"So we have our concerns that she's slipped or fallen into the Young River, or while trying to cross an aspect of that at the upper end she's been swept away."
The river had been flowing much higher than usual at the weekend because of heavy rainfall.
It dropped significantly yesterday, which had helped search efforts, Mr Nicholson said.
Searchers confirmed a number of boot prints on both sides of the pass were those of the missing woman, who was not an experienced tramper.
"She had done a few local walking tracks but not a poled route over a significant pass in bad weather."
While crossing the pass, the woman had fallen behind her companions, who were also students at Otago University.
The pair continued on to Young Hut where they waited for their friend to arrive.
When she had not turned up the following morning, the women tramped to the start of the track and hitch-hiked to Makarora to phone police, roughly 24 hours after they were separated from the missing student.
An aerial search and check of the huts on the route was mounted that afternoon.
Yesterday, about 15 LandSAR Wanaka volunteers and an "air-scenting" wilderness dog carried out extensive ground searches, while others looked for traces of the woman from the air.
Mr Nicholson said the missing tramper had been studying in Dunedin as part of the International Studies Abroad programme.
He had been providing updates to the university and the missing woman's family in Ohio.
Her father is due to arrive in New Zealand today.
Otago University international pro-vice-chancellor Professor Helen Nicholson said the university was "obviously very concerned and worried and hoping for the best" and had been supporting the other students who were on the tramp.
An International Studies Abroad representative was in Wanaka with the women, who she understood were also study abroad students from the United States.
Mr Nicholson said the search for the tramper would continue today.
"We are always optimistic and hoping for a positive outcome. However, as time goes on and with some of the hazards around the rivers and bluffs in that area she had to negotiate ... we have to consider the possibility that we might not find her alive. But we're doing everything we can to find her."
The Department of Conservation website describes the Gillespie Pass Circuit as a 58km, three- to four-day round trip, suitable outside the winter months for relatively experienced parties.
