Laserfiche WebLink
METHODS <br />Historical Changes in Channel Morphology <br />Changes in the morphology of the river channel within the study reaches were analyzed <br />using aerial photographs flown in 1937, 1954, 1968, and 1986. These photographs range in scale <br />from 1:20,000 to 1:24,000. The 1937 and 1986 photo sets were taken when the river was flowing <br />at a discharge of 12,000 cubic feet per second (cfs); the 1954 and 1968 photo sets were taken with <br />the river flowing at 2,500 cfs. The outlines of specific features such as channel banks, islands, <br />emergent bars, and backwaters were digitized on the photographs using a computer aided design <br />system (AutoCAD). Figure 4 shows an example of how these features were differentiated. <br />N islands <br />,aters <br />Figure 4. Differentiation of channel features. <br />Next, the different sets of photographs were adjusted to a common scale. To do this, four <br />or five common points were found on each photograph and on a USGS 1:24,000 topographic <br />map; these points were typically road intersections. The UTM coordinates of these points were <br />then stored as a separate data file. The channel-feature files and reference-coordinate files were <br />then exported to ARC INFO, a vector-based Geographic Information System (GIS), where they <br />were merged. Subroutines within ARC INFO were used to-calculate the area of the main channel, <br />islands and backwaters and to compare changes from one photo set to another. Approximate error <br />estimates are f2% for the water and island areas and ±10% for the side channel/backwater areas. <br />5