U.S. Geological Survey Scientists injected a harmless bright red fluorescent dye into a stretch of the Chicago Sanitary and Ship Canal on Tuesday, November 10, 2009. According to the USGS, "the dye study is aimed at obtaining information on the dispersion and travel times of waterborne contaminants in the canal and characterizing leakage to adjacent water bodies such as the Des Plaines River and I & M Canal. Such information is used by Federal, state, and local agencies for various engineering applications, especially water-quality monitoring and control and invasive species management." One such invasive species of concern is the Asian Carp, which pose a significant threat to the the aquatic communities and fisheries in the Great Lakes.
For more information read the USGS news release and their Red Dye Studies FAQ.