"I obviously don't know the full circumstances, all I know is that one of the guys is missing.
Seventeen other squad members from two coxed eights were rescued, but 21-year-old Leo Blockley, from Lancashire, disappeared after swimming for the bank.
The two Oxford University Lightweight Rowing Club boats were flooded after being hit by sudden gusts on the River Ebro at Amposta, near Barcelona, Spain.
The missing man's sister Rebecca, 23, a student nurse, said from the family home: "We are all extremely worried, we cannot take it in. We are all stunned.
"Leo has been involved with rowing for the last couple of years.
"He went to Cambridge first where he got a 2:1 in maths, then went to Oxford to study for a Master's in applied statistics.
"It was at Cambridge where he took up rowing.
"He is very, very fit and very dedicated and very good at things like that. He is very, very bright and has always done well."
Annual training
Rescuers from the Spanish fire brigade, Red Cross and Civil Guard said they would not give up hope and are resuming their search on Saturday morning.
Steve Royle, the director of rowing at Oxford University Boat Club, said the local boating club had assessed the conditions as safe.
"The tragedy is that Leo swam to the bank and got into difficulties," he said.
"These guys are pretty shell-shocked as you can imagine. It's a close community, they go through a lot together."
A Foreign Office spokesman said it had not yet been established if the vessels collided or whether bad weather conditions were the cause.
Redgrave shocked
The rowing team were on their annual training camp in the Spanish town in preparation for fielding two crews against Cambridge in March.
Rowing club president Jeremy Fagan, speaking from Spain, said everyone was distraught over the fate of the Lincoln College student.
"All other members of the party - rowers, coxes and coaches - are safe and well, but upset by today's events. Our thoughts at this time are very much with Leo's family."
Britain's five time gold medal winning rower Steve Redgrave has expressed shock at the news of the accident.
"I obviously don't know the full circumstances, all I know is that one of the guys is missing.
"If the reports are true and someone is presumed drowned then that is devastating. I feel for his parents and rowing colleagues, you never want news like that," he said.
The OULRC is a separate club from Oxford University's Boat Club and the Blues Eight, which competes in the Varsity Boat Race.
Their rowers, who must weigh under 11 stone, take on a Cambridge lightweight crew at Henley in March.
An Oxford University spokeswoman said it was not known when the squad would fly home from Spain.