USGS, GCA, and American Whitewater gauges have some discrepancy. The graphs below were all taken at the same time on January 5, 2011 for the Upper Chattahoochee River.
The underlying data from USGS is captured here in PDF format.
From the picture above, you can see that the GCA gauge tracks the USGS perfectly for the current level. However, the historical level is broke.
The American Whitewater gauge is doing something different that I can’t explain.