A helicopter search of the Young River in the Mt Aspiring National Park yesterday failed to find a 20-year-old American tramper missing since Saturday.
Wanaka police search and rescue co-ordinator Sergeant Aaron Nicholson said yesterday the river had been searched ''in some detail'' from Lake Wanaka, up the Makarora River, and to the headwaters of the Young River.
''As at this time we have had no other sightings, no other clues, no other items of property found.''
On Monday, searchers found a raincoat and pack believed to be the missing tramper's in the upper end of the Young River, and a sweatshirt, thermal top and sock near the river.
Sgt Nicholson said the expectation was the woman ended up in the river.
The weather in the area had improved, but there was a ''fair bit of flow'' in the river, he said.
The young woman's father arrived in Wanaka from Ohio yesterday and had been briefed on the search.
The missing woman and two companions, understood to be two female students also from the United States, were on a tramping trip up the Wilkin Valley, across the Gillespie Pass and down the Young Valley.
They became separated after entering the Young Valley.
Asked why that occurred, Sgt Nicholson said he understood the ''very severe'' weather conditions meant they wanted to get off the pass into the lower country as quickly as possible, which they had done.
''In terms of why they weren't together, we definitely know that the main reason was ... levels of fitness and they're trying to escape that appalling weather.''
All three were students at the University of Otago understood to be part of the American International Studies Abroad programme.
