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<br /> <br /> 70,000 <br />0 <br />z <br />0 <br />0 <br />w 60.000 <br />r/J <br />a: <br />w <br />a. <br />>- 50,000 <br />w <br />w <br />u. <br />0 <br />"' <br />::> 40.000 <br />0 <br />~ <br />ill <br />" 30,000 <br />a: <br /><( <br />I <br />0 <br />r/J <br />0 20,000 <br />" <br /><( <br />w <br />a. <br />..J 10.000 <br /><( <br />::> <br />z <br />z <br /><( <br /> 0 <br /> 1970 <br /> <br />~ Cross-sectional area <br /> <br />1975 <br /> <br />1980 <br /> <br />1985 <br />YEAR <br /> <br /> 1,000 <br /> 900 <br /> >- <br /> w <br />. 800 w <br />lL <br /> w <br /> a: <br /> 700 <( <br /> ::> <br /> 0 <br /> r/J <br /> 600 ~ <br /> <(- <br /> w <br /> 500 a: <br /> <( <br /> ..J <br /> <( <br /> 400 z <br /> 0 <br /> ;:: <br /> 0 <br /> 300 w <br /> r/J <br /> U, <br /> r/J <br /> 200 0 <br /> a: <br /> 0 <br /> 100 <br /> 0 <br />1990 1995 1997 <br /> <br />Figure 4. Annual peak discharge and cross-sectional area from surveys during indirect measurements of <br />discharge at cross-section 2, Agua Fria River near Rock Springs, Arizona, 1970-97. <br /> <br />of 9,940 ft3/s was measured using a current-meter <br />on February 20, before the peak discharge. This <br />cross section shows that some scour was occurring <br />again in the channel. On February 21,3,160 ft3/s <br />was measured on the recession of the flow. At that <br />time, the channel had filled to a level higher than <br />the level measured on February 10. <br />Surveyed cross sections.-When the <br />gaging station was established at its present <br />location in 1975, one cross section at the gaging <br />station (cross-section 2) was surveyed for rating- <br />curve development. In January 1979 after the large <br />flood on December 18, 1978, four cross sections <br />(1-4) were surveyed as part of an indirect meas- <br />urement of discharge (fig. 5). Cross-section 1 was <br />390 ft upstream from the gaging station, and cross- <br />sections 3 and 4 were 315 and 800 ft, respectively, <br />downstream from the gaging station. Cross-section <br />2 showed substantial fill in the channel at the <br />streamflow-gaging station after resurvey of the <br />cross section in 1979. <br />Following the large floods of 1993, cross- <br />sections 1-4 were resurveyed for another dis- <br />charge computation (fig. 5). The 1993 survey <br />showed that the channel incised by as much as 4 ft <br /> <br />and by varying amounts in the overbank area at all <br />four cross sections. In 1997, cross-section 2 was <br />resurveyed. Since 1993, the only substantial <br />change to cross-section 2 occurred along the 10w- <br />flow channel where some fill occurred. Channel <br />change is common along the reach near the gaging <br />station. Figure 4 shows the annual peak discharge <br />and the cross-sectional area relative to the <br />reference elevation at cross-section 2. <br /> <br />PZF analysis.-The PZF at the Agua Fria <br />gaging station changed by several feet over a per- <br />iod of a few decades. A correlation, however, <br />could not be determined between PZF and annual <br />peak discharge at this site (fig. 6). The highest PZF <br />of5.6 ft was measured in July 1979 when the <br />channel filled with sediment after a large flood on <br />December 18, 1978. The large floods that occurred <br />in 1978-80 were followed by 10 years oflow flow <br />in which no flood exceeded the 10-year recurrence <br />interval, and only two floods exceeded the 2-year <br />recurrence interval. By 1988, the PZF had decreas- <br />ed in elevation to 3.29 ft. Since 1989, only a slight <br />additional decrease in the P ZF has occurred de- <br />spite three floods with peak instantaneous dis- <br /> <br />10 Determination of Channel Change for Selected Streams, Maricopa County, Arizona <br />