Nerodia rhombifer rhombifer
Mingo Swamp, Wayne Co. Missouri. May 12, 1983.
Mingo Swamp in southeastern Missouri is a very good place to observe water snakes. I saw a number of diamondbacks there, almost always basking on half-submerged logs. In later years I encountered them in south Texas as they basked in the willows overhanging the Rio Grande. The top photo is a rhombifer from Mingo, and the close-up photo was taken in central Kansas.
HerpMapper records for this species are here.