The family of a man reported missing from the Royal Welsh Show in Powys have said they are "desperate for news".
James Corfield, 19, a member of Montgomery Young Farmers' Club, was last seen at the White Horse pub in Builth Wells early on Tuesday morning.
He later failed to meet family at the showground where he had been camping with friends.
Police are searching the river and the force helicopter and mountain rescue teams have joined the search.
Supt Jon Cummins asked people to stay away from the search area so specially trained officers and rescue teams could carry out their work.
Speaking on behalf of his family, Mr Corfield's aunt, Gill Corfield, said: "We are desperate as a family for any small piece of information that anyone may have regarding his whereabouts.
"We are beside ourselves as a family and are desperate for news. We love you James, please come back safe."
Supt Huw Meredith of Dyfed-Powys Police said the force had been searching for more than 24 hours and was growing "increasingly concerned".
Brecon Mountain Rescue team said it was called to carry out a specialist search of the river in the area on Tuesday evening and rescue efforts continued on Wednesday.
A team spokesman warned there were "sections of fast moving water, deep pools and unstable banks" and urged the public to stay away from the river.
The Wales Federation of Young Farmers Clubs has asked for volunteers to help look for him and police have asked residents to check gardens, sheds and outbuildings.
Mr Corfield is described as 6ft 2in tall, of slim build, with short brown hair and was last seen wearing a blue Abercrombie and Fitch shirt and jeans.