This is Eleocharis obtusa in the family cyperaceae:
This particular plant was photographed at LeFleur's Bluff state park on May 11, 2010. This plant is a native of the Contiguous 48 States, Canada and Hawaii. It isn't a large plant, rather, it has numerous stalks each topped with the flower picture above. The stalks radiate from a common base that, for this specimen, was obscured by the shallow water/mud it was growing in.