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gaging station, the effect is a change in the stage- discharge relation. Another effect is a change in the conveyance capacity of the stream in the affected reaches. Changes in conveyance can result in changes in water-surface elevations for floods of a given recurrence interval. Similarly, the performance of engineered structures, such as levees and bridges, is changed. METHODS OF DETERMINATION OF CHANNEL CHANGES The quantity of channel change was deter- mined using three methods. The first method used remeasured cross sections. Changes in cross- section geometry were examined over a period of years and during individual floods. The second method examined historic trends in the point-of- zero-flow (PZF) elevation. The PZF is the eleva- tion at which flow ceases to occur at a gaging station. Finally, the third method used was repeat photography. Repeat photographs were used to document qualitative changes in channel shape, vegetation, and sediment characteristics and to provide photographic evidence of channel changes. Site Selection For this study, 10 streamflow-gaging stations in and near Maricopa County, were selected. Criteria for site selection included known changes to channel geometry or characteristics, streamflow records that exceeded 10 years, several measure- ments of large discharge during the same flood, mobile channels, and previously monitored cross sections. Site Survey After site selection, existing cross-section dsta were examined including those from current-meter and indirect measurements of discharge. Current- meter measurements included wading, bridge, and cableway measurements. Cross-section geometry was determined from these measurements by sub- tracting the flow depth at locations along the cross section from the gage height at the time of the dis- charge measurement. At sites that had not been surveyed previously for indirect measurements of discharge, cross sections were surveyed for this study. On the Hassayampa River near Arlington, monumented cross sections surveyed by Parker (1995) were resurveyed. The depth measurements from current-meter measurements were used to determine channel changes during individual floods. The widespread floods of January-March 1993 rank among the largest in the history of discharge measurements in Maricopa County. Channel change during this per- iod was determined almost exclusively using data from current-meter measurements of discharge. A second period of smaller regional flooding occur- red in 1995, and data from selected floods during this period also were used to determine channel change. Long-term changes in channel geometry were determined using survey data from indirect meas- urements of discharge. Because the original pur- pose of these surveys was to determine discharge, rather than monitor channel change, cross-section locations often changed from one survey to an- other. The differences in location resulted in dif- ferences in elevation that needed to be taken into account before cross sections were compared for the purpose of measuring channel change. In these instances, the elevation data from one of the cross sections were adjusted by multiplying the distance of the cross section from a point common to both sections by the slope (in percent) of the stream in that reach. Because a key assumption of a slope- area computation is that the stream channel gener- ally is uniform, these modifications to the original measurement data were considered reasonable for this study of channel changes. Channel Geometry The historic cross-section data and data from cross-section surveys done for this study were compiled into a single data set (table I and tables 2-12 in the section entitled "Cross-Section Data" at the back of the report). From this data set, cross-section plots were created to display cross- section changes. Graphical representations of cross sections in this report are oriented so that the view is downstream and the left bank appears on the left side of the graph. 4 Determination of Channel Change for Selected Streams, Maricopa County, Arizona