Eddies

The maximum potential area of eddy bars (MPAEB) represents the cumulative area of the eddy occupied by sand at different times within the photographic record (1935-2002). The MPAEB is the sum of all areas of eddy bars that have been observed to contain sand in all periods of record; thus, the sand bar need not have occupied all of this area at any given time. Therefore, the MPAEB represents a potential maximum extent for an eddy sand bar within the period of record, corresponding to the persistent eddy concept described by Schmidt et al. (1999). The main objective in monitoring sand bars is to describe changes in their areas over time. The simplest approach to meet this objective is to compare the area of each sand bar at different times. However, there are differences from eddy to eddy in each reach and there are differences in average eddy size from reach to reach (Schmidt et al. 2002). Thus, a reference state is required for a one to one comparison of eddies in different reaches at one time, and eddies in a given reach at different times. The development of MPAEBs allows us to normalize the areas of sand bars and report the areas of sand bars at a point in time as a ratio of an objectively-defined reference state. Thus, we can make statements about how relatively "full" or "empty" eddies in a reach are at a given time and how they compare to eddies in other reaches at the same time. Mapping of surficial deposits was the first step in generating these data. The classification used for the mapping was adopted from Hereford (1996), Schmidt and Leschin (1995), and Hereford et al. (2000). Surficial deposits were mapped using aerial photography acquired in the following years: 1935, 1951, 1952, 1965, 1973, 1984, 1990, 1996 (three series), 2000 (two series), and 2002. Based on their morphology and location within fan-eddy complexes, fine-grained deposits were also classified as channel-margin deposits, separation bars, reattachment bars. The total area of the eddy covered by sand in all photo-series was determined by overlaying coverages of all years of photography, and lumping all areas within eddies that were occupied by sand in any one photo-series. The calculation of this area was done using ARC/Info software. An AML (ARC Macro Language) script was used to create a new coverage containing polygons that were the total area covered by sand in eddy complexes, in all photo years available, using the "union" command. Arcs separating polygons of the same persistent eddy complex are then removed using the "dissolve" command to create a single polygon per eddy bar. The eddy complexes created were then assigned an arbitrary identification number for discerning them from each other. In cases where separate polygons were created by the AML because there was no overlap between the separation and reattachment bars within the same eddy complex, both polygons were assigned the same identification number.