Santee Police believe 38-year-old Arturo Rouillard, and his girlfriend, 41-year-old Adrienne Denney, were night fishing on an ATV, when they fell off a cliff near Crazy Peak, and crashed in to the river.
Denney's body was recovered on Saturday, but Rouillard's body has yet to be found.
They've been searching for for three days.
Santee Police say a fisherman discovered Denney's body near a 75-foot tall cliff and called law enforcement.
After she was identified by police, Denney's body was sent to Omaha for an autopsy.
Authorities are awaiting results.
Denney's family is holding a traditional funeral service for her through Thursday.
Santee Police, Santee Nation EMS, Yankton County Search and Rescue, and Bloomfield and Bureau of Indian Affairs firefighters are aiding in the search for Rouillard's body.
Santee Nation EMS officials say the search area is about 20 square miles downstream from the believed entry point.
They say it's now a recovery operation.
"Where's my son? I need my son," said Arturo Rouillard's mother, Laura Rouillard. "I need my son to come home. I need the river to give him back to me. He calls me in the afternoon, 'How are you doing, Mom? What are you doing?' Calls me at night, 'Good night, Mom. I love you,' I haven't had that call. I just need him home."
Santee Police say plenty of residents night fish at the Crazy Peak, but this is the first time anyone has fell into the river.
They say the couple was last heard laughing atop the peak in the early hours of Saturday morning, before, police believe they accidentally drove off the edge of a cliff.
"We'll just keep searching," said Santee Police Chief, Rob Henry. "We'll just keep searching until he's found."
Police say the river's current is dangerously swift, limiting the dive team's search.
Rouillard's family members believe it's due to the Fort Randall Dam opening into the river.
Laura Rouillard says authorities should use every resource available to find her son.
Adrienne Denney leaves behind five children.
Rouillard and Denney were together for five years.