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<br />18 <br /> <br />GIS Analysis of Aerial Photos Taken <br />Between 1938 and 1995 <br /> <br />Stereo air photos taken in 1938, 1952, 1962, 1985, and 1993 were <br /> <br />analyzed to determine the geomorphic changes that have occurred in a 26.4-km <br /> <br />section of the study reach over time and to determine if the style and magnitude <br /> <br />of narrowing at the cableway was representative of the entire study reach. <br /> <br />Total active-channel surface area of the Green River through the study reach <br /> <br /> <br />has decreased by approximately 14.5 percent since 1938, from 4.20 x 106 to <br /> <br /> <br />3.46 x 106 m2. This corresponds to a reduction in average channel width of <br /> <br />approximately 23.5 m. The decrease in channel width has not been continuous <br /> <br />over the last 66 yrs, but instead has been episodic, and the trends are similar to <br /> <br />those determined from discharge measurements at the present cableway <br /> <br />(compare Figs. 10a and b). Air photos do not provide a continuous temporal <br /> <br />record of channel change, but narrowing occurred between 1938 and 1952. <br /> <br />This was followed by overall width stability between approximately 1952 and <br /> <br />1962. High rates of channel narrowing occurred after dam closure in 1962. <br /> <br />Thus, channel changes measured at the cableway are representative of the <br /> <br />entire study reach. <br /> <br /> <br />GIS analysis also shows that many semipermanent island complexes <br /> <br /> <br />that existed in 1938 are now attached to the bank, and the secondary channels <br /> <br />that once surrounded these islands have become constricted and filled with <br /> <br />sediment (Fig. 13). Mapped surface area of secondary channels decreased by <br />