The Fire Service in Timaru says if an accident on the Temuka bridge had happened a day earlier, the driver of one of the trucks involved could have drowned.
The trucks collided on State Highway 1 near Canterbury town about 1pm on Thursday.
Gary Parker from the Fire Service says one truck ended up with the rear still on the bridge and the front of the cab touching the river bed.
The drivers are in Timaru hospital with serious injuries, but Mr Parker says things could have been even worse if the crash happened on Wednesday when the river was swollen due to heavy rain.
A crane has been brought in to lift the truck out of the river.
Mr Parker says the Fire Service is also watching the load of fertiliser one of the trucks is carrying, which could contaminate the water.
The road reopened on Thursday night.
Listen to Checkpoint interview with Gary Parker