20"-75" long, a dark, heavy-bodied water snake that is olive, brown, or black above, patternless or with serrated-edged dark crossbands.  Flattened head.  Found throughout the southeast and Gulf Coast states and along the Mississippi drainage, typically along aquatic habitats from lowland swamps to mountain streams.  When annoyed, tends to stand its ground and expose its cotton-white mouth; may also wag its tail vigorously.  Unlike the colubrid water snakes, cottonmouths swim with their head above water (colubrids often swim below).  Most active at night, feeding on sirens, frogs, fishes, snakes, and birds.  Also called water moccasin.  Young have a green tail tip which they use to attract prey.