Thanks for posting this. This made me think of some questions -- I realize you may not have all the answers. Would the Chattahoochee be very cold in February in Atlanta? Is it fed by snow melt from the Blue Ridge Mountains? I don't know much about the weather in those mountains or about the sources of the Chattahoochee.
How strong the current of the Chattahoochee is in Atlanta? How deep is the river there?
Another question is whether Cunningham knew how to swim. I gather you can drown yourself even if you know how to swim. You can weigh yourself down with stones as Virginia Woolf did, or you can try to do other things to get around your natural survival instinct, but it may take some doing.
If he did know how to swim, being overweight could make it harder to drown himself because his body would be more buoyant and hypothermia would take longer. Being weighed down by wet clothes, however, might hurry the process. Of course, if he didn't know how to swim, that would make it an effective method of suicide, if that was his goal.
One way people apparently commit suicide by jumping off bridges into rivers is if the fall from the bridge is so great that it knocks you unconscious. Looking at Google Street View, it doesn't look like the bridge across the Chattahoochee at Marietta Blvd NW is high enough to reliably knock one unconscious. The bridge at the James Jackson Parkway has curved metal fences that would prevent one from jumping off. I don't know about the railway bridge because it doesn't have Street View.
But if the Chattahoochee in that area was cold and swiftly flowing at that time, suicide by drowning/hypothermia might have been easier. (Steep banks might also make suicide more possible, but not sure the Chattahoochee has steep banks there.)
Not to say that the body is TC's, or even if it is, that he committed suicide -- just thinking about how doable that would be. JMO