Two children were taken to hospital and a search has been launched for another child and an adult missing after an incident at a weir, West Midlands Ambulance Service said.
A spokesman for the service said the circumstances of the incident were not yet clear but it is thought a boat may have gone down a weir in the Barford area of Warwick, Warwickshire.
Numerous emergency resources were sent to the scene and officers were searching the river for the missing adult and child.
It is not yet known how all four are related, or how old the children are.
The two youngsters who have been located have been taken to hospital and one is more seriously injured than the other, the spokesman said,
Emergency services were called around 5.35pm today and resources including Midlands Air Ambulances from Strensham and Cosford, land ambulances, the Hazardous Area Response Team and a number of senior paramedic officers were deployed.
Chris Kowalik, of the ambulance service, told Sky News that the children had gone to hospital in Warwick.
"The search continues for an adult and a child," he said.
Asked if it was fast running water, he said he was not at the scene and could not say.
"It appears they were at the bottom of a weir, and that's where the search has been centred," he added.