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beds. Farther downstream near Wickenburg, the Hassayampa River enters the Sonoran Desert where the channel is wider, has more sand, and is less steep. Apart from runoff, the Hassayampa River is dry most of the year except for a short reach in Box Canyon, a reach downstream from Wickenburg, and a reach with irrigation return flow near Arlington. The flow in these three areas occurs for only a few miles before the water infil- trates into the sandy streambed or, near Arlington, enters the Gila River. Floods generally are short in duration, although smaller flows (in the hundreds of cubic feet per second) can be sustained for several weeks after winter and spring floods in the watershed upstream from Morristown. This sus- tained flow, however, normally does not reach the gaging station, Hassayampa River near Arlington, because the flow infiltrates the sandy bed down- stream from Morristown. 09515500 Hassayampa River at Box Canyon, near Wickenburg, Arizona The gaging station at Box Canyon, near Wickenburg (sometimes referred to as Box Dam site), is about 8 mi upstream from Wickenburg (fig. 1). The USGS operated the gaging station at the present location from May 1, 1946, to Septem- ber 30, 1982, and the FCDMC began operating the station in 1993. Drainage area at the gaging station is 417 mi2. The channel width past the gaging station is confmed to 80 ft by nearly vertical rock cliffs that are about 50 ft high. The reach generally is straight from 1,800 ft upstream from the gaging station to 2,700 ft downstream. The channel is about 40 ft wide upstream from the gaging station and widens to 100 ft immediately downstream from the gaging station. The streambed is sand and gravel, and bed-sediment size varies in response to flooding. When the bed elevation is low, often following an extreme flood or prolonged moderate flow, the bed appears as predominantly coarse gravel. At higher bed elevations, the bed sediments are composed primarily of medium to coarse sand. Cross sections.-Nine cross sections were surveyed in Box Canyon. Cross-section 1 is at the streamflow-gaging station and cross-section 3 is under the cableway about 200 ft downstream. The first measured cross section-cross-section 3-was measured in 1946 (fig. 11). The other eight cross sections- 1, 2, and 4-9-were established in 1995 and resurveyed in 1996 and 1997. The channel of the Hassayampa River at Box Canyon generally has lowered through time, and the left half of the channel is about 4 ft lower on average than it was 50 years ago (fig. 11 ). Be- tween May 1995 and August 1996, the central and deepest part of the channel filled 2 to 3 ft although the left and right margins did not change signifi- cantly. Other cross sections throughout the reach show similar responses over short periods (fig, 11). PZF analysis.- The PZF elevation changed significantly at the Hassayampa River at Box Canyon, near Wickenburg, as shown by PZF measurements and annual peak discharge (fig. 12). The PZF elevation ranged from 0.85 ft in October 1951 to 7.2 ft in July 1966. The greatest changes in the PZF occurred in relation to large floods, such as in 1951 and 1970 (fig. 12), although the direction of change was not consistent. For example, following a large flood in 1951, the PZF increased more than 4 ft; however, following the peak of record in 1970, the PZF decreased more than 4.5 ft. In addition to the large, single-event changes in the PZF, there seem to be longer-term patterns of change at this site. For example, the PZF generally decreased from the highest PZF measured in 1966 until the peak flood of record in 1970 (fig. 12). Following several years oflow PZF measurements immediately following the large flood, the channel filled, and by 1974 the PZF increased to within 0.5 ft of the PZF before the flood of 1970. After 1975, the PZF again decreased at about the same rate as before the flood of 1970. Longitudinal profile.-Four longitudinal profiles were made of the reach near the gaging station (fig. 13). The three profiles measured between May 1995 and March 1997 generally show parallel slopes and vertical shift of about 0.5 ft. An examination of historic photographs and field observations revealed a distinct white line along the canyon walls in Box Canyon, especially upstream from the gaging station. The fourth plot- ted profile, therefore, is that of a white stain line that generally parallels the surveyed channel pro- files (as plotted in relation to the reference eleva- tion on fig. 13). This white line appears in the same position on the canyon walls in 1998 as it did in photographs of the gaging station taken in 1970. The white line is found at a gage height of 6.9 ft at the gaging station (fig. 12). The PZF elevation of I 16 Determination of Channel Change for Selected Streams, Maricopa County, Arizona