An ambulance crew, the Great North Air Ambulance, the police and the fire and rescue service all attended the scene. Three helicopters - the air ambulance, a police helicopter and a crew from RAF Boulmer in Northumberland - have been involved in searches for Colin along the river.
Teesdale and Weardale Search and Rescue have also been asked to help.
Chief Inspector Sukhsham Sahani, of Durham Police, said: "The police and other agencies are making every effort to locate Colin and our work will continue throughout the night if necessary.
"His family are understandably distraught and are being kept fully updated."