September 5, 1970. Note the ab- sence of the riparian trees along the right bank and the flatness of the channel in comparison to figure l4A. The two PZF measurements made in 1970 were 5.84 ft on March 11 and 6.12 ft on July 29; these are about 1 ft higher than the PZF at the time of the 1964 photograph. Also note the location of the top of the white stain on the rock in relation to the channel (fig. 14B), and compare this to the location of the white stain to the channel in figure 14C. These two photographs show that the channel has lowered about 4 ft in elevation from before and after the flood on September 5, 1970. This change in channel relative to the white stain is similar to the changes in the PZF from 6.12 ft on July 29, 1970, to 1.53 ft on March 18, 1971, and is a change of 4.59 ft. Thus, it seems that the large flood on September 5, 1970, caused the channel bottom to incise about 4 ft along this reach. The second set of repeat photographs were taken 2 days before and 4 days after the flood on September 26, 1997 (figs. 15A-15B). The photographs show the coarsening of the channel sediment immediately following prolonged flows, which occurred after the flood on September 26, 1997. This coarsening of channel sediment has been observed on several occasions at this site. 09516000 Hassayampa River at U.S. Highway 60, at Wickenburg, Arizona The Hassayampa River has a drainage area of about 711 mi2 at the crossing of U.S. Highway 60 in Wickenburg, Arizona (fig. 1). The channel is about 400 ft wide under the highway bridge. The channel sediment is primarily sand and is highly mobile. Flow velocities during large discharges are high, and standing waves are common. The USGS operated a crest-stage gaging station on the bridge in the 1960s and 1970s. Only maximum stage values were reported from this station. In 1994, the FCDMC installed a continuous streamflow-gaging station on the bridge. Cross sections.~ The earliest cross-section measurement (fig. 16) was taken from the Flood Insurance Study HEC-2 model of 1988 (Cella-Barr Associates, written commun., 1989). A second cross-section measurement was constructed from a current-meter measurement of discharge made from the bridge on September 26, 1997, at 3,800 ft3 Is on the recession limb of a flood. The peak discharge earlier that morning was estimated at 15,400 ft3/s at this site. A third cross section was surveyed on September 29, 1997. A comparison of the dry riverbed in 1988 with the riverbed on September 29, 1997, shows that the cross-sectional area increased by 21 percent relative to the reference elevation. The cross- sectional area measured during the flood on September 26, 1997, was 12 percent greater than the cross section of the post-flood channel. 09516500 Hassayampa River near Morristown, Arizona The gaging station, Hassayampa River near Morristown, has a drainage area of 796 mi2 and is at an elevation of 1,831 ft (fig. 1). The gaging station, which is attached to a bedrock cliff that overlooks the channel on the left bank, is about 15 mi downstream from the gaging station, Hassayampa River at Box Canyon, and 8 mi down- stream from the gaging station, Hassayampa River at U.S. Highway 60. Bedrock cliffs also are im- mediately downstream from the gaging station. Beyond these bedrock controls, the river enters the alluvial fill of the valley called the Hassayampa Plain. The river channel is flat, and narrows as it approaches the gaging station. Bed material is primarily sand. The narrowing increases stream velocity past the gaging station, and most high flows are near or at critical depth. The cross sec- tion was measured nine times at the site during and Results 21