St. Louis District U.S. Army Corps of Engineers Natural Resources Specialist Angie Smith explains macroinvertebrate sampling at the 2013 St. Louis Earth Day Festival in St. Louis, Mo., Apr. 21, 2013. Macroinvertebrates are useful indicators of the health of rivers, lakes and streams. (U.S. Army Corps of Engineers photo by Public Affairs Specialist Romanda Walker)

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Photo by: Romanda Walker |  VIRIN: 130421-A-HT646-001.JPG