The woman was last seen near this spot on the River Usk in Brecon on Saturday evening
Rescue teams are searching for a 21-year-old woman, who fell into the River Usk in Powys after crossing a bridge.
A police helicopter, rescue boats and a mountain rescue team have joined the search of a mile-long stretch.
The local woman was reported missing at Watergate bridge in Brecon at 1900 GMT on Saturday by a male friend.
Police are trying to establish how the woman ended up in the river but said the banks are muddy and "very slippery" and urged the public to stay away.
Matt Murray reports from the search area, and Insp Mark Davies explains how the rescue is progressing.
Mid and West Wales Fire and Rescue service are using two rescue boats, a rescue dog and patrol team in the search.
Inspector Alun Samuel, of Dyfed-Powys Police, said: "We are running a full search of the river in Brecon.
"A member of the public called the police when they saw what happened.
"It is a very serious search with dog handlers, specialist units and there have been helicopters."
Police say they are still trying find out how she got into water and are talking to a male witness.
She lives nearby and it is understood she was walking across the pedestrian bridge when the incident happened.
BBC Radio Wales reporter Hefina Rendle said it is believed she fell into the river, after crossing a bridge.
The section near where she was last seen has railings apart from a gap of about six feet (1.83m).
Inspector Mark Davies said the Dyfed-Powys force helicopter left its Carmarthen base at 1030 GMT on Sunday to rejoin the search, which is concentrating on the river banks within a mile of where the woman
The woman was reported missing near a bridge in Brecon, Powys
Insp Davies said: "We are investigating the full circumstances of why the female was in the river and are concentrating all efforts on trying to find her.
"The helicopter and police search dogs are involved as well as appliances and assistance from other agencies."
Police have had a number of calls from the public asking if they can help in the search but they say its too dangerous and they are asking them to stay away.
"The conditions of the river bank are very slippery and muddy," said Insp Davies.
"The last thing we would want is for someone else to end up in the river. That could make the situation even more difficult."
Mid and West Wales Fire and Rescue said they have sent patrol units, two pumping appliances and two rescue boats.
A search and rescue team on the River Usk in Brecon
The river was too swollen for the boats to be used initially.
An Environment Agency Wales spokesman said: "River levels in the Brecon area dropped overnight but due to further rain in the early hours of this morning they have risen again.
"With more rain due, the levels are not expected to drop again before lunchtime.
"A flood watch is currently in place on the River Usk and we are continuing to monitor the situation."
At 1640 GMT, there were flood watches on 18 river areas in Wales, including the Usk in Powys.
There are also three flood warnings, with the River Towy at Abergwili in Carmarthen and in north Wales, at Bangor-on-Dee and the lower Dee valley between Llangollen and Chester.
High winds saw the Cleddau Bridge in Pembrokeshire and the M48 Severn Bridge eastbound closed to high sided vehicles, with speed restrictions on the Britannia Bridge over to Anglesey.
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