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SUMMARY This report has presented channel cross- section changes over short and long periods of time. Most of the sites show changes in channel geometry spanning both time scales. Point-of- zero-flow data also suggest changes in channel geometry over time. Short-Term Channel Change AIIuvial stream channels in arid regions are dynamic, and channel changes can occur over short time periods ranging from hours to weeks. A channel that scours during increasing discharge and fiIls during decreasing discharge is an example of a site experiencing short-term channel change. Results of the examination of short-term channel change from current-meter measurements indicate that some reaches are more dynamic than others. Several gaging stations that are in dynamic reaches include the Agua Fria River near Rock Springs; Hassayampa River at D.S Highway 60, at Wickenburg; Gila River at EstreIla Parkway, near Goodyear; and Verde River below Bartlett Dam. Current-meter measurements made at the Agua Fria River near Rock Springs from January to February of 1993 show that several feet of scour and fiIl can occur during changes in discharge. Channel changes were observed at two sites on larger rivers, Gila River at Estrella Parkway, near Goodyear, and Verde River below Bartlett Dam. Most of the short-term channel change at these two sites occurred in the low-flow channel area. The cross-sectional area and hydraulic radius at the Gila River site increased between flows. Long-Term Channel Change Long-term channel change was measured in time scales ranging from years to decades. These channel changes were determined by cross-section surveys and usually represent general scour and fill of the stream channel reach. Short-term change is the type that occurs during an individual flood. Long-term channel change represents lasting changes that would have more of an effect on flood-hazard regulation. Most sites in this study had substantiallong-term channel change. Three sites appeared to have substantial 10ng- term channel change. For example, Hassayampa River at Box Canyon, near Wickenburg, scoured as much as 6 ft at the gaging station during the peak-of-record flood in 1970. This scour extended for a reach several hundred feet upstream and downstream from the gaging station, and the streambed at the gaging station did not recover to within I ft of the preflood elevation for 3 years. Conversely, a large flood in 1979 at Agua Fria River near Rock Springs caused several feet of aggradation at the control reach of the gaging sta- tion. The cross sections measured at the Hassayampa River near Arlington also show sev- eral feet of fill from 1992 to 1996, during which only two flows occurred at this site. At Waterman Wash near Buckeye, as much as 2 ft of vertical change occurred between surveys and 3.5 ft occurred from 1964 to 1982. Because base flow does not occur at this site, this change and changes observed over the period of record can be attributed to individual floods. The fmal site that showed long-term channel change was Tiger Wash near Aguila. Like stream- flow at the Waterman Wash gaging station, streamflow at Tiger Wash at Aguila is ephemeral, and all channel change is caused by the few floods that occur at this site. Between 1972 and 1996, the bed elevation decreased by about I ft. Further incision into the channel followed the peak-of- record flood in 1997. Point-of-Zero-Flow Method The PZF method, which is the examination of the changes in the PZF through time and in relation to annual peak discharge, was developed for this study. Although the PZF method has limitations for examining channel change, this method may provide useful information under certain circumstances. The PZF method is most useful for streams in which channel control exists throughout most of the range in discharge. The gaging stations Hassayampa River at Box Canyon, near Wickenburg, and Hassayampa River near Summary 41