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, 5 . Aerial Photo2raDh I GIS Analvsis Aerial photographs are an excellent source of data for studying historic changes in river morphology and fish habitat. These changes can be measured with a Geographic Information System (GIS) and increases or decreases in the features indicating fish habitat can be quantified. , , GIS Methodology Aerial photographs were chosen from data in the U. S. Geologic Survey (USGS) Aerial Photograph Summary Record System (APSRS), a catalogue of the aerial photographs taken for government agencies. Two factors were considered when choosing the best aerial photographs. First, in order for adequate detail to be detected, a scale of 1:24,000 or larger was required. Second, the photographs needed to be taken at similar flows since the morphology of the river and the amount of fish habitat changes with discharge. Appropriate photographs were found for comparisons between 1937 and 1986, as well as between 1954 and 1968. The 1937 photographs have a scale of 1 :20,000 and the discharge at the Cameo gauge, upstream of the site, was 210 m3/ s (7,400 ft3/ s). The 1986 photographs have a scale of 1:24,000 and the discharge at the Cameo gauge was 196 m3/ s (6,915 ft3/s). The 1954 and 1968 photographs both have a scale of 1:20,000. In 1954 the river had a discharge of 54 m3 / 5(1,900 ft3 / s) at Cameo while in 1968 the discharge was 60 m3/ s (2,130 ft3/ s). All photographs are black and white and cover both the 15 and 18 mile reach of the Colorado River. Important features on these photographs were digitized using Auto CAD (A CAD) and a digitizing tablet with a precision of 0.001 inch. A hand- held puck with cross hairs was used for data input of the boundaries of the features. The input of points for a given length of channel varied with the It fa ~ . . . .