Several witnesses heard the Chicago woman calling for help near the Orleans Street Bridge around 1 a.m.
Only her head was above the surface, they said.
"Got in the middle of the bridge, and there she is right in the middle of the water, saying, 'Help! Help!' I ran to this thing over here to try to get the life tube. We couldn't get it open. I whacked it and it broke. Thank God it didn't cut it," Kevin Fryer, a copy editor at the Chicago Sun-Times, said.
He was leaving work when he heard people calling for help. He said the woman made no effort to get to the life preserver they threw to her.
The Chicago Fire Department rescue diver Jamie Delgado swam to the woman and brought her to shore, where other rescuers were waiting.
"Thank God they came here because I don't think she had much left," Fryer said. "The fire department came and the guy grabbed her. It was scary. Yeah. Crazy night."
The woman was taken to Northwestern Memorial Hospital, where she is listed in good condition.
Delgado said the woman was cold when he got to her and estimated the temperature of the water is about 50 degrees.